From the outside, there is nothing unusual about the stylish new gray and orange row houses in the Kranichstein District. But these houses are part of a revolution in building design: There are no drafts, no cold tile floors, no snuggling under blankets until the furnace kicks in. There is, in fact, no furnace. In Berthold Kaufmann's home, there is, to be fair, one radiator for emergency backup in the living room - but it is not in use. Even on the coldest nights in central Germany, Mr. Kaufmann's new passive house" and others of this design get all the heat and hot water they need from the amount of energy that would be needed to run a hair dryer. "You don't think about temperature - the house just adjusts," said Mr. Kaufmann. His new home uses about one-twentieth the heating energy of his parents' home of roughly the same size, he said. The concept of the passive house, pioneered in this city of 140,000 outside Frankfurt, approaches the [energy efficiency] 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. New passive houses use an ingenious central ventilation system. The warm air going out passes side by side with clean, cold air coming in, exchanging heat with 90 percent efficiency.