Northwest Harvest House Week 20: George Ostrow's New Green Design Project

To build a net zero energy house it is essential to reduce air leakage to the bare minimum. Doing so requires a comprehensive effort by the contractors to fill every gap between the heated inside and the outside. 

Typical problem areas are at electrical outlets, light fixtures and switches, pipes and ducts, around windows and doors and wall intersections. For this house, we used a combination of spray polyurethane foam and caulk to seal all these gaps.

During design, the architects determined the boundary of the air barrier surrounding the whole house. At the walls and floor overhangs it is the plywood sheathing, which had all its joints covered with black butyl tape. At the floor and foundation it is the concrete slab and walls. And at the roof it is the sloped gypsum board ceiling, which had a dozen light fixtures hung from it that were enclosed in a special plastic box.

To confirm air tightness, a device called a blower door is used to depressurize the interior to 50 pascals. The blower fan is trying to suck outside air in, which can be felt by the test technician using a damp hand and an infrared camera. A gap as small as that between two uneven pieces of wood is significant.

Our specs targeted 2.0 ACH50 (air changes per hour at 50 pascals) as an ambitious maximum, but the team wanted to get much lower. Our first blower door test resulted in 1.6 ACH50, which is a terrific result. 

George Ostrow is principal of VELOCIPEDE architects and member of Northwest EcoBuilding Guild, U.S. Green Building Council, Passivehouse Northwest and Built Green.