03 May 2011
Passive House - more observations
Passive House is a pretty rigorous system:
-annual energy consumption for heating and cooling is limited to 4,755 Btu per square foot;
-air infiltration is set at a maximum of 0.6 air changes per hour at 50 pascals of pressure;
-annual primary energy usage (energy consumed by appliances, lighting, and other devices not directly related to heating and cooling) is capped at 11.1 kilowatt hours per square foot.
There are also stringent standards regarding what is considered square footage. Only living space is considered in the calculation. Thus, larger houses with wasted space don't get to consider that waste in their energy budget.
A house in Maine (built by my cousins-in-law, actually) has R-49 walls, R-57 roof, and R-74 beneath the concrete-slab floors. I don't quite know structurally how you float a building on 16" of foam, but I'll talk to the engineers about that. I suppose that the conductivity of direct earth is a significant heat loss. It seems counter-intuitive somehow, since the ground can be a significant source of heat if the freezing can be isolated. In this case, the mass of the slab is being used to maintain the interior temperatures. I believe they indicated that there was no mechanical plant in the building.