How Your Windows Lose Heat | Homeowner Guide | Design/Build Kitchens, Baths, Additions and Home Remodeling
Your Old Windows, Part 2:
How Your Windows Lose Heat
Heat moves inexorably from hot to cold. Heat in something hot will migrate to an adjacent cooler object until the temperature of the two 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.
Thermal Conductivity of Common Materials
The conductivity of a material is a measure of the rate at which heat moves through a material. This is more complex than it sounds since conductivity is affected by factors such as temperature, and by the specific composition of the material. Ordinary carbon, for example, is not a particularly good conductor, but Graphene, a slightly different physical form (or 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 (often abbreviated W/mk and informally called a K-value) which reflects the general conductivity of the material. The higher the number, the more conductive the material.
R-Value and K-Value are related. R-value is a measure of how much time is required for heat to move through a material from one side to the other. Obviously it takes less time for heat to more through a highly conductive material. K-value is the speed of a car and R-Value would be the amount of time it takes to get from point A to point B at that speed. Traveling at constant 30 mph (K-Value), a car would travel 1 mile in 2 minutes (R-Value). At 10 mph, it would take 6 minutes. The faster the speed, the less time it takes to travel the distance.
Similarly, it takes heat a considerable amount of time to pass through one inch of a low-conductivity material like aerogel (R-10), but almost no time at all to get through aluminum (R-0.61), a highly conductive material.
Graphene (The most conductive material)
Hardwood (oak, maple)
Mineral wool (insulation)
Plaster (wood lath)
Soft woods (fir, pine)
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-15 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.)
Windows as Heat Sources?
Not all modern windows are just "energy holes" in an otherwise well-insulated wall. High-performance windows are 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 radiation 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 understood if converted to a percentage: A rating of 0.4 means 40% of the sun's radiation gets through 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 they 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, beginning 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 blistering 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: close 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 windows the Germans can make.
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 the area of 7 million BTUs 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 System 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 houseful of Canadian or German super-windows at $70,000 and more, but it's a start.
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.
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 conductivity 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.
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,
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. Aerogel produced in laboratory can have an R-value of up to R-30 per inch. — 10 times greater than fiberglass insulation. Aerogel now in commercial production has much lower R-value, about R-10 per inch. — still higher than any other form of insulation other than a pure vacuum (R-45/in).
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 is used in premium 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 glazing. 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.
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 estimates1, 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 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."
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.
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
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 year2.
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.
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.
2. The study most frequently quoted to support the claim that gas leakage is as less than 1% per year is Plavecsky, J. "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". Leaking was found to be 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.
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.