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.

We replace a lot of windows. We probably replace nine old windows for every old window we restore and save. The fact is that most windows made in the past 60 years 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.

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 Photo: Andersen Windows. Andersen Woodwright Window

Nearly all major window manufacturers market windows designed to look like heritage windows. This Andersen window looks like an original Craftsman window. But, most replacement windows last at best 40 years while original windows were built to last generations with proper care. This window starts at about $600.00 — not including installation. Restoring the original window is about half that and since the window is already installed, no installation is required. 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.

But, then 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.

They 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 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 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. Artic Ranger Tab

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.
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 low-e single pane windows and new storm window?$2.57
…new double pane thermal windows?($0.37)
…new low-e double pane thermal windows?$4.45


The low-e coating seemed to be the key to the slight improvement in thermal efficiency of the double-pane replacement window over the restored wood window with storm. The restored windows did not have low-e coatings. But, what if low-e coatings were added to either the restored window or a new storm window?

This was one of the issues addressed in a study by the Lawrence Berkeley National Laboratory (LBNL) under contract with the Department of Energy to field test the thermal performance of single-pane wood windows with storms and low-e coating against dual pane vinyl windows with low-e coating3.

This study is interesting for the way it was done. LBNL has developed an "Accurate Window Thermal Test Facility" that is a mobile laboratory unit that can be moved into various climate situations. For this study it was moved to Reno, NV and faced north. The test windows were installed using normal installation and weather sealing practices, then left for the winter. An instrument called a calorimeter was used to measure temperature, wind speed and direction, solar intensity, and so on — all the factors needed to accurately determine thermal performance in the field. Measurements were taken every ten minutes.

What Others Say


Tom Silva "... [i]f your windows are single-paned, look into the cost of adding storm windows for a fraction of the cost of new windows."

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
 The LBNL study found that:
  1. There was little measurable difference between the overall thermal performance of the two window systems in the field test environment, but the low-e vinyl double-pane window did outperform the single panel wood window slightly in overall thermal insulation.
  2. Air infiltration around and through the two window systems was about the same, with the window/storm window combination performing slightly better.
  3. Air infiltration made little difference to the overall thermal performance of the windows.
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?

....If it is your intent to pay for your re­place­ment win­dows out of energy cost sav­ings, think again. Independent field studies have repeatedly shown that the payback period is far longer than your lifetime… This is precisely the question researcher, architect and engineer Keith Haberern set out to answer in his study of heating costs in New Jersey. He published a chart of energy savings based on various window improvement strategies that appeared in an article in the Old House Journal in October, 20074. The article 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 double-pane replacement thermal window and a restored old wood window are insignificant. In fact, an old wood window with a new storm window outperformed a new double pane window, and was a lot cheaper with a short 4.5 year payback period (for the cost of installing the storm window). If low-e coating was added to the new dual-pane window, then it did outperform an old window/storm window combination — but not by very much. In fact, if you already had a wood window/storm combination, replacing it with a double pane window with low-e resulted in such meagre energy savings that the payback period was 240 years. Ouch!
How Long to Pay Back Your Re­place­ment Window Investment?
According to Keith Haberern, 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).
ActionResulting
U-value/
R-value		Cost*Annual Energy Savings (BTU)Annual Savings per WindowPayback Period (Years)
Add a storm window to a single-pane windowU-0.50
R-2.00		$50.00722,218$13.504.5
Replace a single-pane window with double-pane thermal windowU-0.58
R-1.72		$450.00625,922$11.0740.5
Replace a single-pane window with double-pane thermal window, low-e glassU-0.35
R-2.86		$550.00902,772$15.1034.0
Replace single-pane window/storm window combination with double-pane thermal window, low-e glassU-0.35
R-2.86		$550.00132,407$2.29240.00

* Costs include installation and are based on costs in New Jersey, in­clud­ing heat­ing costs, at the time of the stu­dy.


These results almost precisely duplicate the findings of the 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.

Energy consultant Michael Blasnik found similar results in his field study of window performance in New York.

The methodology of his study 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 alone5. Ouch, once again!

A 1990 study by William Hill6 at Indiana State University found much the same. 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 of Lincoln, Nebraska, this would amount to $33.12 for an average home. This is about what you would save replacing 10 incandescent light bulbs with compact fluorescent bulbs, at a cost of about $30.00, whereas new windows 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 200 years where you live.

Lies, Damn Lies and 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. 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 in a device called a “hotbox” is carried out in a laboratory to confirm the model. At no time is the window actually installed in a house and the results evaluated. And, 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.

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 on 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 they all operate all the time and at the same time. Of the three, convection is the most important, followed closely by conduction. Radiation is a distant third. Very little can be done to combat any of these processes in windows — as we will discuss in detail below — but of the three, window engineers have had the most success retarding transfer through radiation, the least important of the three. The very best "super insulated" windows barely reach R-7 using three panes, a super insulated frame, low-e coatings and Argon fill gass (compared to a minimum R-13 in you wall and R-40+ in you ceiling). And, these are windows that cost $1,500 and more per window. A more typical thermal window tops out at R-2 to R-2.8. By comparison, an uninsulated house wall is about R-3.2.

Radiation
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.

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 form of the material. Ordinary carbon, for example, is not a particularly good conductor of heat, 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.

Here are the Thermal Transfer Coefficients of common materials.
MaterialThermal 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
Graphene600.000
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
Styrofoam0.006
Vinyl (PVC) 0.034
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 out are your room walls, furnishing and the room air — and this is 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 in 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 the heat in your house.

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 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 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 grows.
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.

Conduction


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


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 in a vacuum to move around. But, in gases such as air, convection is one of the prime movers of heat, and one of the hardest to combat.

The movement of heat through your windows by convection is not just one, but a series of four convection processes. Heat moves with air, so air exfiltration through leaks in and around your windows moves heat out of your house. 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 are the primary mover of heat from warm inner pane to cooler outer pane.

Air 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 estimates7, 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.

Fill Gas is 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."

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 Guardian Industries which since 2005 has been talking publicly about its new vacuum window glass. It may increase the thermal efficiency of windows to as much as R-12. The release date has been pushed back several times, and is now slated for 2013. If it works, it would be a tremendous improvement in window technology, and also a very expensive one. Guardian is so far not talking about the cost of this patented window glass.

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 year8.

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 3 years. So that extra $5,000 you spent for a house full of Krypton-filled windows is, at best, a temporary solution.

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.

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.

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-13. 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-13 wall.

For more information on R-values and U-values, see Will the Real R-Value Please Stand Up?. 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 them as much as possible.

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 Additional Heat Loss (AHL) 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 AHL of the window.

Let's say Window "A" has an Additional Heat Loss of 4,500 BTUs over a 24 hour period, while window "B" had an Additional 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.

Single Heat Source

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?


"[We recommend] the retention and repair of original windows whenever possible...."

National Park Service
Technical Preservation Services 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.

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 windows9 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, especially with repeated use. After a few years they become brittle, break and need to be replaced.

Where to Find Window Parts


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.

AA Window Parts & Hardware, 800-804-0147.

All About Doors & Windows, 816-221-8543

Blaine Window Hardware, 800-678-1919

Phelps Company, 802-257-4314

Pickens Window Parts, 513-931-4432

Smith Restoration Sash, 401-351-1222

Strybuc Industries, 800-352-0800

Swisco, 856-283-0390

 

Are any of these links broken?
Please report broken links.
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.

Links & Resources
from The National Trust for Historic Preservation
Replace or Restore Wood Windows: Repair or Replace: A Visual Guide. Repair or Replace: A Visual Guide
Download PDF.
Window Restoration or Window Replacement: Window Types, Residential Field Guide. Window Types:
A Residential Field Guide
Replace or Restore Wood Windows: Historic Wood Windows Tip Sheet. Historic Wood Windows Tip Sheet
Download PDF
Window Restoration or Window Replacement: Window Know-How - A Guide to Old-Building-Friendly Contractors. Window Know-How:
A Guide to Old-Building-Friendly Contractors
Replace or Restore Wood Windows: Weatherizaton Resources. Directory of Weatherization
Links & Resources
Window Restoration or Window Replacement: Directory of Window Links and Resources. Directory of Window
Links & Resources
Are any of these links broken? Please report broken links.

Leaking Spacers

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 in even 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.

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


Other Voices

National Trust for Historic Preservation


Click to Read the Report
Click to Read the Report.
   Is this link broken? Please report broken links.

"...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


Click to Read the Report
Click to Read the Report.
   Is this link broken? Please report broken links.

"[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). 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.

An old wood window is put together so it can be easily taken apart and any of the individual parts repaired or replaced. 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.


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

Paste this Home Workship Reference Sheet, including the head above, in your scrapbook in the section marked windows

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 pully 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 pully 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 pully 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 pully. 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.
 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. Without the weights, the window will not stay up. With the weights and sash joined with new ropes, they will work just like new. The cotton ropes used when the window was made had a life expectancy of just 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 ropes we use today are not the ropes of 100 years ago. The new nylon/cotton ropes 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.

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.

Weatherproofing

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-13 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.

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.

How to Restore an Old Wood Window


Learn the basics of restoring old wood windows from this video by Preservation North Carolina. It won't make you a window preservation expert, but it will get you started safely. If you have never restored a window before, you might check out our article Can I Do It Myself before you begin.

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 SpencerWorks 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


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.

December 17, 2012, Lincoln, Nebraska

 


References:

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):

3 Joseph H. Klems, "Measured Winter Performance of Storm Windows", Lawrence Berkeley National Laboratory, (2002) (Download (PDF).

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

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

6 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

7 U.S. Department of Energy (2005). www.energystar.gov/windows.

8 The study most frequently quoted to support the claim that gas leakage is down to as little as 1% 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.

9 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.

More Good Reading


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.

Selecting Windows for Energy Efficiency. 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

How to Restore Windows


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

 

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