We finally did a blower-door test done on our old (early 19th c.) house yesterday, and the results were quite a revelation. I figured I knew where the air leaks were: the perimeter of the stone foundation, the joints of the lousy replacement sash, the poorly weatherstripped doors. And, sure enough, when our energy auditors fired up the fan and depressurized the house, the foundation did leak like mad; when I turned the knob on the cellar door, the door practical flew open. The surprise was where else air was leaking in.
Gaps in the floor five feet away from an exterior wall, innocent-looking baseboards on interior partitions, light-switch boxes on interior walls--some of the last places I would suspect are among the bigger gaps in our building envelope, because they serve as outlets for circuitous air channels hidden in the structure of the house. The stack effect sucks cold air through gaps in the foundation into the cellar; an open chase at the chimney carries it up to the attic; positive pressure up there forces it down through an open top plate into a partition wall and out the electrical box. Fascinating. And now we’re going to make it all stop! --b.d.s.