Your Old Windows:
Testing & Rating Windows

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.

What is a U-Value?

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

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

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

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

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

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

For more information on R-values and U-values, see Will the Real R-Value Please Stand Up?.
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". 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, 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 of atmospheric convection. It takes a long time and a super-computer to produce even a rudimentary model.

A Direct Measure of Heat Loss Through Windows

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

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

Here's the test.

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

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

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

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

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

This test has the advantages of nearly exactly duplicating an actual home environment, being easy to understand, and a more direct and forthright test of window thermal performance. So, if it is easier to understand and a more direct and more honest test, why don't window manufacturer's use it? Well, it's easier to understand and a more direct and more honest test — it would be very hard to spin.

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.

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 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 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 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 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 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. 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 and durability. One of the objectives of restoring an old wood window is to preserve and extend its longevity while improving its energy performance.
Are any of these links broken? Please let us know.




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

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

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

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

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

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

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

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

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

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

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

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