You probably think of your ceilings as solid surfaces, but the truth is that ceilings leak air into unfinished attic spaces through various gaps and openings, such as around pipes and lighting fixtures. Air leaking into your attic could be costing you money—up to 30% of a home’s heating and cooling energy is lost due to air leaks. Annually, that’s $300 out of your wallet that you can prevent. Therefore, fixing air leaks is close behind attic insulation as must-do attic tasks.
Once this air sealing work is done, you may want to add more insulation to your attic floor. If you want to add insulation, remember that air leaks have to be sealed first.
The easiest way to find air leaks is with a blower door. In some cases, a theatrical fog machine is also very useful.
If you can’t find a blower door, you’ll have to find attic air leaks using your eyes and powers of deduction, as well as a powerful flashlight. If you don’t like balancing on joists, don’t forget to bring a few 2ft. by 3ft pieces of plywood to step on while inspecting your attic. You don’t really want to step between the floor joists and end up punching a hole in the ceiling.
Big holes in your attic floor — holes above soffits, dropped ceilings, and utility chases — can be patched up using gypsum drywall, plywood, or OSB. Perhaps the easiest material to use, however, is foil-faced polyisocyanurate, since it’s easy to cut and easy to tape.
Whatever type of sheet material you use to seal your large holes, cut a piece so that it covers the hole, and secure it in place with nails or screws. The perimeter of each piece of material can be sealed with caulk, non-hardening acoustical sealant, or canned spray foam.
The gaps around brick chimneys are dealt with in a different manner than holes above soffits. Because chimneys can be hot, these gaps should be covered with sheet metal, not rigid foam. After nailing four pieces of sheet metal in place (one on each side of the chimney) the seams where the metal pieces overlap and the gaps between metal and chimney can be sealed with high-temperature silicone caulk.
Gaps around metal chimneys are sealed with techniques similar to those used for brick chimneys. The easiest way to seal around a metal chimney is with two overlapping pieces of sheet metal; of course, you'll need to cut each piece with a half-moon hole that corresponds to the chimney’s diameter.
Manufacturers of metal chimneys and most building codes require a 2-inch air space between the chimney and any framing lumber. Respect this air space; avoid the temptation to fill the air space with insulation.
Before you begin air sealing work, check whether the chimneys are in use or not. Unused chimneys represent a thermal bridge as well as air leakage path, so all unused chimneys—brick and metal—should be removed. At the very least, the top section of unused chimneys should be demolished to a level that is lower than the ceiling air barrier, so that the penetration through the attic floor can be patched up.
In a typical attic, here are some cracks and small holes to watch out for:
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