We replaced our old windows with double-glazed ones, but now we seem to use more energy for heat because we've lost the nice warmth in our front room on sunny winter days. What's going on?
--Barbara in Lafayette, California
Double-glazed windows provide terrific insulation, but many of them are not designed for insolation — that is, letting the sun's heat in. So it turns out that some people who replace south-facing, single-pane windows with double-glazed ones end up running their heater more often than they used to.
The problem lies in air-conditioning. The window industry and the EPA seem to have decided that it's more important to block solar heat in summer — thus cutting A/C use — than to exploit the sun for heat in winter. The result is a one-size-fits-all approach that can disregard local climates and site-specific features.
I learned this when I got into a wrangle with a window salesman who was clueless about the benefits of solar heat gain. A single-glazed window lets in about 85% of solar heat, which explains your once-cozy daytime situation.
Some double-glazed windows admit lots of solar energy (up to 70%) and others very little (perhaps only 20%); this varying heat is expressed as the solar heat gain coefficient. You'll want the highest gain if you stand to benefit more from heat gain than from the cooling effect.
But even where air-conditioning is common — and solar blockage desirable — the devil's in the details. If you've got large overhangs (or shade trees or functioning shutters) that block the summer sun, you might save more energy by installing windows that admit more solar heat during cloudless cold snaps.