How do you test windows for heat loss?

Your windows occupy an environment filled with air convection. Room air convection moves heat to your windows. Air convection between glass panes helps pass the heat through your windows to the outside air, and wind whisks heat away from your windows into the great beyond to add your tiny contribution to the problem of global warming. This is not, however, the environment in which windows are tested and rated for thermal performance.

Heat moves through your windows in three ways: radiation, conduction, and convection. All of these processes operate all the time and at the same time.

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, there is not much that 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 $3,000+ 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 nothing more than a gaping hole in your house's insulation. [But, there are exceptions. See: Windows as Heat Sources, elsewhere on this page.]

Radiation

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 your 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 it is not a prime mover of heat out of your house. 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 to come into the room, warming it up, but does not allow long-wave radiation originating in your house to escape, 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 American South, a different configuration of low-E coatings is used to keep heat-producing UV radiation out of your house but still allow light to come in.

Unfortunately, low-E coatings are not tuneable. They cannot be designed to let heat 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. Here we usually opt for allowing radiation in to help combat winter cold, although as global warming gets worse, we may revisit that decision.

Conduction

Conduction is the movement of heat on a microscopic level from molecule to molecule inside 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 by conduction 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.

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 a laboratory setting can have an R-value of up to R-30 per inch. — 10 times greater than fiberglass insulation. Aerogel now in commercial production has a 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 windowpane with an R-value of 12.5, which is approaching 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 online and competition intensifies.

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

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

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.

Windows as Heat Sources?

Not all modern windows are just "energy holes" in an otherwise well-insulated wall.

Very-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.

SHGC 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 window is admitted — a rating never reached because none of the solar radiation striking the frame gets through, and the frame is counted.

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 staying cool in summer.

In frosty climes, 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 Maine to New Mexico. Typically the SHGC rating of the glass is in the range of 0.27 to 0.35. This is too 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 tunable low-E coating.

And, amazingly enough, there is such a thing.

Low-E coatings can be tuned to take advantage of the fact that infrared [heat] energy is transmitted at different wavelengths. Infrared coming from the sun has a fairly short wavelength, 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 wavelength [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 shortwave infrared into your house, but block long-wave infrared from leaving your house. This type of low-E coating is ideal for heating

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, now 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 superwindows 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. It's about 2,000 kWh of electricity. In our hometown, Lincoln, Nebraska, 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 $450.00. Still, probably not enough savings to offset the cost of a houseful of Canadian or German super-windows at $100,000 and more, but it's a start.

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 the warm inner pane to the cooler outer pane.

Air Infiltration and Exfiltration

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

Room Air Convection

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

Most heat moves to your windows piggy-backed on moving air. Cool window glass attracts air convection currents like ants to honey which do their best to 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 estimates, Note 1 in a well-insulated house as much as 25% of your total heat loss is through 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 truly is. Heat views your windows are just large holes 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. It is not possible to alter the compositon of glass making it a better insulator without affecting transparancy, the property of glass that makes it useful in windows.

All of the current solutions invove using two [or more] panes of glass with some sort of insulating material sandwiched between them. The usual sandwich material at the moment is air. It's not a particularly good insulator, but better than glass. A dual pane window/air sandwich has an R-balue of about 2.2.

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, however, pressure builds and it gets crowded against the colder outside pane. It gives up some heat. Now colder and heavier, it falls, and soon there is a constant convection current moving inside the glass, conveying heat from the warm inside glass to the cold outside glass. The process is, unfortunately, very efficient and ruthlessly unstoppable.

But, while convection cannot be stopped, it can be slowed. So, this is an area where window engineers try to retard heat flow.

One solution that has been tried is to eliminate most of the air between the panes, creating a partial vacuum. We know that vacuum is an excellent insulator — in fact, one of the best insulators, rated at R-45 per inch. Neither convection nor conduction works 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 the promise of solving the vacuum problem. And, every year it turns out that it also does not work in the long term.

Fill Gases Are Guaranteed to Leak Out

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

....The gradual dissipation of the [fill] gas may occur naturally over time and is not a defect.....

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

Fill Gases

So, the solution most often adopted by window manufacturers at present is to replace the air between the glass panes with a heavier gas such as Argon or Krypton [which has nothing to do with Superman® — that's Kryptonite, not Krypton]. Heavy gases are more and flow more slowly. They are also less conductive. Heavy gases can greatly slow down convection currents and reduce conduction, slowing heat transfer by as much as 50%, achieving R-values of up to R-4.

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 their way through the seal material, while air molecules, in turn, work their way in. And, unlike your 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 year. Note 2

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% per year also showed a possible leakage rate of "up to 18%" per year. 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 temporary boost in your windows' energy performance.

Eventually, fill gas or no fill gas, heat will reach the outer glass pane. Conduction takes over again 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 summer 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.

Heat Movement and Window Testing

The complexity of heat movement through windows makes accurate models of the process very difficult. And, in fact, windows are tested in a very artificial environment that does not even remotely resemble your home — the place where the windows will actually be used. For testing in a laboratory, a window is placed between a hot plate and a cold plate inside a tightly sealed, environmentally controlled chamber, colloquially known as a "hotbox".

One side of a window is heated. The window's thermal performance is then estimated based on how long it takes for the temperature on the non-heated side of the window to equal the temperature on the heated side. After a few calculations on the computer, the result is then stated as the U-value of the window.

This method of testing heat transfer is well-established and reliable when it comes to testing simple materials. For more complicated assemblies like windows, it is less reliable because it eliminates much of the complexity that exists in the real world. In testing windows, the two most important simplifications are the elimination of the air that supplies convection currents from the test environment, and the substitution of a single heat source for the multiple sources that affect heat flow through windows in the real world.

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, harder yet to standardize in a test environment. It's even very hard to model. Climate scientists have been trying for decades to develop an adequate model with only marginal success even when using supercomputers to do the calculations.

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

Single Heat Source

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 or hot air register inside the room could contribute quite a bit.

What is a U-Value?

Windows are not rated by R-values 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 steel block, for example, has a relatively high U-26.2. A material with a low U-value transmits little heat. A block of Styrofoam® has a U-value of just U-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 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 R-11 to R-19. So, it's not much insulation.

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

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

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

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 the Laboratory, Not for Your House

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

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

The most detrimental 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 on 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 modest 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 tell you how well the window will perform in your house, UNLESS, your house is 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 almost nothing useful. The test of the window in a normal house environment has not been done. 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, although this would seem to be right in the Department of Energy's bailiwick.

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. Yet, in lab tests, old windows always perform poorly.

There are substantial differences in the design and manufacturing of modern windows compared to traditional wood windows. The important difference is that modern windows are designed and made for energy efficiency above all other considerations while old wood windows were designed for longevity, endurance, durability, and ease of repair and restoration. One of the objectives of restoring an old wood window is to preserve and extend its longevity while improving its energy performance.

How this is done is the subject of the next section, Restoring Your Old Wood Windows.

Rev. 08/17/20

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