“In reality, everything is related [to thermal performance and comfort],” says Saxler. “You need a total home evaluation if you want to fine-tune your windows.” An energy audit by a local rater certified by the Residential Energy Services Network (www.natresnet.org) is a good place to start, while most window manufacturers and/or their dealers employ software programs to determine ideal specifications based on other, whole-house factors, such as wall insulation values.

Gaiser is even more blunt about the role of windows in lowering a home’s energy demand. “If you want to save energy, put in fewer windows,” he says. Upgraded insulation costs far less than high-performance windows, he says, and results in a better thermal envelope and a faster return on investment. “Windows rarely pay back as well as other energy-efficiency measures. Put them in for light and views, but not for energy efficiency.”

http://www.efficientwindows.org/

Energy Star (Qualified Windows):
www.energystar.gov/windows

Lawrence Berkeley Laboratory, Windows & Daylighting Division:
http://windows.lbl.gov/

National Fenestration Rating Council:
http://www.nfrc.org/

Residential Windows, A Guide to New Technologies
and Energy Performance
by John Carmody, et. al., available at http://www.wwnorton.com/

Which takes us back to the original question of whether specifying window performance for each orientation is worth the effort, at least in terms of thermal value. For builders, the decision may come down to marketability. “It’s an opportunity to stay one step ahead of the competition, to offer a house that’s more comfortable and efficient than the next guy,” says Saxler. “If you do the building science, you’re in the top 10% of all builders.”