How it works:
The Passivhaus is basically a super-insulated box that is sealed tighter than any drum ever was. Heating and cooling are both limited to 15kWh per square meter, just under 1.4 kWh per square foot. The reason they need that much is because they are as much about being a healthy house as they are about being energy efficient; the key mechanical feature of the passivhaus is the air to air heat exchanger, that constantly brings in fresh air to keep the humidity down and the oxygen levels where they should be.
the New York Times did a great explanation of it here: Passive Houses Get Good Graphic Explanation
Credit: Passivhaus institut