What is Passivhaus?
Passivhaus is essentially a standard for energy efficiency which results in ultra low-energy buildings that are very cheap to heat. This is achieved by high levels of insulation, triple-glazed windows and a very efficient mechanical ventilation and heat recovery system. This MVHR, which ensures very little heat is lost to the outside, keeps the house well ventilated.
For more information see this explanation by the Passivhaus Trust.