The homes use "ultra thick insulation and complex doors and windows" to form an airtight shell that allows only a bare minimum of heat to escape and cold to seep in. As a result the house is warmed not only by the sun, but by occupants' body heat and the heat generated by home appliances. Occupants get all the heat and hot water they need from the amount of energy that would be used to run a hair dryer, says The Times. That's about one twentieth of the energy that would be used by a comparable-size conventional home.
Some 15,000 of these "passive" houses have been built, mostly in northern Europe. Building costs in Germany run only about 5% to 7% more than conventional construction, says The Times.
Air quality and moisture problems associated with a too-tightly sealed building envelope are solved with a central ventilation system, in which outgoing warm air passes by cold air coming in, exchanging heat at some 90% efficiency.
Although the number of passive houses is growing, there are several hurdles to more widespread utilization of the design. The sophisticated, highly-sealed windows and heat-exchange ventilation systems needed are not readily available in the U.S, for one thing. Proper site selection is vital; plenty of south-facing windows to pick up solar heat are a must. It has yet to be demonstrated that the design will work in warmer climates, where chilling warm outside air is needed. Finally, the design may not adapt easily to the styles and sizes of homes more common in the U.S. However, with the need for energy conservation and reduction of green house gases becoming more accepted every day, it's likely that an increasing number of passive houses are bound to be built. Click to read the complete article.