This article was originally published in the September/October 1998 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online September/October 1998
Window Rating Lessons from Around the Globe
by Peter Lyons
Window labels are a work in progress, as we improve our ability to measure window performance. This article shows the progress-and the problems-in three window labeling systems from four different countries.
With such advances in window performance comes the need for independent standards and education. It's difficult for most people to sort through the claims and counterclaims of window manufacturers. Since windows are now optimized for specific climates, choices are harder to make. For example, does someone who lives in a cold climate like that in Madison, Wisconsin, need a window that insulates well, or one that lets in lots of sunshine-or one that does both? What about someone in a mixed climate like that in St. Louis? In Florida, does it really matter if the window has a good R-value, or is it more important to cut the solar heat gain? How much does the frame matter, compared to the center-of-glass properties? What about house orientation?
Such questions challenge designers of today's window-rating programs in the United States, Canada, Australia, and New Zealand. South Korea, Russia, and the United Kingdom have also indicated their intention to begin such programs.The Window as an Appliance I think of windows as appliances. Like refrigerators, they are almost always manufactured off-site and then installed, and like refrigerators, they affect the house's total energy use. The way in which the fenestration system (including shades, awnings, and other accessories) is operated also affects the final energy bill for heating, cooling, and lighting.
Like other appliance rating systems, window ratings are helpful because they summarize scientific data that are largely meaningless to most people. They offer consumers an alternative to the confusing array of technical data found in trade literature and advertising-such things as center-glass R-value, center-glass U-factor, and shading coefficient. For example, some window manufacturers advertise R-8 windows, which have a center-glass U-factor of 0.125. However, these manufacturers provide no information about the frame, solar heat gain, or more fundamentally, which characteristics really matter in a particular climate.
Even a window that faces north (or south in the Southern Hemisphere) can admit more energy than it loses through the winter if its heat losses are small enough to be offset by its solar heat gain, which is free. In temperate climates like St. Louis, windows can easily be net energy gainers instead of losers. But consumers who select windows only by U-factor cannot take advantage of this fact.
In contrast, new ratings systems provide a consistent means of giving consumers the information they need. Ultimately, these ratings will be used internationally.
All the window ratings programs described in this article have been designed around energy efficiency. When correctly implemented, they cut residential energy use. Each program:
The annual energy ratings in these programs are derived from predictions from computer programs that model how much energy typical houses use for heating and cooling. The NFRC program, ER, and WERS all aim to rank windows on a comparative scale, not an absolute one. The heating or cooling ranking of a set of different candidate windows doesn't vary much with factors such as insulation, house orientation, HVAC settings, or even climate.
Energy rating systems have developed separate heating and cooling ratings. A window's heating (or cooling) ranking is usually the same irrespective of whether the house is in Phoenix or Madison, but in Phoenix, cooling matters, while in Madison it is heating that counts. NFRC and WERS help to tell the consumer what weighting to give to heating versus cooling in their area.
The U.S. program, set up by the NFRC, is a voluntary national energy rating system for windows, doors, and skylights. Under the present system, three properties are rated by accredited test or simulation agencies. These properties are thermal transmittance (U-factor), solar heat gain coefficient (SHGC), and air infiltration. Revisions to the testing and rating procedure may soon add condensation resistance, annual energy performance (over four seasons), and long-term energy performance. Long-term energy performance quantifies the effect that degradation of sealants, weatherstripping, and insulating glass units have on the energy efficiency of a product. Rating agencies use ASHRAE standardized weather conditions-one for summer, one for winter-so that performance of different windows is compared on an apples-for-apples basis.
NFRC uses simulations to come up with almost all its ratings, making the tests cheaper and the results more consistent. Whole windows are modeled with modeling software FRAME 4.0 and WINDOW 4.1 (unrelated to Microsoft Windows). New windows in many regions now have NFRC window labels; these contain simple performance numbers but don't say what's good or bad. Some NFRC labels show only U-factor; others add SHGC and air leakage (see Figure 1). However, NFRC- commissioned market research has shown that these labels are too technical and too abstract for nontechnical consumers to follow. They also make no reference to climate appropriateness.
The latest and most ambitious goal of the NFRC is to add a new annual energy rating to a redesigned, consumer-friendly label. The annual energy rating procedure, NFRC 900, will feature two simple numbers on a scale from 0 to 10-one for heating rating (HR) and one for cooling rating (CR). Builders and consumers can use these numbers to compare the energy impact of different vertical windows in hot, cold, and mixed climates. It will state on the label that a higher heating rating will lower the energy costs in the winter, and a higher cooling rating will lower energy costs in the summer. Even if a home isn't mechanically heated or cooled, a high rating will most likely improve comfort.
The U.S. ratings system will succeed only if it is championed by industry itself. In the United States, many of the best-known companies are members of the NFRC, but thousands more need to join. As of May 1998, 26 states or jurisdictions already used, or were likely to adopt, the 1995 Model Energy Code. This code references the NFRC ratings.
Canada's ER-Where Heating is Key
Canada's Energy Rating (ER) program is similar to the program in the U.S. Like the NFRC's new heating rating system, ER starts by establishing fundamental ratings for the thermal, solar, and optical performance of whole windows. A cooling rating procedure exists in Canada, but it has not been promoted because Canada is overwhelmingly heating dominated.
ER uses the same reference weather conditions as the NFRC except they are expressed in SI (metric) units. However, ER's annual energy rating is expressed not in terms of costs saved during the winter or summer, but in terms of whether, taken over the whole heating season, the window is a net energy gainer or loser for the home (see Figures 2 and 3). Like the NFRC heating rating, ER takes account of average solar heat gain nationwide, and allows for that solar gain in reducing heating needs. A variant of ER, called the Specific Energy Rating (ERS), can account for the way available solar energy varies with orientation. However, ERS is used for design guidance, not as a rating tool. Like NFRC ratings, ER ratings are based on computer simulation of U-factor and SHGC plus air infiltration measurements.
For most windows, ER doesn't require physical testing. Canada is a smaller market than the United States, and the government was concerned that expensive testing would cause manufacturers to pull out of the Canadian program. It was also felt that computer modeling was sufficiently accurate for almost all products.
ER does offer the option to rate complex products (such as double glazing with between-the-panes venetians) by physical testing. A similar compromise has been adopted in Australia, as described below.
The Australasian Window Council is comparable to the NFRC. Its system-called WERS-rates and labels residential windows for energy performance in the same way as NFRC 900 and Canada's ER. Australian and New Zealand climates range in severity from cold alpine to humid tropical, so WERS must be adaptable.
WERS gives certified ratings for U-factor, SHGC, visible transmittance, and fading transmittance. It then uses WINDOW 4.1 and the FRAMEplus Toolkit to model whole windows.
In the past, WERS has calculated its standard ratings using typical Australian environmental conditions. In the interests of maintaining compatibility with North America, WERS will soon switch to metric versions of the ASHRAE weather conditions. In any case, the conditions for the nominal U-factor and SHGC on the label has no effect on annual energy ratings. In WERS, as in NFRC and ER, the annual numbers are based on real hour-by-hour weather data.
WERS rates on a scale of one to five stars, with higher ratings representing better heating and cooling performance (see Figure 4). The ratings are based on the relative improvement in annual heating and cooling energy for each window compared to a base-case single-pane clear window in an unbroken aluminum frame. The heating scales range from a single-pane, tinted, aluminum-framed window, which gets one star, to a double-pane, low-e, argon-filled window with a wood frame, which gets five. The cooling scale ranges from a clear single-pane aluminum window, which gets one star, to a double-pane, spectrally selective, low-e window with a wood frame, which gets five. There are already windows that go off the scale-ones that deserve more than five stars. WERS is now struggling with how to show the relative performance of high-end windows.
The energy impacts are calculated using the Nationwide House Energy Rating Software (NatHERS), based on a typical Australian model house and regional weather data. Unlike the U.S. and Canadian procedures, the heating and cooling numbers are derived by direct simulation and not from a regression equation built on earlier simulation results (see Software: The Key to Effective Ratings). The heating and cooling indices-and the stars derived from them-take U-factor and SHGC into account in all situations.Uncertain Predictions All three energy rating programs must deal with controversy. They need to find agreement between research groups, industry, and many end users, so some dissent is inevitable. Most controversy has been sparked by the assumptions that underlie the annual energy performance ratings. In Australia, some in industry have argued that no annual heating or cooling energy predictions for whole houses (using the rated windows) should be made because they are too uncertain to be meaningful. In the United States and Australia, others have maintained that the heating ratings give too much credit for solar heat gain, or that the orientation of the window is ignored. (Some members of the debate don't understand that rankings are largely insensitive to house design, climate, and orientation.) In Australia, some people have argued that the summer rating should ignore U-factor.
Raters need to make many assumptions about what is typical. Is it typical to have 25% of the windows located on each of the four cardinal orientations? How efficient is typical HVAC equipment? Is it typical to have refrigerative cooling and gas-fired heating? What shading obstructions are typical? Any specific case can be close to or far from the typical case.
Label designs have also led to debate. Some hold that consumers want labels kept very simple. Others argue that oversimplification can mislead.
In the end, ratings try to offer good advice, but there is no way to make the advice exactly appropriate for every specific case. For specific cases, software like RESFEN (U.S.) or NatHERS (Australia) is available.Energy Star Windows In the U.S., the Department of Energy (DOE) and Environmental Protection Agency (EPA) have launched the Energy Star Windows program, in which windows meeting certain energy performance criteria will qualify for a special logo. Since the energy efficiency performance of windows, doors, and skylights varies by climate, product recommendations will be given for three U.S. climate zones: heating, cooling, and mixed. Unfortunately, Energy Star makes reference only to certain combinations of NFRC-certified U-factors and SHGCs. Although a number like the U-factor is important, rating a window on this alone is like trying to rate a car's fuel economy solely on its engine capacity while ignoring important factors like weight, size, and aerodynamics. The window with the lowest U-factor and the lowest SHGC is not necessarily the best choice and also presupposes that everyone uses refrigerative cooling and gas-fired heating.
For example, the window manufacturer who ignores SHGC in the winter rating throws away the opportunity to market bigger windows that return a better net energy performance than smaller ones. Even a north-facing window (south-facing in the southern hemisphere) can take in more energy than it loses through the winter if its thermal losses are small enough to be offset by free solar heat gain through the windows. In temperate climates, it is possible for the population of windows to become net energy gainers instead of losers. In other words, they take in more heating energy from the sun than they lose. This situation is now a stated goal of the DOE and the EPA. But if windows are selected only by U-factor, the consumer may make a much less informed purchasing decision. A much more sophisticated annual energy rating is embodied in the NFRC 900 procedure. If DOE and EPA really want windows to become net energy winners, they should adopt NFRC 900 for vertical windows as soon as it is finalized.Challenges Yet to Come Window ratings still must continue to evolve as new technologies appear. It is likely that annual performance measures will take over from fixed data as window systems become more dynamic and high-tech. The new technology creates problems for the ratings systems, because the performance indices are variable, not fixed.
For example, smart, or electrochromic, windows may change in just seconds from dark to light by means of a small direct current. This causes a change in SHGC. Only annual energy ratings will be able to quantify the performance of these windows in typical conditions.
Another challenge for NFRC, ER, and WERS is to involve all window manufacturers, not just the prestige brands. Many residential windows are still sold like commodities, that is, on price, with performance coming a poor second. But the invigorated push toward energy efficiency in buildings means that no country can dismiss windows as being unimportant. All climates impose solar and thermal demands on the skin of buildings. Certified, independent ratings are critical for the informed selection of fenestration products. Energy ratings complement ratings for air, water, and structural performance. And with customers who are prepared to pay more for products that work well, manufacturers get a chance to sell more profitable products.
In the United States, half of residential windows go into remodeling and renovation, a market that is less price sensitive and better disposed toward using high-performance products. In fact, in all the countries that are embracing window energy labeling, new construction and the first home buyer market is losing share to the retrofit and replacement market.
Once energy labeling for window products becomes the norm, it will have addressed the last really difficult aspect of the energy performance of the building envelope. And it will have provided a framework for rating the new generation of window technologies.
Peter Lyons is a visiting researcher at Lawrence Berkeley National Laboratory and chair of the International Liaison Subcommittee of the National Fenestration Rating Council.
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