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Energy Codes for Windows
by Dr. Oneida Cramer

Windows are a primary source of energy exchange in the home. One square foot of window, for instance, conducts 5 to 10 times as much heat as one square foot of wall, according to the Texas Window Initiative. By increasing the energy efficiency of the windows, a home can expect to reduce its requirement for air conditioning?a strategy that the new Texas state codes expect will lower total electric air conditioning loads during the hot summer season and (because electricity is a major contributor of nitrogen oxide) lower levels of nitrogen oxide emissions.

In the new program of energy codes that regulate the auto emissions testing put in effect this month, there is one section pertaining to building codes and an effort to increase the energy performance of residences. The building codes took effect on September 1, 2001 in all the rural districts, and many Texas municipalities (for instance Fort Worth) have begun enforcing them. But, the entire state must be in compliance as of September 1, 2002, which means basically that all new home construction and remodeling must meet energy efficient code guidelines. These codes target primarily home insulation, heating/cooling systems, water heating, building tightness, and windows.

?Most people should be looking for the National Fenestration Ratings Code (NFRC) stickers on windows,? said Tom Fitzpatrick, energy code specialist with the Energy Systems Laboratory. ?It?s going to be the easiest way to find out the performance characteristic of a window. And manufacturers are required to test and label their windows.?

NFRC is a non-profit public/private organization created by the window, door, and skylight industry, according to Jim Benney, NFRC Director of Education. NFRC is comprised of manufacturers, suppliers, builders, architects, and designers, and their goal is to provide uniform and accurate information about the energy performance of windows, doors, and skylights. Currently, their labels list three technical factors (1) visible transmittance, (2) solar heat gain coefficient, and (3) U-factor.

But the issue, as far as compliance with the energy codes, comes down to just two numbers, according to Dan Miller, vice president of Action Window Technology, Inc., window manufacturers for new homes, and president of Texas Best Windows, a distributor for replacement windows.

One of the numbers is the U-factor, otherwise known as thermal transmission. The value of the U-factor indicates the amount of heat that is allowed to transfer through a window system from the inside to the outside. U-factor requirements are dependent upon the total amount of window space in the home and the climate. For the Dallas area, permissible U-factors vary from 0.90 for a home with less than 8% window area to 0.50 for a home with 25% of the area in windows. As a rule of thumb, a U-factor of 0.65 will accommodate most homes. Note, however, that the new codes put a ceiling of 25% on the upper limit of allowable prescriptive window area per total area of the home.

The second number to get acquainted with is Solar Heat Gain Coefficient (SHGC), which is a measure of the fraction of solar heat that passes through the window compared to the total radiant solar heat that strikes the window. When the sunlight hits the window, some of the sunlight gets reflected back to the outside. Some sunlight continues through the window system as radiant energy. And part of that energy becomes a different type of energy, known as heat. SHGC is a measure of that change.

SHGC is an important measurement for residents in warm climates like Texas because the solar energy that gets transferred into heat becomes the heat that the home then has to cool off. Window systems with low values of SHGC are preferred in warm climates because these windows allow less solar heat to transfer into the building. To be in compliance with the energy codes, look on the window sticker for an SHGC of 0.40 or less, which means that that window system allows 40% (or less) solar heat to pass though the window.

?As you go up in price and quality of windows, the U-value is fairly easy to obtain,? Miller said. ?But the solar heat gain number is the one that gives manufacturers a little difficulty. That?s overcome by this energy efficient low-e glass.?

The double pane low-e is the most frequent way that a low-e coating (the soft coat) is insulated and sealed for protection.

?There are other kinds of coatings called hard coats and other ways of getting the low solar heat gain with tints and things that don?t require soft coating,? Fitzpatrick said. Also available are the spectral selective windows that allow the passage of visible light wavelengths, but block long infrared and short UV wavelengths.

?These (energy efficient) windows help people to do any sort of (home) design they want and stay within the energy code,? Fitzpatrick said. The demand for these windows is high; currently Action Window Technology, Inc. is manufacturing and shipping about 90% low-e windows.

?If you want to use a different kind of window or exceed the amount of window area that is allowed on the homes, there are a variety of prescriptive options,? Fitzpatrick said. ?You can do a systems analysis of the house, which is a computer simulation that lets you compare many kinds of design. And it?s intended to be for people who are using either very complex designs or very unusual new plans. You compare a new design against a design that?s of the same basic shape but meets all the requirements of the energy code for its envelope characteristics, its hot water characteristics, etc.?

If you have an older home and you want to replace the glass, you can still replace with single pane glass, according to Fitzpatrick. But if you plan to remodel and change out the window or build an addition with new windows, the project must meet the new energy codes in regard to the high efficiency windows.

?If you have extra shading or you also shade a portion of the wall then you improve the performance of the window as well, and the whole window system benefits from the extra shading,? Fitzpatrick said. ?The bottom line is these windows work in our area, and they should be looked on as a potential candidates for all homes, even passive solar designed homes.?

Note that a passive solar home is carefully designed with a minimum number of glazed openings on the east and west, not too much on the north, and has a very balanced exposure on the south so that the home has a relatively large, but carefully planned amount of glass that is shaded properly in the summer and in the winter gets heat from the sun. In this case, the home can take advantage of solar energy simply though passive organization on the site.

-by Dr. Oneida Cramer
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