The Participants
John Carmody: Director of the Center for Sustainable Building Research at the University of Minnesota in Minneapolis
Ross DePaola: National Fenestration Rating Council–accredited simulator at WESTLab in Madison, Wis.
Chris Mathis: Code expert and building performance consultant in Asheville, N.C.
Stephen Selkowitz: Program head of the Building Technologies Department at the Lawrence Berkeley National Laboratory in Berkeley, Calif.
JLC: Which is most thermally efficient for window frames — wood, vinyl, or fiberglass?
DePaola: It's really about how the window is built, not the material. Vinyl is a budget material that's not as strong as fiberglass, so the frame material is thicker and usually has more internal webs for strength. If you compare a hollow vinyl frame to hollow fiberglass, those webs might cut heat loss a little by reducing convection. But the webs in the vinyl frame make it harder to fill with foam. Wood is a good insulator, but it's going to vary with the species. If you want to compare thermal performance, look at the NFRC label — the frame material is more an aesthetic issue.
Carmody : I don't take sides on that. There's not a huge energy distinction between those three until you get into the foam-filled fiberglass "super windows" or the wood-framed triple-glazed windows from Europe, both of which are very expensive. It's more a matter of what people feel — their perception of what looks right, the perceived durability, and other things that are hard to measure.
JLC: Since blower-door tests have become common, builders have learned that air leakage through the building envelope can be a bigger driver of comfort problems and wasted energy than insulation levels. On a smaller scale, how big a problem is air leakage through window seals and weatherstripping?
DePaola: Some types of windows leak more air than others. Double-hungs and sliders are leakier than casements, and casements are leakier than fixed windows. But overall, windows aren't a big source of air leaks. Lots of new homes with vinyl double-hungs qualify under the Energy Star air-infiltration requirement. It's thermal bypasses, like attic openings and chimney chases, that are the real problem. Builders are getting better at installation, but air leaks around windows are likely to be bigger than leaks through the windows themselves.
JLC: There's not enough space to include a comprehensive window installation guide here, but what about some general recommendations on preventing air and water leaks?
Mathis: Once you leave the factory, you leave the world where things are controlled by the window industry and enter a world where things are controlled by lawyers. That gap between the window and the wall is something manufacturers have no control over, but it's the source of most of the callbacks and problems they get called in to fix.
There are literally thousands of little mom-and-pop window shops in the U.S. If you take the big guys together — the Andersens, the Pellas, and a few others — they've got maybe 30% of the market. A lot of the small manufacturers make excellent products. But the big players spend more money on testing, and they usually have really good installation advice, especially companies that have their own replacement crews. In the end, though, the builder has to understand the building science and figure out the installation for himself.
Carmody: Installation is a messy area, and there's a disconnect between the guy in the field and the window manufacturer. But compared to 10 years ago, there are a lot of good information resources out there. For example, the Building Science Corporation [buildingscience.com/index_html] has some good directions based on their long involvement with the DOE's Building America program. The information is out there, but you have to go find it.
JLC: Is there a useful distinction to be made between the low-E coatings used in the north, which are designed to prevent heat from radiating outward through the glazing, and those used in the south, which prevent solar radiation from overheating the interior?
Carmody : The original low-E coatings that came along in the 1980s were designed to keep heat indoors. The only way to reduce solar heating at that time was with tinted glass, but no one really liked the resulting gray or green or blue windows. In the second wave, we figured out how to use low-E coatings to block radiant heat gain without losing too much visible light. In the '90s, people used to talk about what they called "northern low-E" and "southern low-E" glazing.
But it's not that simple anymore. In a place like Minnesota, where there are big heating loads, a northern-type window seems like an obvious choice. But it gets hot here in the summer, so if you also have air conditioning, as a lot of people do, switching to low-solar-gain glazing works to your advantage for part of the year. Depending on the situation, you might end up saving more in air conditioning costs than you give up in wintertime heat loss. If you don't have air conditioning, low-gain windows might make you more comfortable in the summer even if they don't save any money.
DePaola: The original hard-coat low-E windows were coated on the inside surface of the glazing. Sputtered coatings came later and made it possible to put on enough metal to reject heat from outdoors. But sputtered coatings are softer, so the first low-gain windows had the coating on the outer surface of the inner layer of glass, where it would be protected by the outer glass [see illustration, next page]. In really hot areas, that caused some seal failures because the reflected heat built up between the inner and outer glass, like in a greenhouse. Now most low-gain windows are coated on surface 2 — the inside of the outer glass — so the heat is absorbed by the outer layer and reradiated. Glazing that's optimized for heating climates usually has the coating on surface 3.
