It is estimated that air leakage in buildings represents up to 40% of heating and cooling costs and consequently is a very important part of any insulation/weatherization project, from both a performance and a durability standpoint. Naturally, the effectiveness of insulation is severely compromised when air is allowed to circumvent it. Condensation, mold and rot are often the result of leaks that allow conditioned air into walls or unconditioned areas like attics.

If you took all imperfections in the air barrier of the average home and put them in one spot, you could easily end up with the equivalent of a hole in the side of your building one or two feet square. The effects of these imperfections are amplified by the “stack effect”, which is the tendency of buildings to act like a large chimney. In the winter, warm air rises, passing through leaks in the attic deck and pulls cold air through leaks lower in the building. This process can even be reversed, to some extent, in the summer.

Obviously you can’t hope to deal with every air barrier flaw, particularly in a finished building, but it certainly makes sense to deal with those that can be reasonably and economically dealt with.

In a basement/ crawlspace area, this usually involves sealing and insulating the rim joist, either with spray foam or rigid insulation sealed around the edges. Batt insulation in this area is not effective. All penetrations in the ceiling/floor for wires or pipes, chimney chases etc. are also sealed with appropriate materials. The attic deck should be sealed in a similar manner, but in addition the top plates of the walls below should be sealed, the reason being that after the sheetrock is installed, the framing lumber usually shrinks,
leaving a small gap that can have a large cumulative effect. Can lights, bath fans etc. should also be sealed with appropriate materials, which sometimes involves constructing a box out of sheetrock or rigid insulation, to ensure proper clearance from insulation.

(Moist air leaking from bathroom below caused this mold growth.)

 

(The top plate of a wall, viewed from the attic, that has been air sealed.)

(The open space between the framing and the chimney represents a huge energy loss and fire hazard. The seal formed with sheet metal and fire caulk is effective and inexpensive and will literally save hundreds of dollars per year.)

(Viewed from attic, a light fixture that has been air sealed.)