The concept of the passive house, pioneered in this city of 140,000 outside Frankfurt, approaches the challenge from a different angle. Using ultrathick insulation and complex doors and windows, the architect engineers a home encased in an airtight shell, so that barely any heat escapes and barely any cold seeps in. That means a passive house can be warmed not only by the sun, but also by the heat from appliances and even from occupants' bodies.
And in Germany, passive houses cost only about 5 to 7 percent more to build than conventional houses.
"The myth before was that to be warm you had to have heating. Our goal is to create a warm house without energy demand," said Wolfgang Hasper, an engineer at the Passivhaus Institut in Darmstadt. "This is not about wearing thick pullovers, turning the thermostat down and putting up with drafts. It's about being comfortable with less energy input, and we do this by recycling heating."