These days everyone wants living spaces that are flooded with natural light, and most new houses, whether traditional or wildly contemporary, have big windows.
With all those windows, though, getting ones that are energy efficient is a must. Otherwise those sun-bathed interiors can become an architectural albatross–expensive to heat and cool and still, depending on the time of day, the season, and the orientation, too hot, too cold, too drafty or just plain uncomfortable.
Getting information about the energy efficiency of a specific window and comparing energy efficiencies between different windows is relatively easy, thanks to the work of the National Fenestration Ratings Council (NFRC).
After a window brand has been tested and certified, an NFRC sticker stating the results must be posted on every window sold.
The NFRC data on energy performance for all windows tested through their program is posted on its website (www.nfrc.org).
Since the NFRC is a voluntary program, only about 25 per cent of the nearly 1,000 window manufacturers in the US participate. Nonetheless, the NFRC listings are extensive. For example, browsing in the double-hung window category (which NFRC calls “vertical sliders”) you will find 912 different window products from 215 different window manufacturers.
Besides posting a U factor for each window listed, about half the NFRC window ratings also include a solar heat gain coefficient (SHGC), a measure of how much solar heat passes through a window, and Visible transmittance (Vt), a measure of how much sunlight passes through a window.
Since sunlight streaming through the windows accounts for about half the heat buildup in a house where cooling is the major concern (for example Florida, much of Southern California and Washinton), the SHGC can be a critical factor when choosing a window in those areas.
When comparing the ratings posted on NFRC’s website, however, you will find that within a given category, such as double hung windows with a low e glass coating and argon gas between the panes, most of the windows have a similar profile for energy efficiency.
Since there are substantial price differences between windows with similar ratings, what else differentiates them and what should consumers look for?
The first thing is additional testing from other certification programs. The American Architectural Manufacturers Association (AAMA) and the Window & Door Manufacturers Association (WDMA) run a testing and certification program for structural properties, air and water penetration and forced entry of windows
To qualify for the Hallmark WDMA or the AAMA Gold designations, a window must meet or exceed the same specific criteria. A permanent sticker is placed in the window jamb and a temporary sticker may be placed on the windowpane.
Of the nearly 1,000 window manufacturers in the country, only about one third have certified their windows through these two programs.
Besides testing the windows according to established protocols, the NFRC, AAWA, and WDMA certification programs require that inspectors make periodic unannounced visits to the manufacturer’s assembly plants to insure that manufacturing standards are maintained.
Energy Star, a public/private-sector program, jointly sponsored by the Department of Energy and the Environmental Protection Agency also rates windows for energy efficiency. Unlike NFRC’s program, Energy Star Windows correlates energy efficiency with climate, and rates windows accordingly (Energy Star window information listed on its website, www.energystar.gov).
The three Energy Star designations are “N” for northern climates where heating concerns predominate, “S” for southern climates where cooling is the primary concern, and “C” for central climates where both heating and cooling are considerations. When checking Energy Star window ratings, make sure that the window is rated for the climate where you plan to build.
A critical difference between windows that does not show up in the testing is the warranty. If your builder is using a window manufacturer that you have never heard of–a likely possibility given how many manufacturers there are–and it’s not one that participates in the testing and certification programs, the warranty may be the only way that you can judge its quality.
Most firms warrant their glass units and non-glass parts separately. Industry leaders such as Andersen, Pella, Hurd and Marvin offer ten years on the non-glass parts, but differ on the glass. Hurd offers a lifetime warranty on the glass, but limits its coverage to 50 per cent after ten years. Pella and Andersen warrant the glass for 20 years, Marvin for 10 years. Certainteed offers a limited lifetime warranty for both glass and non-glass parts for its New Haven and New Castle lines.
Some warranties are fully transferable to subsequent owners, but many limit coverage to the original purchaser. The worst ones cover the frame but not the glass which is akin to a car with no warranty on the engine.
If the energy efficiencies, certifications and warranties for the windows you are considering are similar, the choice may come down to price and appearance.
The two most commonly used framing materials are vinyl and wood, which are about equal in terms of energy efficiency Though many people think that wood is inherently superior to vinyl, it can be a nightmare to maintain, requiring repainting every two to five years. To avoid this, the extra cost of an exterior vinyl or aluminum cladding for a wood window is worth every penny.
Though some buyers may be apprehensive about vinyl, the quality of this material has vastly improved over the last ten years. Though the first vinyl windows made 15 to 20 years ago did have problems with yellowing, cracking, and warping so that opening and closing a window was hard, the formulations for the vinyl have been changed and these problems are no longer an issue. We frequently see vinyl products in mid to higher end homes these days due to improved looks and quality
One thing to note, however, a vinyl window with welded frame and sashes will be stronger and more durable than a cheaper one in which these are mechanically joined.
Aluminum is still the least expensive framing material available, but aluminum is an excellent conductor and aluminum frames channel indoor heat straight into the great outdoors in the winter. They should be avoided in areas with cold weather. In Florida and Southern California, aluminum frames are acceptable, though not optimal; if used the frames should have a thermal break.