Your Old Windows

Where an old wood window is restored and equipped with a good storm window, it has been repeatedly shown in field studies to perform at least as well as a typical thermal replacement window — and at a lower cost.

If the craftsmanship and charm of your old windows is quickly being eroded by cold drafts and frost on the panes, it may be time to consider doing something about them. There are two options: restore or replace.

We replace a lot of windows. We probably replace nine old windows for every old window we restore and save. Many windows cannot be restored. Most windows made in the past 60 years or so are not traditional wood windows. The post-war push to build a lot of new housing quickly virtually eliminated the traditional wood window. It took too much time to build and install the window, and required a level of craftsmanship that just was not available. Builders opted for steel and aluminum windows, and factory made, self-contained wood window units that did not need a lot of site preparation and wall modification. Just put them in the opening and nail them up. It was faster, it was cheaper, and builders then, as now, were for anything faster and cheaper. (For more on the post-war housing boom, see Postwar Styles: Cape Cod, Colonial and Ranch.)

But, the sad consequence is that most of the windows installed since the World War are not worth saving even if they could be saved, and most cannot be. Manufacturers that are not out of business don't make those windows any longer, so parts just are not available. If these windows were installed in your house, the only option to fixing your window problems may be to replace the windows.

But, most pre-War housing, and some better housing built since the World War have shop-crafted wood windows. These most often can be saved, and as for parts — if you have a glass company, hardware store and lumber yard in your town, then you have all the parts you are likely to need.

OK, so old windows can be saved, but can they be made as energy efficient as modern windows?

The answer is "yes".

While old wood windows cannot be upgraded to rival the finest (and incredibly expensive) super-windows, they can be restored to rival the performance of a more typical (and more affordable) dual-pane thermal replacement window, and usually at a fraction of the cost.

Plus, there are other, nearly as important, advantages of restoring rather than replacing1 you old windows. You not only save on your own heating and cooling costs, which reduces waste and your carbon footprint on the planet, but you also save the resources and energy cost required to manufacture new windows — which considering what new windows are made out of, is not an inconsiderable savings.

You also preserve, not just wonderful old-time workmanship, but the superb old growth wood from which your windows were made. We can't build windows like that any more. It's not that our craftsmen do not have the skill and experience. Any of our master carpenters or cabinetmakers could build a traditional window. But, we can rarely get that dense, heavy old growth wood, and the new wood is… well, we're pretty sure it's wood, but it's not very good window wood.

If you own a home with old wood windows, then you are one of those fortunate folks that live in one of the 20 million American houses that do. You are one lucky soul. You have a choice of what to do about your windows that most homeowners do not have. You can restore them or you can replace them with a modern factory-made window. Those who live in houses with steel, aluminum or vinyl windows don't have the choice. These windows cannot be restored. They can only be replaced. Sorry.

What Others Say

Tom Silva "A good storm stops air infiltration about as well as most replacement windows, and the upfront costs are much lower. It's like putting money in your pocket."

Tom Silva
General Contractor
This Old House

Remodeling in Lincoln, Nebraska: Renovating Old Houses Cover
"While the thermal performance of a refurbished single-glazed window fitted with a tight storm can never quite equal that of the best factory-made double-glazed windows, the difference is not so great as to merit the replacement of old windows solely for reasons of improved energy efficiency..."

George Nash
Renovating Old Houses

Remodeling in Lincoln, Nebraska: Save American's Windows cover
"Homeowners tell me they know something is wrong with ripping out all their old windows and throwing them away, but they don't quite know what the alternative might be. They cannot find trades people to do the work.... If you are not a do-it-yourselfer don't worry. More and more savvy trades people are recognizing this new market for traditional window maintenance and repair services…

John Leeke
Save America's Windows
This article takes a look at the case for restoring rather than replacing heritage wood windows. We would like you to consider, just consider, the possibility that your old wood windows can be saved and restored to as good as new and even better than new condition for less money and at a much lower cost to the environment than replacing them. There are advantages to doing so, but also some disadvantages. We look at both and leave it up to you to make up your mind.

Are Replacement Windows a Good Investment?

Before 1996, primarily as a consequence of the pervasive and unceasing marketing of replacement windows after the energy "crisis" of the 1970's, it was nearly universally thought that replacement windows were vastly superior energy performers.

The Army-Vermont Study

But, in 1996 the State of Vermont and the U.S. Army Cold Regions Research and Engineering Laboratory joined together to actually test the performance of restored heritage wood windows, not by using computer simulations and a laboratory model, but with a field test during actual winter weather conditions2.

The researchers repaired and restored 150 old wood windows all over Vermont, then tested them against replacement windows in similar homes. What they discovered was completely unexpected. They found that…
Army-Vermont Study Results:
Annual Heat Cost Savings
Assuming your house already has wood windows with storm windows installed, how much would you save each year in heating and cooling costs by replacing your existing wndow/storm window combination with… Annual Savings Per Window
…new single pane windows and new storm window?0.00
…new single pane windows with low-E coating and new storm window?$2.57
…new double pane thermal windows?
(This option actually costs money.)
…new double pane thermal windows with low-E coating?$4.45

Well, this was absolutely not the result anyone expected.

Everyone had assumed that the thermal replacement windows would be more efficient than restored windows. The only question to be answered by the study was "how much more efficient". The last thing anyone expected to find was that restored old wood windows could be just as efficient, and often more efficient, than new thermal windows.

Why Does the U. S. Army Care About Windows?
The Pentagon is the biggest landlord on earth, public or private, owning over 300,000 housing units throughout the world, in every climate you can imagine from the frosty arctic to the torrid tropics, and every point in-between.

This does not include barracks, office buildings, guard shacks, bunkers, hangers, or storage structures, all of which have to be repaired and restored from time to time.

The average military housing unit contains 14 windows. The cost to replace the windows would be about $8,400. The cost to restore the windows, about $2,800. This amounts to a savings of $1.6 billion — which is not chump change even by Pentagon standards.
The study concluded that while replacing a restored wood window/storm window combination with a low-E coated dual-glazed window did slightly improve thermal efficiency, the improvement resulted in a heating cost savings of only $4.45 per window per year.

Given that any improvement in thermal performance obtained by replacing your old windows with a new double-panel wood window would be very slight, if any. Is the energy saving worth the substantial cost of replacement windows?

The Haberern Conclusion

This is precisely the question researcher, architect and engineer Keith Haberern set out to answer in his study of heating costs in New Jersey. His study results were published in the form of a chart of energy savings resulting from various window improvement strategies. His results appeared in an article in the Old House Journal in October, 20073. The study caused quite a stir at the time, and its results are quoted (and misquoted) widely even today.

Mr. Haberern concluded that the difference in energy savings between a modern replacement double-pane thermal window and a restored old wood window are insignificant. In fact, an old wood window with a new storm window slightly outperformed a new double pane window, and was a lot cheaper with a short 4.5 year payback period.

How Long to Pay Back Your Replacement Window Investment?
According to the Haberern study, the most cost-effective option, with a payback in 4.5 years, was simply adding a good storm window to the existing single pane window. The worst option is replacing a single-pane window that already has a storm window with a new double-pane thermal window. This option had a payback of 240 years.

The most interesting finding was that a single pane/storm window combination with a U-value of 0.50 (R-Value=2) slightly outperformed the basic double-pane thermal window with a U-value of 0.58 (R-value = 1.72).
Cost* Annual Energy Savings (BTU) Annual Savings per Window Payback Period (Years)
Add a storm window to a single-pane window
$50.00 722,218 $13.50 4.5
Replace a single-pane window with a double-pane thermal window
$450.00 625,922 $11.07 40.5
Replace a single-pane window with a double-pane thermal window with low-E glass
$550.00 902,772 $15.10 34.0
Replace single-pane window/storm window combination with a double-pane thermal window with low-E glass
$550.00 132,407 $2.29 240.00
* Costs include installation based on actual costs in New Jersey, including heating costs, at the time of the study.
The modern window outperformed the restored old window only if a low-E coating was used in the modern window. And, even then, the difference was very small with such meagre energy savings that the payback period was 240 years.

These results almost precisely duplicate the findings of the Army-Vermont study which also found that a restored old wood window with storm outperforms a new double-glazed thermal window, and that only when low-E is added to the thremal window does it very slightly outperform the wood window/storm window combination.

The Blasnik Result

Energy consultant Michael Blasnik found similar results in his field study of window performance in New York State using methodology that could hardly have been simpler, or more elegant. He simply checked building permits in his upstate New York community to see which houses had installed replacement thermal windows, then examined utility bills both before and after replacement to see if any actual savings had resulted. He found the actual average annual savings per household was just $40.00, not per window — but per house.

Based on these findings, Blasnik calculated that it would, on average, take 250 years for the cost of the replacement windows to be repaid from energy cost savings alone4.

The Hill Study

An earlier stury, in 1990, by William Hill5 at Indiana State University found much the same. The study did not get much attention at the time, but when combined with other similar studies, its results are significant. Looking at actual window performance in the field rather than in the laboratory, Hill concluded that merely replacing old windows, without any other energy improvements in the home, results in an annual savings in energy costs of just 1.4% per year.

He did not bother to translate this into dollars, but we did. In our town, Lincoln, Nebraska, this would amount to $33.12 for an average home which is what you would save replacing 10 incandescent light bulbs with compact fluorescent bulbs, at a cost of about $30.00, whereas replaement windows with installation would cost you $10,000, at least.

It would take about 311 years to repay the costs of replacement windows from energy savings alone. Of, course, this is using our local, Lincoln, Nebraska, rates for electricity and natural gas — some of the lowest in the country. So, it might take as little as 250 years where you live.

National Trust for Historic Preservation

A national study published in October, 2012, funded by the National Trust for Historic Preservation, looked at various retrofit measures used to restore

Reading a Window Label

Windows sold in the U.S. are rated by the National Fenestration Rating Council, an industry-sponsored trade association, and the results of the ratings are shown on a label attached to the window. These ratings are not exactly intuitive, and need a little interpretation.

 A   The "U-factor" is a measure of how much heat is transmitted through the window. The lower the U-factor, the less heat transmitted. U-factor is an archaic measure of heat transfer, and has been replaced in almost everywhere else by the R-value rating. The R-value can be calulated by dividing 1 by the U-factor. In this case, the window's U-factor of .30 tranlates to an R-value of ( 1.00/.30 = 33.3333... ) 3.33. Not much compared to an average insulated wall at R-13.

 B   The "Solar Heat Gain Coefficient" (SHGC) measures the amount of radiant heat from the sun admitted through the window. Rated from 0 (no gain) to 1, the rating is best understood as a percentage — simply multiply by 100. This window admits 42% of the solar heat radiation striking it. For more information, see Windows as Heat Sources, elsewhere on this page.

 C   "Visible transmittance", (VT) as the name suggests is a measure of how much visible light is transmitted through the window. The range is from 0 (none) to 1 (all). Again, it is best understood as a percentage. Multiply the rating by 100. This window is rated to admits 51% of the visible light striking it. No window is rated at 100% since the frame always blocks some light.

 D   "Air Leakage" (AL) is an optional test, and many windows do not include it, but if present, is a measurement of how much air leaks through the window in its fully closed position. The rating is in in cubic feet per minute of air flow for 1 suare foot of window. The lowest possible rating, indicating the least air loss, is .1, meaning 1/10 of a cubit foot per minute or less. The maximum allowed leakage is .3. The test does not measure air leakage around the window, for example, leakage between the window frame and the surrounding wall, so the actual leakage in your house will likely be greater.

 E   "Condensation Resistance" (CR) is an optional and relatively new test developed by the National Fenestration Research Council (NFRC) that measures a window's resistance to the formation of condensation. The NFRC describes the test thusly:
"The NFRC Condensation Resistance scale is 1 to 100, with a higher number representing more resistance to the formation of condensation. The Condensation Resistance rating is determined based on outside conditions of approximately -18°C (0°F) with a 6 m/s (15 mph) wind, and inside conditions of approximately 21°C (70°F) with relative humidities of 30%, 50%, and 70% taken into consideration. The Condensation Resistance rating is a value that considers the relative area under condensation at these three humidity levels, which are then normalized, and the degree to which the surface temperatures are below the dew point for the frame and for the glazing are taken into account. The Condensation Resistance rating specified in the NFRC rating is based on the lower of the frame, center-of-glazing, or edge-of-glazing values."
More simply: The range is from 1 to 100, with a higher number indicating a higher resistance to condensation. The problem with the measure is that it is hard for the average window buyer to translate the result to the real world. A lot depends on the average humidity level in your house, and if you install new windows, the average humidity level is likely to rise because you have probably sealed a lot of old air leaks. So the risk of condensation also rises. Bummer!

Generally, however, you should avoid windows with a CR rating of less than 50. The window CR rating here barely makes the cut. A high-rated CR is less likely to condensate, but not guaranteed against condensation. Any window will show condensation under the right conditions. If the CR is not shown, you should probably avoid the window. If the CR rating was high, the manufacturers would brag about it.
and enhance old windows and compared them to new replacement windows in five cities, representing distinct climate conditions across the U.S. The study concluded that old windows restored using commonly available materials and processes easily exceed the energy performance of typical replacement windows and can be nearly as effective as very expensive high-performance replacement windows, at a fraction of the cost of replacement windows.

The study's authors concluded that…
…upgrading windows (specifically older, single-pane models) with high performance enhancements can result in substantial energy savings across a variety of climate zones. Options that retain and retrofit existing windows are the most cost effective way to achieve these energy savings and to lower a home’s carbon footprint. Replacing the windows with high performance new windows is considerably more expensive.
(To read the full study report, download "Saving Windows, Saving Money: Evaluating the Energy Performance of Window Retrofit and Replacement".)

Lies, Damn Lies & Computer Models

All of these studies have something in common: So, where is the big savings on my heating bill everybody keeps talking about? We've all seen the ads in magazines, and on television: "Save 35%, 40%, even more on your heating bill. Replace your old, tired wood windows with our new Magnifico plastic windows!"

How can replacement window manufacturers claim such huge energy savings? Are they just lying?

Not really (well, maybe just a little).

First, in defense of window manufacturers, no reputable manufacturer makes such claims, and those companies that do are usually out of business pretty quickly. Unfortunately, however, local window sellers often do make very exaggerated claims, as evidenced, for example, by a recent advertisement we found on the web from Clear Choice windows of Orggon.

Second, window manufacturers don't do actual field testing of their windows. They rely almost entirely on computer models or simulations and laboratory testing. Computer analysis is used to build mathematical models of the various window components and then calculate the window's resistance to heat transfer. Then, a physical test is carried out in a laboratory to confirm the model. The laboratory testing environment, the "hotbox", is very artificial. It bears almost no resemblance to the actual environment of your house. This artificiality skews the computer models, making them inaccurate. Heat is not actually lost the way the computer simulations assume it is.

At no time is the window actually installed in a house and the results evaluated, so the computer models themselves are rarely compared to the real world experience of windows in a house. But, when they are, they are found to be very inaccurate. The Earth Advantage Energy Performance Score Pilot project of the Energy Trust of Oregon found that the most commonly used energy models were wrong as much as 96.6% of the time, regularly overstating energy savings.

So, how is heat actually lost from you house, and why are these models so inaccurate?

Thermal Conductivity of Common Materials
Conductivity of a material is determined by measuring how long it takes heat to move through a specified thickness of the material. This is more complex than it sounds since conductivity is affected by temperature, and by the specific composition of the material. Ordinary carbon, for example, is not a particularly good conductor, but Graphene, an allotrope of carbon, is the most conductive material so far discovered. Generally a material being tested is held at a specified uniform temperature to make results as universal as possible. The result of the test is a value called a Thermal Transfer Coefficient which reflects the general conductivity of the material. The higher the number, the more conductive the material is.
Material Thermal Transfer Coefficient
(W/m K)
Air (gas) 0.004
Aluminum 44.028
Argon (gas) 0.003
Copper 70.620
Fiberglass insulation 0.008
Glass (window)0.169
Graphene (The most conductive material)600.000
Gypsum drywall0.030
Hardwood (oak, maple)0.028
Krypton (gas) 0.001
Mineral wool (insulation)0.007
Plaster (wood lath)0.049
Silica aerogel0.001
Softwoods (fir, pine)0.021
Vinyl (PVC) 0.034

Window Thermodynamics 101

Heat moves inexorably from warm to cold. Heat in something warm will move to a nearby cooler thing until the temperature of the two things is exactly equal, Then heat will stop moving. This is "thermal equilibrium", the state that heat constantly strives to reach.

As we indicated in our article Insulating Your Old House, heat moves in three ways, radiation, conduction and convection. All of these processes are involved in moving heat through your windows, and all three operate all the time and at the same time.

Windows as Heat Sources?

Not all modern windows are just "energy holes" in an otherwise well-insulated wall. High-performance windows are quickly approaching the point at which they can be winter heat sources. How is that possible? Well, a wall can only lose heat — although perhaps very slowly — while a window both loses and gains heat. It gains heat because, under the right circumstances, it can gather enough solar heat to offset any heat loss through the window. But it takes a rather special, and, at the moment, a rather expensive window.

Solar Heat Gain Coefficient (SHGC) is a measure of the amount of solar heat radiation admitted through a window. In the U.S. the measure takes into account the whole window, frame included, and not just the glass. It is expressed as a number between 0 and 1. A rating of zero means no solar heat radition is admitted, and a rating of 1.0 means that all of the solar radiation striking the glass is admitted. The rating is more easily inserstood if converted to a percentage: A rating of 0.4 means 40% of the sun's radiation gets throught the window.

Low solar-gain glass shades the house from the sun's heat, so a low SHGC is desirable in very sunny areas like Arizona and Florida where keeping warm in winter takes second place to keeping cool in summer. In cold climates, like ours in Nebraska, a high SHGC rating is preferable. A window without a low-E coating typically rates an SHGC of about 0.7, just fine for cold climates. But, adding a low-E coating to reflect radiated heat back into the house, also tends to keep solar rays from entering the house because thay are reflected back to the outside. Most window manufacturers offer just one kind of low-E glass and use it in windows installed from Canada to the Mexican border. Typically the SHGC rating of the glass is in the range of 0.27 to 0.35. This is to much gain for sunny climates and not nearly enough for cold climates.

The ideal low-E coating is one that blocks radiant heat coming from one direction, but admits heat coming from the other. In winter, we want to block heat from leaving the house, and admit all the heat from the sun coming into the house. In summer, we want to do the opposite. So we need, what is in effect, a tuneable low-E coating.

And, amazingly enough, there is such a thing. Low-E coatings are designed to take advantage of the fact that infrared (heat) energy is transmitted at different wave lengths. Infrared coming from the sun has a fairly short wave length, begining at about 700 nanometers. This is usually referred to as "short-wave infrared". Heat energy radiating from warm objects, such as the furniture and walls in your rooms has a longer wave length (3000 nanometers and longer) and is called "long-wave infrared" Certain types of low-E coatings (generally called "passive" or "hard coat" low-E) are tuned to admit short wave infrared into you house, but block long-wave infrared from leaving your house. This type of low-E coating is ideal for heating climates.

In summer, or climates where cooling, not heating, is the problem, the opposite result would be better: blocking short-wave infrared while letting long-wave infrared out. And, there is a low-E coating for that. This one is called "solar control" or "soft" low-E. This is manufactured using a more expensive process than that required for passive low-E, and is, therefore, somewhat more expensive (and less durable — hence the "soft" in "soft low-E"). But, it is the ideal coating for net cooling climates.

But, what if you live in a place, like Nebraska, which "enjoys" both frigid winters and blazing summers? A compromise might be in order, and a lot of study has been done on just how to arrange windows on the north and south sides of the house to best suit both seasons. In the past it was thought best to put high solar-gain windows on the south walls and low solar gain windows facing north. But, emerging science, mostly from Canada, how suggests that high solar gain windows on all sides of the house gives the best result. So, how do you block that blazing summer sun. The very low-tech, but still the best solution: pull the shades.

Combine high solar gain and very low heat loss through the window (R-values of 7.0 and higher), and you have a window that is a potential net heat source in winter. There actually are such high-performing windows. Most of these are made in Germany, where the push for super windows is bolstered by strict laws making low-performing windows illegal to sell. But, the Canadians are coming on fast, and some Canadian super windows are beginning to rival the best German windows..

The problem, however, is the cost of these super windows. As a heat source even the best performing window is not great: the net heat gain is generally in te area of 7 million btu per year per house. This sounds like a lot of energy, but it isn't. Its about 2,000 kWh of electricity. In our home town. Lincoln, where the publicly-owned Lincoln Electric Ststem charges some of the lowest rates in the country, that 2,000 kWh savings amounts to $120.00. Elsewhere it may be as much as $300.00. Still, probably not enough savings to offset the cost of a house full of Canadian or German super-windows at $70,000 and more, but it's a start.
Of the three, convection is the most important, followed closely by conduction. Radiation in cold climates is a distant third.

Unfortunately, because windows are mostly glass, very little can be done to combat any of these thermal transfer processes. Glass is a good conductor and a poor insulator, and since glass makes up 90% of most windows, keeping heat from transferring through your windows is a losing battle. The very best "super insulated" windows barely reach R-7 (using three panes, a super insulated frame, low-E coatings, and Argon or Krypton fill gas) compared to a minimum R-13 in your walls and R-48+ in your ceilings. And, these are windows that start at about $2,000 per window. A more typical thermal window tops out at R-2 to R-2.8 for around $500.00.

So, to heat, even a well insulated window is usually nothing more than a gaping hole in your house's insulation. (But, there are exceptions. See: Windows as Heat Sources, elsewhere on this page.)

Radiation is the movement of heat in electromagnetic waves. It is how the sun's heat gets to Earth through the near vacuum of space. Anything warm, even air, radiates some heat. If it is very warm, it radiates a lot of heat. Put your hand in front of a radiant heater, and you can feel the radiated heat. Put you hand in front of a warm wall and you won't feel any heat. Your wall is, in fact, radiating heat, just not enough for you to feel.

While radiation operates all the time, during the winter is is not a prime mover of heat out of your house. In winter the only things radiating in your house are your walls, furnishings and the room air — not much radiation. So there is not much radiation going out of your windows (unless you happen to have a radiator right under a window, then there will be more). But, there is some, and window engineers combat this form of heat loss using low-E (for "low-emissivity") coatings.

Low-E coatings are metal or metal oxides thinly deposited on the surface of the window glass. They reflect thermal radiation, but allow light waves to pass through, so you can still see out. Properly used, they reflect back most of the long waves of heat radiation that originate inside your house, but do not reflect as much of the short UV waves that come from the sun. This allows sun rays in to warm up your room, but does not allow long-wave radiation originating in your house to escape, thus helping retain heat in your house.

In summer it is a different story. Radiation from the sun is a prime source of heat gain in the summer. In net cooling climates, like most of Arizona, a different configuration of low-E coatings are used to keep heat-producing UV radiation out of your house, but still allow light to come in.

Unfortunately, low-E coatings are not "turnable" to let heat in in the winter, but keep it out in the summer. In climates that have both a hot summer and cold winter, you have to pick one of the other type of low-E barrier. Here we usually opt for allowing radiation in to help combat winter cold.

Aerogel: Almost Science Fiction
Aptly nicknamed “frozen smoke,” aerogel is a solid material so lightweight that it is just slightly heavier than air. Invented in 1931 by chemical engineer Samual Kistler to win a bar bet, it is, despite its name, not a gel, Remodeling in Lincoln, Nebraska: Aerogel Block A block of aerogel, almost, but not quite transparent. It's so light — only a tiny fraction heavier than air — that a small puff of air would blow it across the room. but a dry solid that feels more or less like Styrofoam®.

Aerogel is almost all air: millions of tiny cells filled with air. This makes aerogel an excellent insulator. One inch of aerogel has an R-value of about R-30 — 10-15 times greater than fiberglass insulation.

It would be a great insulator in windows if it were transparent. It's not. It's translucent, but not transparent. And, while silicon aerogel has been used in skylights, it is not yet suitable where clear viewing is needed.

It's also very expensive. Its production is slow and costly, although recent advances in production methods will probably bring the cost down soon.

There is little question that eventually researchers will come up with a transparent aerogel — they're pretty close now. In fact, a company called Aspen Aerogel has already produced a modified aerogel that in thickness of 1/2" or less is as transparent as ordinary window glass, but it's still in the largely experimental stage.

Sandwich a thin slice of transparent aerogel between two pieces of glass, and you have super-insulated window glass. A mere 1/4th inch of aerogel could potentially yield a window pane with an R-value of 12.5, which is approaching with the R-value of a 4" wall.

This could make windows net heat sources in winter. Most house windows would take in more heat from solar radiation than they would lose. Large window walls eventually could be a building's primary source of heat.

Initially aerogel windows will be very expensive, but, like early LCD television screens, the price will come down fairly quickly as new production methods come on line and competition intensifies.

But, don't look for it in your neighborhood window store. It won't be there for a few years yet.


Conduction is the movement of heat on a microscopic level from molecule to molecule within a material. Heat one end of a steel bar with a propane torch and soon the other end is too hot to touch. Heat moved through the metal until it reached the other end of the bar. To reduce heat loss by conduction, materials that do not conduct heat very well are used. Most insulation, for example, is made of low-conductivity materials.

Unfortunately for window engineers the primary material in a window is glass, and glass is not a low conductive material. If it were, then the simple solution to window insulation would be to install thicker glass. But, while glass is not as good a conductor as most metals, it's good enough to move heat right along. There is not much that can be done about it until science develops a less-conductive glazing material. Such materials are in the experimental stage, but none is quite ready for widespread commercial use (See "Aerogel: Almost Science Fiction", this page).

Heat also moves through window framing materials by conduction. This is where today's efforts to reduce condutivity are concentrated, simply because it is an area in which something can be done. Most of today's window frames are very insulative, but this has little effect on the overall conductivity of the window since non-glass parts make up no more than 20% of a typical window.


Convection is the movement of molecules within fluids (i.e. liquids and gases). Convection does not take place in solids because solids do not flow, nor in a vacuum, because there are no molecules to move around. But, in gases, like air, molecules move freely and with that movement transfer heat and cold.

Most of the literature on controlling the movement of heat into and out of your house concentrates on air moving through gaps in your building envelope. But convection is actually more complex. It is not just one, but a series of four processes. Heat moves with air, so air infiltration and exfiltration through leaks in and around your windows moves heat out of your house and cold air in. Room air convection currents are the force that moves room air to your windows. Atmospheric air convection is responsible for whisking heat away from your windows. And, finally, air convection between panes of glass in a window is the primary mover of heat from warm inner pane to cooler outer pane.

Air Infiltration and Exfiltration
Air moving out of your house can be a major cause of heat loss, and not just through your windows. Warm air escapes from your house and cold air gets into your house through tiny cracks and gaps in walls and roof, and through gaps in and alongside your windows. A lot of homeowners are motivated to replace their old windows because they are "drafty". The proper cure for drafts is weatherization, not window replacement. Weatherization is simple and costs little — certainly much less than replacing your windows, which is akin to buying a new car because the old one has a flat tire.

Room Air Convection
To get through your windows heat must first be moved to your windows.

Most heat moves to your windows piggy-backed on moving air. Cool window glass attracts air convection currents which do their best move all the heat in the room to your window glass. Some heat is lost through the walls and ceilings, of course, but, according to Department of Energy estimates6, in a well insulated house as much as 25% of your total heat loss is through thermal windows. A 2000 sq/foot house has about 3440 square feet of walls and ceilings, and about 200 square feet of windows. Windows comprise just 6% of the building envelope. If six percent of your building envelope loses 25% of your heat, you can see how weak the thermal protection of a window really is. Heat sees your windows are just a large hole in your wall's insulation. And, if the windows are the easiest way out of your house, that's the path heat will take.

Your room air molecules give up heat to the glass in your windows by bumping up against the glass molecules. Then the heat moves, molecule by molecule, by conduction, through the glass where it is again in contact with air. The journey does not take very long. Window glass is only about 1/8" to 3/16" thick. How to slow heat transfer through glass is one of the perplexing problems confronting window engineers. Insulating window glass may not be far away (See "Aerogel: Almost Science Fiction", this page), but it's not here yet. The current solution is to add another (or two more) panes of glass, trapping air between the panes.

Inter-Pane Air Convection
The air trapped between two panes of glass is often described as "dead air", but it is anything but dead. It is, in fact, a very lively convection current that draws heat from the inside pane of glass and conveys it to the outside pane. The air touching the warm inside pane picks up some heat and starts to rise — warm air, as you know, rises. It soon reaches the top of the window sash where it cannot rise any further. Eventually it gets crowded against the colder outside pane and gives up some heat. Now colder and heaver, it falls, and soon there is a constant convection current moving inside the glass, conveying heat from the warm inside glass to the cold outside glass. The process is, unfortunately, very efficient and ruthlessly unstoppable.

But, while convection cannot be stopped, it can be slowed. So, this is an area where window engineers try to retard heat flow. One solution that has been tried is to eliminate most of the air between the panes, creating a partial vacuum. We know that vacuum is an excellent insulator — in fact, one of the best insulators, rated at R-45 per inch. Neither convection nor conduction work in a vacuum. The problem is that so far no one has been able to create a vacuum in a glazing system that will hold up over time. The extremes of climate and the brutal environment in which windows live always defeat the vacuum seal in the end, usually in fairly short order. Every year there is a new technique with promise of solving the vacuum problem. And, every year it turns out that it also does not work in the long term.

The current effort is being made by the UK's Guardian Industries which unveiled its new vacuum window glass in 2012. It may, according to company sources, increase the thermal efficiency of windows to as much as R-12. If it works, it would be a tremendous improvement in window technology, and also a very expensive one.

Fill Gases Are Guaranteed by Window Manufacturers to Leak Out

No manufacturer warranties in-fill gas from leaking, because they know full well it will leak out over time. This warranty language from Milgard Windows — which offers one of the best warranties in the business — is typical:

....The gradual dissipation of the [fill] gas may occur naturally over time and is not a defect..... "For Milgard Products with argon or krypton gas-filled insulating glass, Milgard injects the gas at the time of manufacture. The gradual dissipation of the gas may occur naturally over time and is not a defect. Other than gas loss due to seal failures, this warranty does not cover the gradual dissipation of inert gas or the amount of inert gas remaining in the Milgard Products at any time after manufacture."

Fill Gases So, the solution most often adopted by window manufacturers at present is to replace the air between the glass panes with a heavier gas such as Argon or Krypton (which has nothing to do with Superman® — that's Kryptonite, not Krypton). Heavy gases are more viscous and, thus, flow more slowly. They are also less conductive. Heavy gases can greatly slow down convection currents and reduce conduction, slowing heat transfer as much as 50%.

Unfortunately, however, the gases are not permanent. They're like the shine on a new car. Eventually the shine dulls and the car has to be polished. Gases leak out. Not because of seal failure, but because the gas molecules simply work they way through the seal material, while air molecules, in turn, work their way in. And, unlike you new car shine, the gases cannot be renewed. Once they're gone, they're gone for good. Seals have certainly improved since gas-filled IGUs were introduced over 40 years ago. But, they still permit gas exchange. Window companies say they have it under control, and that leakage is down to as little as 1% per year7.

That's what they say, but how confident are they in that say? Their confidence can be judged by the fact that not one single manufacturer guarantees its fill gas against leaking, not one. The same laboratory study that showed a leakage rate of "as little as" 1% also showed a possible leakage rate of "up to 18%". At that rate you would lose half of your fill gas in just 32 months. So that extra $5,000 you spent for a house full of Krypton-filled windows is bought you nothing more than a very temporary boost in your windows' energy performance.

Eventually, fill gas or no no fill gas, heat will reach the outer glass pane. Conduction takes over again as the primary transfer process to move it through the glass, and now (unless this is a triple pane window, (in which case you need to go back a few paragraphs and start over) the heat is outside.

What is a U-Value?

NFRC Logo Windows are not rated using R-value like every other insulation product in the U.S. Windows use an obscure measure called a U-value or U-factor, based on a testing and rating protocol established by the National Fenestration Research Council, a window-industry-sponsored association.

U-value is not a measure of how well a material insulates. It is, in fact the opposite. It is a measure of how well a material transfers heat. A material with a high U-value permits a lot of heat flow. A 1" steel block, for example, has a relative high U-26.2. A material with a low U-value transmits little heat. A 1" block of Styrofoam® has a U-value of just 0.15.

Originally all insulation materials were rated using U-values. Then insulation makers realized that showing resistance to heat flow rather than heat flow itself would better help the public understand insulation effectiveness. So they created the R-value rating in use today in North America. R-value is nothing more than the inverse or reciprocal of a U-value rating.

U-value rating of a window is more understandable once it is translated into the corresponding R-value. The calculation is easy, just divide 1 by the U-value. For example, a double-pane thermal window typically scores at about U-0.45, which converts to an R-value of 1÷0.45 or R-2.2. How much insulation is this? Your house walls, without any insulation at all are about R-3.2, and with insulation are at least R-11 to R-19. So, it's not much insulation.

Why do window companies use U-Value rather than the more familiar R-Value? Simple, U-values are less well understood, so they can be made to sound impressive. One young and enthusiastic window salesman told us that his company's windows were rated U-0.45 which meant that they "allowed only 4.5% of heat to pass through." We had to explain to him what the U-value really meant before we gave him the bum's rush.

Thermal window U-values when translated to R-values are anything but dazzling. As far as heat is concerned, a window with U-0.45 or R-2.2 is just a large thermal hole in your otherwise well-insulated R-19 wall.

For more information on R-values and U-values, see Will the Real R-Value Please Stand Up?.
Atmospheric Convection The heat jumps from the window glass to the outside air by again bumping molecules. If the heated air molecules now stayed put — pressed against the window glass — it would act as pretty good insulation.

But, because of atmospheric convection, they don't stay put. They are immediately whisked away by atmospheric convection currents, or, what we usually call the "wind". Wind is constant. It may be just a mild breeze, or it may be a tornado. Even if you can't feel it on those hot, muggy summery days, it's still moving, if only a little. These air currents keep moving cold air molecules against your warmer window glass where they keep drawing heat out of your window. It's a never-ending process, or, more accurately, it will end someday, but not before the sun explodes or the Earth just runs out of air.

Testing and Rating Windows

So, as we have seen, your windows exist in an environment filled with air convection. Room air convection moves heat to your windows. Air convection between glass panes helps pass the heat through your windows to the outside air, and wind whisks heat away from your windows into the great beyond to add your tiny contribution to the problem of global warming.

This is not, however, the environment in which windows are tested and rated for thermal performance.

To test for thermal performance, a window is placed between a hot plate and a cold plate inside a tightly sealed, environmentally controlled chamber, colloquially known as a "hotbox". Heat flow between the two plates is measured with a device called a heat flux sensor. The window's thermal performance is then estimated based on how long it takes to heat to transfer from the hot to cold plates, and then stated as a U-value.

What's missing from this test environment is the air that supplies convection currents.

The Elimination of Convection

While it may seem illogical to eliminate a process known to contribute substantially to heat loss through windows, there is a reason. Convection is much too hard to control. It's even very hard to model. Climate scientists have been trying for decades to develop an adequate model of atmospheric convection. It takes a long time and a super-computer to produce even a rudimentary model.

A successful testing process must be relatively simple, inexpensive, and easily replicated not only from window to window, but from lab to lab. Air convection is far too complex. It is almost impossible to duplicate air convection currents reliably from one test environment to another. So the solution adopted by the testing protocol is to eliminate convection from the tests as much as possible.

Single Heat Source

A Direct Measure of Heat Loss Through Windows

The current window thermal resistance testing process results in a U-value rating of a window. What a U-value tells us is how readily heat flows through a window, but in an abstract and arcane language that most of us don't understand and cannot interpret. What I really want to know is how much heat will be lost through my window. And the window's U-value does not tell me that, at least not in a straightforward way. Window Restoration or Window Replacement: Thermometer.

Testing how much heat a window loses is actually much less complex than the convoluted combination of hot box tests, computer simulations, and estimates now needed to come up with the U-value. It's a simple and straightforward measurement of heat loss. If we know how much heat a window loses, we know how much heat we have to add to make up for the loss.

Here's the test.

Build a a big well-insulated, air-tight box. This will be the testing chamber. Maintain a constant 0°/F outside the box. Add heat to bring the air temperature inside the box to 70°/F and maintain that temperature for 24 hours. For heating and cooling purposes, heat is usually measured in North America in British thermal units or BTUs. Any number of highly accurate devices called calorimeters can measure the number of BTUs added to the testing chamber.

The number of BTUs of heat added to the box to keep the air temperature at 70°/F is always equal to the number of BTUs lost. If 2,000 BTUs must be added to keep the temperature constant, it's because the test chamber lost 2,000 BTUs through the floor, ceiling and walls over the 24 hour period. The number of BTUs lost is the Base Heat Loss.

A British Thermal Unit (BTU) is the amount of heat energy needed to raise the temperature of one pound of water one degree Fahrenheit (1°/F) at a constant pressure of one atmosphere. It is equal to 1055 joules (the metric measure of heat energy, and the one most used in scientific circles). In the U.S. and Canada the humble and largely outdated BTU it is the traditional measure of heat energy in the heating and air conditioning industries, and the measure most people are familiar with.
Now cut a hole in the wall and install a window to be tested (in accordance with the manufacturer's installation instructions). Repeat the test with the window installed. The additional BTUs required to heat the box is the Window Heat Loss (WHL) due to the test window. If the Base Heat Loss is 2,000 BTUs, but with the window installed, the chamber lost 6,500 BTUs, then 6,500 - 2,000 = 4,500 is the WHL of the window.

Let's say Window "A" has a Window Heat Loss of 4,500 BTUs over a 24 hour period, while window "B" has a Window Heat Loss of just 2,200 BTUs, do you have any trouble telling which is the better thermal window?

Isn't this all we really want to know about the window? Do we care which window has the lower U-value or higher R-value? Do we need to know what features of Window "B" made it the better window? Maybe it leaks less air, perhaps it is less conductive, or its low-E coating is more effective. The point is we don't know or need to care why it is a better thermal performer. We just know that Window "B" is a better thermal window because it retains heat better than Window "A".

This test has the advantages of nearly exactly duplicating an actual home environment, being easy to understand, and a more direct and forthright test of window thermal performance. So, if it is easier to understand and a more direct and more honest test, why don't window manufacturer's use it? Well, it's easier to understand and a more direct and more honest test — it would be very hard to spin.
Another simplification used in laboratory testing is the heat source. In your home, the heat source is complex and multi-faceted: Most of the heat lost through your windows comes from warm room air. But, your inside walls, floor, ceiling, and furniture contribute a little radiant heat (not much, but some) and a radiator inside the room could contribute quite a bit.

In the laboratory the sole and only heat source is a bank of radiant heaters aimed at the window. Radiant heat can be precisely controlled and greatly simplifies measurement, so it is the standard for laboratory testing. But, it over-produces the one heat transfer process, radiation, which in the real world of window heat loss, is the least important process. In the lab world, however, it is the most important.

Windows Designed for Laboratory Tests, Not Your House

As a consequence laboratory test results are very skewed. Lab tests have almost uniformly concluded that about 70% of the heat loss through your windows is by radiation. Conduction and convection account for only 30% or so. This is true, keep in mind, in an environment in which almost all air has been eliminated and radiant heaters are aimed directly at the inside of your windows. How close is this to the actual environment of your house?

The consequence of this elimination, however, is that the testing model is an unreliable predictor of how windows will actually behave in the real world full of air and convection currents. And, this has unfortunate results.

The most consequential result is that it affects how windows are designed and built. Window manufacturers tend to build windows that score high in the test environment, but do not necessarily perform well in the real-world. For example, most window manufacturers place a lot of emphasis in their low-E coatings to block radiation. Yet, outside the laboratory setting, radiation plays only a small role in heat loss. Field studies have shown that low-E coatings have very minimal effect in winter, and just a small effect in keeping our houses cool in summer. But, in lab tests, windows with better low-E coatings score well because the sole heat source is a blazing radiant heater, so window companies emphasize low-E coatings rather than working on measures to reduce convection and conduction, which are the process by which most of your heat is lost in the real world.

So, a low U-value rating for a window does not actually tell you how well the window will perform in your house, UNLESS, your house is actually a hotbox chamber with a blasting radiant heater located just behind the window. If your house is more conventionally arranged, say with furniture, carpets and other typical house-type stuff, heated by forced air or a room radiator, the U-value tells you very little. The test of the window in a normal house environment has not been done, although it would be simple enough to do (See: "A Direct Measure of Heat Loss Through Windows", above). Window manufacturers are not the least bit interested in showing that their windows do not perform in the real world as well as advertised, and no government agency seems to have been aroused enough to do a comprehensive formal field study — even though going Green is now officially the government's policy.

From the limited field studies that have been done, however, we know that actual thermal window performance is well below that predicted by U-value ratings. There is plenty of evidence that properly restored old wood windows with storms perform at least as well as new thermal windows, and in the long run, as seals start to leak and the fills and coatings that temporarily boost new window thermal performance start to degrade, restored old windows may perform better.

Modern Window Design and Materials

Back in the day, windows were designed for longevity, endurance and durability. Now, they are engineered for energy efficiency, with a long service life being a lesser consideration. This focus has drastically changed how windows are designed and manufactured, and how long they last. Most modern windows have an expected lifespan of about 15-25 years — some of the best may reach 40 years — some of the worst only 5-10 years. Twenty years is about the average. The problem is not that the windows are poorly made. Many of today's windows are very well made. The problem is the engineering and the materials used. They just don't last like old wood windows.

Complex Spring Balances

A comparison of old and new sash balances, for example, illustrates this basic difference.

If you raise the sash of an old double hung window, it stays in place at whatever position you leave it. This is possible because the weight of the window sash is counter-balanced by two iron weights that ride in pockets built into the wall alongside the window, attached to the sash by ropes. The mechanism is simple, and works by gravity. And gravity, at least so far, has never failed.

There is nothing to break but the ropes, which can be easily replaced and once replaced last between 50 and 100 years, (or even 200+ years if bronze chain is used in place of rope).

Modern, self-contained, replacement windows cannot use sash weight pockets built into the wall, so different balancing mechanisms had to be developed. These are all some form of metal spring. The tension in a spring is what holds the sash in place when the window is open. Spring ballasts, however, unlike simple iron weights, are complex mechanical devices prone to breaking. Metal springs themselves are subject to metal fatigue which can cause the spring to lose tension over time or even fail completely. We have replaced some worn out spring balances less than four years old.

Other Voices

National Trust for Historic Preservation

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"...Given that an average house has between 24 and 30 windows, and the typical replacement window unit costs between $500-1,000 each, does an investment of $12,0000 or more make sense? On the flip side, the cost to restore an existing window and add storm windows (where appropriate) will generally be much less....

Many window replacement manufacturers claim greater savings than actually occur. Since windows account for at most 25% of heat loss, the payback and time to recoup your investment in terms of energy savings could take between 40 and as much as 200 years, based on various studies. A study from Vermont show the saving gained from replacement windows as opposed to a restored wooden window with a storm is only $.60. The added problem is most replacement windows will not last as long as 40 years, much less over a hundred years. And, some are being replaced only after 10 year of service."

National Park Service

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"[We recommend] the retention and repair of original windows whenever possible. We believe that the repair and weatherization of existing wooden windows is more practical than most people realize, and that many windows are unfortunately replaced because of a lack of awareness of techniques for evaluation, repair, and weatherization. Wooden windows which are repaired and properly maintained will have greatly extended service lives while contributing to the historic character of the building. Thus, an important element of a building's significance will have been preserved for the future." (Emphasis supplied).

Vinyl (PVC) Frames

Vinyl is another culprit. Vinyl expands and contracts nearly twice as much as wood with changes in temperature, and seven times more than glass. This amount of expansion and contraction makes it hard to keep gaps from developing between vinyl components of the windows. The Canadian Natural Resources Consumer Guide from 2005 states “The disadvantages of vinyl framing material is that vinyl expands and contracts with temperature, opening up cracks for air leakage.” The Canadian Center for Mineral and Energy Technology, study of long term performance of operating windows8 concluded that “Air Leakage in vinyl windows increases 136%, significantly more than aluminum or wood. Un-reinforced PVC profiles have a lack of rigidity and a high coefficient of expansion; PVC profiles are subject to distortion.” In their studies, they found that vinyl windows have a tendency to bend, distort, and even crack, especially in "regions of Canada that experience cold winter months". (What regions of Canada don't experience cold winter months?)

Vinyl deteriorates when exposed to ultra violet (UV) rays. Those of us of a certain age remember well how vinyl dash boards in the ol' Chevy used to crack and split after only a few years exposure to sunlight. The vinyl is better these days. UV inhibitors retard deterioration, but nothing can stop it entirely. Vinyl window parts, especially the thin, flexible vinyl used in balance mechanisms, will deteriorate over time with repeated use. After a few years they become brittle, break and need to be replaced.

Vinyl softens at temperatures greater than 165°/F, and its not that uncommon to see such temperatures in the unventilated space between window panes on a bright, sunny day. Excess heat can also cause vinyl to warp and twist. While vinyl window manufacturers say they have that problem under control, few manufacturers actually warrant vinyl windows against warping, twisting or cracking, and those that do limit the warranty to just a few years.

Leaking Insulated Glass Units

Double- and triple-pane glass in thermal windows are manufactured in what are called Insulated Glass Units (IGUs). Spacers between the glass panes not only separate the panes, but seal in the gas between the panes. Air between panes of glass must be very dry. If the spacer holding that two panes of glass together springs a leak, moist room air will get between the panes and condensation will form, which may lead to mold, mildew and other nasties inside your IGU where you cannot get at it. There is no cure for this problem. It cannot be repaired. The entire unit has to be replaced.

Seals are much better now than they were when first IGUs were first marketed in the 1930s as Thermopane®. Still, no one has yet invented a seal that does not leak. All spacers are going to leak eventually. Some leak sooner, some later, but they all will leak someday. The environment that spacers have to survive is brutal. Temperatures can be as hot as 180° in summer, plummeting to -30° in winter, or even worse. It's very hard to come up with an adhesive that works for a long time in that kind of environment. It is common to find spacer leaks even in very good windows within 10 years of installation and in some poorly made windows within one or two years.

Window Sashes that Cannot Be Repaired

Like today's Integrated Glass Units, modern window sashes are often single units that cannot be taken apart for repair. They can only be replaced using a new sash supplied by the original manufacturer. You have to hope the window manufacturer (1) is still in business and (2) still makes the part. Plus, you can expect the replacement parts to cost nearly as much as a new window. The manufacturers have a monopoly on replacement parts for their own windows and are not at all bashful about charging what the market will bear, and a little more.

Old Windows: Built to be Re-Built and Re-Built and Re-Built and…

Replacing a Broken Sash Cord
Popular Science Monthly, December, 1928

What is the quickest and easiest way to replace sash cord?

Few defects around the house cause greater annoyance than a broken window cord. Yet, like many other things that go wrong, it may be easily remedied witout callin in the aid of a mechanic, if you approach the task with confidence and have some degree of ability to use your hands. Few tools are required.
  1. Inspect the cords of both upper and lower sash. If one is broken and one or more are worn and frayed, it will be an obvious economy to replace them at the same time.
  2. Obtain a good grade of braided sash cord from the hardware store. It will be more economical to buy an entire hank if several cords are to be replaced, but if only one or two, you can figure roughly how much you need by allowing 5 ft. for each cord of any window of ordinary size.
  3. We will assume that only one cord is to be replaced and that in the lower sash, for that is more likely to give trouble. Remove the sash by the method described in a previous reference sheet (Nov., 1928). If there is a broken cord on each side, remove both stop strips, but if only on one side, the other side need not be disturbed.
  4. If but one cord is broken, swing that side of the sash out and place a kitchen chair, bot, or other support under it. The lower sash may be pushed under one end of the upper sash to help hold it, as suggested in Fig. 1. It is, however, the best and safest practice to take the good cord out of the sash, tie a knot in it, and allow it to run up to the pulley as in Fig. 2, and then set the sash aside.
  5. Remove the pocket facing. It may be necessary to take out the parting strip to do this. Lift the weight out of the pocket as in Fig. 3, cut the rope away from it and dig out the knotted end from the sash as at A of Fig. 1. Observe how each of the knots is tied and fastened, for the new cord may be fastened the same way.
  6. The easiest way to put the new cord through the pulley is to make a “mouse.” Wrap several narrow pieces of thin sheet lead around a piece of strong, flexible line perhaps 8 ft. long, as in Fig 4. Pound lightly, or press each piece of lead so it stays in place about as shown. A piece of chain, a bent nail, or other light weight will answer the purpose as a makeshift. A 4-in. length of sash chain makes a particulrly convennient “mouse.”
  7. Tie a knot near one end of the cord like Fig. 2, and tie the long end of the mouse line to the other end using half hitches as in Fig 5. Push the mouse through the pulley from the front; allow it to drop down the pocket behind the pulley stile until it can be reached from the pocket opening. Pull out the mouse through the opening and at the same time coax the cord through the pulley from the front. Pull the cord down until the knot (Fig. 2) stops it. Remove the mouse line line and tie the end of the cord to the weight with a knot that will not slip. Use the knot shown in Fig. 6, for example, or use the same knot that was on the old cord. Nearly every workman has a pet knot for this purpose, but any knot that does not allow the cord to pull directly over the axis of the weight will permit the latter to turn and swing clear of the back side of the pulley or the sash ccnot be closed, though the stretch of the cord will soon make it right if not more than 1/2 in. has to be gained.
  8. To find the length of the cord, pull the weight up until it strikes the back of the pulley. Untie the slip knot (Fig. 2) and swing the sash back again until it is as nearly as possible in place. Hold the cord against the edge of the sash and cut it off 6 inches below the hole at A, Fig. 1. Lay the cord in the groove, tie the knot and push it into its hole. Drive a small nail through the knot, if necessary to hold it there and push the sash into its place. Raise the sash, put the pocket face in its place, fasten it, and put the stop strips on.
  9. It is obvious that if the upper sash is to have new cords, they must be put in before those of the lower sash. In this case, remove the lower sash entirely and do not merely swing it around and attempt to hold it as suggested in operation No. 4 above. Pull the upper sash down, take out one or both parting strips, pocket facing, and weights as may be required. Put the cords in by the methods already described, being sure the cord of the upper ash is not too long or the sash may not stay up. The lower end of the weight should swing at least 3 inches above the window stool when the upper sash is in place to allow for the stretching of the cords. Replace the parting strips, pocket facings and stops.
The Sinclair Inn Museum in Annapolis Royal, Nova Scotia can claim what is possibly the oldest double-hung wood window in North America. Built sometime between 1690 and 1710, it was probably recycled from a still older building.

After 300 years, it still works.

Unlike modern windows, old wood windows were made to last for many generations, so they were built to be repaired, over and over again if necessary. The old-time craftsmen knew that their windows would last a good long time, but not forever. So they built windows that could be easily fixed when something finally did give way.

There are no exotic materials in an old window, just wood, glass, iron, rope and a little bronze or brass for the hardware. No vinyl, no unpronounceable chemical compounds, just basic stuff available almost anywhere that has a lumber yard and a hardware store.

There is no chance the parts won't be available even 200 years from now, unless we simply run out of trees — which, despite the the hysterical alarms of the more radical elements of the environmental movement, is not all that likely.

Anyone with some basic carpentry tools, a little understanding of how windows work, and decent eye-hand coordination can restore an old window.

Restoring Window Function

Unlike modern windows, old windows rarely break. They may stop working, but the culprit is seldom the window itself. It is (1) broken pulley ropes and (2) accumulated layers of paint that have glued the sashes to the frame. These are easy fixes that usually take less than 2 hours to complete. Freed from decades of paint, with new pulley ropes, and with a little scraping and sanding, most old windows work like new.

First, with light taps with a hammer on a painter's 5-in-1 tool inserted into the gap between the window frame and sash, break the window free of the paint that has glued it shut. The window will then move up and down. This will take time and a little elbow grease, but it's not hard, and does not require great skill. Keep the taps light and watch out for the upper sash when it breaks free. These have a tendance to just drop, mashing fingers, etc.

Step two is to replace the pulley ropes. These join the window sash to the counter-balancing iron weights, and are known in the trade as "sash cords". Without the weights, the window will not stay up. With the weights and sash joined with new cords, they will work just like new. The cotton cord used when the window was made had a life expectancy of about 25 years. The amazing thing is, that after nearly 100 years, most are still working. But, some are not, and in either case, it's past time to replace them, broken or not.

The sash cords we use today are not the ropes of 100 years ago. The new nylon/cotton cords last up to 100 years. If you want the repair to last nearly forever, use bronze sash chain instead of rope. Some people don't like chain because it's noisy, but considering that a properly installed sash chain never has to be replaced again, the trade-off is, in our humble opinion, a no-brainer. One caveat is that chain cannot be used with pulleys originally intended for cord, so if you are migrating from cord to chair, you will also have to replace the pulleys.

Repairing Deterioration

The next step is to check for rot and deterioration. Water is a window's worst enemy. Although poor design, sloppy installation, wood-loving insects and baseball-loving kids can contribute to a window's demise, the usual culprit is wood rot caused by standing water.

We find this primarily on the sashes and stool or sill of the window. It's not hard to fix. If the problem is minor, and exterior (which is where it usually is), then a little outdoor spackling and some new paint solve it. Otherwise there are special two-part epoxy fillers that are — or so their manufacturers' say — even stronger that the wood being replaced. If the problem is even more serious we may splice in some wood or even remove and replace the deteriorated part with a new, matching, part made in our cabinet shop. If necessary, we can build and entirely new sash that duplicates the old one exactly. In 40 years we have had to do this exactly twice.


Now we need to look at weatherproofing. Over the years the wood in your window has dried out and shrunk a little. This is the reason your sashes may be loose in their frames and sometimes rattle in the wind. Since the sash is now smaller, air can creep around the sash.

The weatherstripping may also need attention. A lot of old window makers used bronze for weatherstripping, and it may be intact, but often it has come loose because the nails used in those days to attach the weatherstripping have worked themselves out — or the bronze may have been removed by some old painter too lazy to mask it off when painting. We use new spring bronze slipped around the sides of the sashes to eliminate air leaks, tighten them up in frame and provide a nice slick surface to ride on. Horsehair felt made specifically for windows, or silicon bulb weatherstripping (but not rubber or plastic which do not last) can be used where the sashes meet the frame at top and bottom and at the meeting rail to bar air infiltration.

Insulating Around Windows

Once the weatherstripping is done, we look at the insulation where the window meets the wall and in the sash weight pockets. Even if you have had your old house insulated, the insulators usually miss that small 1/2 inch or smaller gap between the window frame and the wall stud. We seal this area with low-expansion foam.

Hopefully your insulators did miss the sash weight pocket. If they did fill it with blow-in foam, cellulose of fiberglass, you just may have sussed out why your window suddenly stopped working. If it is filled with insulation, the weights don't move. If the weights don't move, the window does not work. But, the sash weights do not take up all of the space in the pocket, and the space they don't use can be insulated using a high R-value rigid board like polyisocyanurate (we don't try to pronounce it either — its "poly-EYE-so" to us) which has a rating of R-7.2 per inch. We use 2-1/2 to 3" of it in total together with expanding foam to arrive at a total R-value of R-18 to R-22 in the sash pocket — which is probably more than the R-11 to R-19 you have in your walls.

Replacing Single-Pane Glass

What we never do, and don't recommend, is replace single pane glass with double-pane IGUs. We used to try this, but it never worked very well. Old sashes are just not milled for the extra thickness of the dual pane glass, and the sash pulley/weigh system is not designed for the increased poundage of extra pane. There are also technical difficulties in installing the glass. New Integrated Glass Units are generally not installed in the same manner as the traditional single-pane glass. And, in the end, the gain in energy performance, if any, is tiny. A much better choice is to install a good storm window over restored sashes. Unless its cracked or broken, leave the glass alone.

Restoring Your Wood Double Hung Windows

Here are the seventeen steps required to rebuild a typical old wood window and put it back in working order. Not all of these will be required for every window. Non-typical repairs are indicated with a *.

1. Remove sash stops. Reserve for reinstallation. *Sash stops that cannot be reused are discarded and new sash stops fabricated in our door and window shop match the existing sash stop as closely as possible. We do not have cutters for all of th heritage sash stop profiles, but we have most of them.

2. Remove the lower sash, cutting dnd removing any existing sash rope.

3. Remove the parting bead. This is usually destroyed when removed due to accumulated layers of old paint, and will be replaced with a new parting bead. Parting bead is waxed to ensure smooth operation, but never painted.

4. Remove the upper sash, cutting and removing any existing rope.

5. Remove any hardware (locks and handles) from the sashes for cleaning.

6. Inspect the frame for damage.

7. Strip all paint and varnish in our steam box. Steaming removes most paint and glazing comppound, and does no harm to the underlying wood.

8. Prime the exterior of all sashes.

9. Replace glazing compount with acrylic silicon glazing compound. This compound remains flexible and does not harden over the years like the old linseed oil compounds.

10. *Repair any broken or rotten sash parts: muntins, rails or stiles.

11. *Repair any broken or rotten frame members.

12. Lubricate pulleys with dry silicon lubricant (does not attract dust to turn into goo. *Replace missing and repair inoperable pulleys.

13. Replace existing sash ropes with Sampson** Red Spot pulley ropes — the best available. Most old windows have a pulley access cut into the frame. *If your windows do not, then we will need to remove the interior window casing to replace the ropes, and they reinstall the casing.

14. Install spring bronze strip weatherstripping at sides of sashes. This is the original 100 year weatherstripping. We use Dorbin 25B Bronze Spring Weatherstrip, made in the U.S.A. and the best available.

15. Reinstall sashes and hardware. *Replace missing hardware with matching or complementary hardware as needed. ***Replace missing sash weights.

16. Lubricate sash sides with bowling alley wax. (We recommend re-waxing every Spring — when the time changes from standard time to daylight savings time, so you will remember.)

17. Test for smooth, “one finger”, operation.

* Not included in the standard window restoration because not every window needs this atypical repair.
** Sampson Cordage is the oldest rope maker in the U.S., and the owner of the oldest registered trademark still in use.
*** Old sashweights are increasing difficult to find, but modern replacements are available from Smith Restoration Sash or Window Weights.

Lead Paint and Putty

Before April, 2010 no special precautions were required, other than common sense, in dealing with old lead-based paint and glazing compound. Now, elaborate measures are mandated by the EPA, most of which are still, however, just good common sense.

Never grind off lead paint so that the paint particles discharge into the air. The best course is to use removal methods that do not produce dust, such as heat or steam removal. But, if you must grind (and you may have to grind out the old, hard putty), do it outside, and make sure any dust is captured in a HEPA (Not HEPA-like, but actual HEPA) vacuum. Cover the ground with heavy plastic.

Keep lead dust confined to the work area. Cover any doorway into the rest of the house with plastic, and seal all air intake vents in the room. Use reasonable care to ensure that lead particles are not tracked into other rooms on boots or shoes — usually a brush left by the doorway for dusting off shoes is all that is needed. We hang ours on a string. Wear a N-95 dust mask. Vacuum thoroughly at the end of each day, and at the end of the project, fold the ground-dover plastic edge to center to trap dust particles, put the plastic in a heavy plastic bag, and tie the neck of the bag using a "gooseneck" tie.

Adding Storm Windows

If this seems like a lot of work, it is. But, restoring your old window is about half the price of replacing it with a new thermal window, and wastes nothing. Old growth hardwood is saved from the landfill, and a lot of good old-time craftsmanship is preserved. A typical window can be restored for between $200 and $300. Of course it is not yet as energy efficient as a new window. For that we are going to have to add a storm window.

A good quality white aluminum storm window installed will run about $80. An upscale wood combination storm window from a company like Spencer Works will cost a bit more. If you already have storm widows, then you are just that much ahead. But, assuming you don't, your cost to repair your old wood windows and add a good storm window is about $325.00 vs. $500 and more to replace them. This is a savings of $5,500.00 in a 20-window house. For your investment you get a window that should be good for another 100 years, while a replacement window is doing very well to last 30 years. Your window performance is just as good if not slightly better and you saved 45% of the cost of installing replacement windows.

How to Really Save Energy Costs

Replacement widows are, according to most energy consultants9, high on the list of energy measures that don't work. The notion that new, thermal windows save significant amounts of home energy is a myth fostered primarily by those who sell replacement windows.

Independent studies starting in the 1980s have repeatedly shown that the most effective course of action for those with old wood windows in any sort of reasonable shape is to simply add a good storm window. If you already have a window/storm combination, your window system is already as energy efficient as a replacement thermal window.

If you really want to save energy costs, assuming your attic and walls are already insulated at least to code, your heating and cooling system is already very high efficiency and all your doors are weather-stripped, go buy a high efficiency water heater ($1,000), put $4,400 in the bank against a rainy day, and treat yourself to a really lavish steak and lobster dinner ($100.00), for being "energy smart". Just the water heater alone will save many times more energy dollars than a whole houseful of replacement windows.

Rev: 11/19/2014



1 W. Sedovic, J. H. Gotthelf, "What Replacement Windows Can’t Replace: The Real Cost of Removing Historic Windows" ( Journal of Preservation Technology, Vol. 36, Number 4, pp25-29, 2005. (Download PDF)

2 B. James, A. Shapiro, S. Landers, D. Hamenway, "Testing Energy Performance of Wood Windows in Cold Climates", (1996.) University of Vermont, U.S. Army Cold Regions Research and Engineering Laboratory. (Download PDF))

3Keith Haberern quoted in Noelle Lord, "Embracing Energy Efficiency" The Old House Journal pp 40-45, Oct, 2007.

4 Michael Blasnik quoted in R. Yagid, "Should Your Old Wood Windows be Saved", Fine Homebuilding, Issue 210, March 11, 2010. (Download PDF)

5 William W. Hill, "Replacement Windows and Furnaces in the Heartland: Indiana's Energy Conservation Financial Assistance Program", Center for Energy Research, Ball State University, Indiana, 1990. (Download PDF)

6 U.S. Department of Energy (2005).

7 The study most frequently quoted to support the claim that gas leakage is as little as 1% per year is J. Plavecsky, Leaking Out the Facts, Door and Window Maker, Sept-Nov, 2000, which found that the amount of leaking was determined by quality control in the factory, the method of inserting the gas, the type of seal used, and the care with which the seal was sealed. He tested seals in a laboratory environment using a process of "accelerated aging....said to represent five years of normal field exposure". The results were average leaking as high as 18% and as low as 0.9% over the simulated five years. Quite a range, and there is no way for a consumer to judge whether his replacement window is at the high or low end of this leakage rate.

8 Canadian Centre for Mineral and Energy Technology. A Study of the Long Term Performance of Operating and Fixed Windows Subjected to Pressure Cycling. Republished by CANMET Efficiency and Alternative Energy Technology Branch. Publication # M91-7/214-1993E. 1993.

9 Michael Blasnik, Mythbusters: What Works, What Saves?, Proceedings of the 2008 National Energy and Utility Affordability Conference.

Articles and Books

The National Trust for Historic Preservation: Windows "Have you ever wondered why there are no replacement fireplaces? Fireplaces with ill-fitting or missing dampers leak more heat than windows do, but salesmen don't leave flyers for new dampers in your mailbox, do they?" Learn the answer in this well-written and concise statement of why old windows should be preserved, and a scathing indictment of the practices of the replacement window industry.

John Leeke's Historic Homeworks. Has a number of helpful videos and articles on restoring old windows, and a discussion forum where you can ask questions and get helpful answers. If you are serious about restoring your windows, you will want to invest in Leeke's Save America's Windows book which is pretty much the old window bible, and has, among other useful information, a list of window restoration experts organized by region.

Beth Goulart, "How To Restore Sash Windows" Old House Journal.

Thomas Baker, "How to Repair Sash Windows" This Old House.

William T. Cox Jr., "Sash Window Clinic: Maintaining the Mechanics of Double-Hung Windows" Old House Journal.

John H. Myers, "Preservation Briefs 9: The Repair of Historic Wooden Windows". Technical Preservation Services, National Park Service., U.S. Department of the Interior.

John Michael Davis, "New Life for Old Double-Hung Windows" Fine Homebuilding

Understanding Energy-Efficient Windows. Fine Homebuilding, The Taunton Press.

Energy Savers: Tips on Saving Energy and Money at Home. U.s. Department of Energy. (PDF)

Do-It-Yourself Home Energy Assessments. U.S. Department of Energy.

Alvarez, Kimberly K. & John D. Alvarez II, AIA, "Restoring Our Appreciation of Historic Wood Windows: Making a Case for Restoration Versus Replacement", The Local Landmarket, Issue 11, March 2009. New York State Office of Parks, Recreation and Historic Preservation, Field Services Bureau, Division for Historic Preservation.

Window Repair and Weatherization Guidebook: A Handy Guide for Owners of Portland, Oregon Homes (PDF), Bosco Milligan Foundation/Architectural Heritage Center With support from the Irvington Community Association Portland, Oregon, ©2012.
Parts & Supplies

Some of these sources are not used to dealing with consumers, and expect you to know exactly what you want. Others, like Blain Hardware, even have technical support departments that can help you decide what you need.

Smith Restoration Sash. Hard to find traditional wood window hardware. Each part well illustrated and it use and installation explained. A valuable resource for both the novice and experienced window restorer. Our go-to guy for shash cord and chain. Also, buy a can of Butcher's Bowling Alley Wax. It may not improve your bowling score, but will make your windows slide easier. 401-954-9431.

Click to Enlarge Killian Hardware, Chestnut Hill, Philadelphia, Penna. Wm. A. Kilian Hardware Co. The digital age version of the old corner hardware store with its pot-bellied stove and nails in kegs, Killian is a full line hardware source for all things old, including an extensive selection of basic wood window hardware. It has things you won't see for sale elsewhere. 215-247-0945

SpencerWorks. Wood storm combination windows. All the functionality of a modern storm window combined with the heritage appearance of a tratidional wood storm window. 402-499-7848

AA Window Parts & Hardware, Carries parts for most spring balances, including complete replacement kits and specialized tools. If you have spring balances rather the the traditional iron shash weights in your windows, this is a source for repair or replacement parts. 800-804-0147.

All About Doors & Windows, has been serving homeowners, contractors, and everyone in between for over 35 years. The company offers a vast selection of parts and will work with you to get you what you need. Articles and videos on the site help navigate do-it-yourself projects with minimal hassle. 816-221-8543

Blaine Window Hardware has been the generally recoginzed leader in door and window hardware for several decades. 800-678-1919

Phelps Company designs and manufactures the finest traditional brass window hardware available. The product line includes brass sash pulleys, brass, bronze, and stainless steel sash chain, sash weights, brass sash locks and lifts, push out casement window hardware, ventilation locks, window spring bolts, transom hardware, screen and storm door latchsets, stainless steel storm/screen hangers, and more ... 802-257-4314


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