Seal gap around bathroom exhaust fan. Pull down the trim cover at the fan and detach it by compressing the small spring clips at each side. Remove the light cover and light bulb. When working new construction I would go in to the attic and acousta-seal and tape a piece of poly over and around the fan housing to seal it tight. Seal holes in the fan housing with caulk sealing putty or metal tape. Stand a stepladder at the area of the fan. Seal around the exterior fan duct vent with caulk or a pre-made exterior wall gasket.
Spray foam works too but requires more care to manage the expansion and keeping it off places you dont want it. Inside if the trim covers the gap no worries since the attic air is blocked by the Poly. Make sure the exterior gaskets are properly combined with the house wrap. Install the fan then seal around the fan with caulk or canned spray foam.
In other cases we can add some Roxul insulation and use that as a buffer. Install the fan then seal around the fan with caulk or canned spray foam. Use housewrap tape to attach the gasket to the existing vapor barrier then follow the steps above to seal the plastic to the fan housing. Use more of whatever kind of insulation is in your attic now.
From the inside with the grill off the fan housing use more aluminum tape to seal. You dont seal between the vent and the roof at least for this purpose–rainwater is another issue. Build up insulation around the fan housing. How to Hide Gaps Around a Ceiling Light Box. Kitchen bathroom and clothes dryer vents that exhaust moist air directly into the attic space instead of outdoors A history of ice dams in the winter an indication of serious air leaks Little or no attic ventilation see Page 42 A Note About Attic Ventilation Knob and tube wiring pre-1930 which can be a ire hazard when in. Use foil tape to seal the holes in the exhaust fan housing.