Things You'll Need
Minimal expanding foam
Bathroom vents require openings in the ceiling, wall or roof which create a potential for air leakage around the penetrations. If you have air leakage, it is either due to the age of the fan or improper installation. The leakage over time will result in higher energy bills and discomfort if cold or hot air flows into your bathroom. With a little maintenance, the air leaks in your bathroom vent can be stopped.
Clear any insulation away from the unit, and use metal tape to cover areas where air may enter the bathroom. If you don't have access to the top of the fan, then caulk and tape from below.
Caulk around the base of the exhaust fan with acrylic siliconized caulking, and replace the insulation around the exhaust fan. For the caulk to stick, you must sweep or vacuum the area to be caulked.
Inspect the vent pipe for leaks, and tape all penetrations or replace the vent pipe or hose. Tape all joints including the connections to the fan itself. Before taping the exhaust hood to the vent pipe, check the gravity operated back draft damper to see if it is working properly. A properly working damper will close and open easily with little effort.
Wrap or cover the vent pipe with 24 inches of fiberglass insulation from any warm wall or ceiling area. Then cover the exhaust fan box with at least 12 inches of fiberglass insulation. Use minimal expanding foam or caulk for smaller gaps to fill the area around a wall vent that could be exposed to water.
If you don't have a walkway in the attic, lay boards down to make movement easier and to prevent your stepping through the ceiling.
Wear safety glasses and a mask when working around and with fiberglass insulation to keep insulation fibers out of your lungs.