A key feature of a passive house is that they incorporate very high standards of insulation. This reduces the amount of heat lost through the building fabric to a very low level. When achieving these very high standards of insulation the purpose provided heating requirement, even on the coldest days, is reduced to a minimum and hence it is possible to adequately heat the dwelling by just preheating the fresh air entering the rooms.
The heat loss through a regular construction (an external wall, a floor to the basement or a slab on ground, a ceiling or a roof) is characterised by the thermal heat loss coefficient or U-value. This value shows, how much heat (in Watts) is lost per m2 at a standard temperature difference of 1 degree Kelvin. The international unit of the U-value therefore is “W/(m²K)”. To calculate the heat loss of a wall you multiply the U-value by the area and the temperature difference.

A leaky house is the most poor insulated example. It consists of solid walls, an uninsulated floor, single glazed windows, and openings for draughts. In order to combat the high level of heat loss the leaky house requires 300 kilowatt hours of heat energy per square metre per year to remain at a comfortable temperature. Shrinkthatfootprint.com claims that it is possible for a home to be so well insulated against heat loss or gains that only the sun, our bodies, a few simple appliances, and a basic heat recovery system would be enough to provide all the necessary heat for a comfortable existence.

Is the typical new built house that many people own. It has insulation between the walls, in the ceiling and the floor, double glazing windows, and far fewer draughts. This all helps to drastically reduce the energy needed to heat the property to 150 kilowatt hours per square metre per year.