You probably think of your ceilings as solid surfaces, but the truth is that ceilings leak air into unfinished attic spaces through gaps and openings, such as around pipes and lighting fixtures.
Therefore, fixing air leaks is on par with insulating an attic as must-do attic tasks. Find air leaks From below your attic, check the ceilings and note the locations of all light fixtures, ceiling fans, and electrical outlets. From the attic-side of your ceiling, find the fixtures you noted. Electrical connections for fixtures, fans, and outlets require a hole cutout in your ceiling drywall. Each of these cutouts is a likely air leak.
You can stop air leaks by sealing the cutouts from above with acrylic latex or silicone caulk, or with low-expansion polyurethane foam, depending on the size of the gap.
Gaps around these locations should also be sealed from above. Plug large gaps. The plastic straw applicator seals shut within two hours of the first use, so to get the most mileage out of a can, squirt a lubricant such as WD onto a pipe cleaner and stuff that into the applicator tube between uses. Plug small gaps. Fire Caulk costs the most but works better next to nonporous materials, such as metal flashing, or where there are temperature extremes, as in attics.
Seal air leaks around flues and chimneys. Building codes require that wood framing be kept at least one inch from metal flues and two inches from brick chimneys. But that creates gaps where air can flow through. Cover the gaps with aluminum flashing cut to fit and sealed into place with high-temperature fire block caulk. To keep insulation away from a hot flue pipe, form a barrier by wrapping a cylinder of flashing around the flue, leaving a one-inch space in between. Weatherstrip the attic access hatch.
