Exhaust fans are typically installed in bathroom ceilings and in kitchen range hoods, or sometimes kitchen ceilings or walls, to provide spot ventilation. Generous holes are often cut in the ceiling drywall for installation of bath exhaust fans and kitchen exhaust fan ducts, leaving gaps where the fan box or duct is installed. While these gaps may be covered by decorative trim in the case of the exhaust fan box or be hidden in cabinets in the case of range hood exhaust fan ducts, those coverings will not stop air leaks. When the drywall is not sealed to the edges of the exhaust fan box or ducting, a considerable amount of conditioned air can leak through these gaps and into unconditioned attic space. The boxes themselves can also be leaky. Pressure and temperature differences between conditioned and unconditioned spaces encourage this air flow. These air leaks represent energy losses; they could also potentially allow warm, moisture-laden air into unconditioned attics where it can condense on cold surfaces, creating moisture problems. Air barriers need to be continuous to be effective; this means sealing all penetrations in exterior walls, ceilings, and floors adjoining unconditioned spaces.

Be sure to schedule sealing around exhaust fans and ducts after fans and drywall have been installed. Responsibility for sealing air leaks around exhaust fans and ducts should be included in the contract for the appropriate trade, depending on the workflow at a specific job site.

The gap around this kitchen exhaust duct represents a significant source of air leakage to the unconditioned attic

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How to Air Seal Holes around the Kitchen Exhaust Duct

1. Cut openings for the duct that are no bigger than needed to fit the exhaust duct through the ceiling or top of the kitchen cabinet. Make clean, even cuts.
2. After the exhaust duct is installed, air seal with caulk between the duct and drywall from the room side. If gaps are larger than a ¼ inch, use canned spray foam that is carefully applied. Do not use pieces of fibrous insulation; this insulation does not air seal. If gaps are larger than one inch, they can be sealed from the attic side with air-blocking material such as rigid foam that is cut to fit and sealed in place with caulk or spray foam. 
3. Use caulk or pre-made exterior wall gaskets to air seal the exterior fan duct vent to the exterior wall. Ensure that exterior gaskets are properly integrated with the housewrap.

How to Air Seal the Bathroom Fan Housing

1. Cut openings in the ceiling that are no bigger than needed to fit the fan box. Make clean, even cuts in the drywall.
2. After the fan is installed, air seal with caulk between the fan housing and drywall from the room side before installing trim. 
3. If gaps are larger than a ¼ inch, use canned spray foam carefully applied so that trim will fit over it.
4. If gaps are larger than a half inch, they can be sealed from the attic side with air blocking material such as rigid foam that is cut to fit and sealed in place with caulk or spray foam.  Do not use pieces of fibrous insulation; this does not air seal. 
5. Seal holes in the fan housing with caulk or metal tape.
6. Use caulk or premade exterior wall gaskets to air seal the exterior fan duct vent to the exterior wall. Ensure that exterior gaskets are properly integrated with housewrap.

How to Create an Insulation Shield for the Exhaust Fan

The bathroom exhaust fan box may have air leaks and holes in the casing. You may want to cover the fan to stop air leaks and to allow you to install insulation over it.

1. Create a 5-sided box from a solid air barrier material such as rigid foam, gypsum board, or plywood. Tape the seams of the box with housewrap tape (not duct tape) or seal with mastic. Cut an access in the box for the exhaust duct.
2. Seal the box to the ceiling gypsum board and seal around the exhaust duct with caulk or canned spray foam.

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4. Cover the box with attic insulation.