According to this interesting and informative article from the New York Times:
Passive houses are airtight buildings that use heat from appliances and even the occupants’ bodies for warmth. They have thick insulation, are oriented to maximize winter sun and use a heat exchanger to warm outside air that circulates throughout. The result is a house that needs little or no extra energy for heating, even in very cold climates. While they are increasingly popular in Germany and Scandinavia, passive houses have yet to make inroads in the United States.
