An exhaust fan in your bathroom, kitchen or laundry room can at times allow cold air to leak into your home. This is generally caused by insulation issues with the fan. An exhaust fan is typically a necessary device, particularly in bathrooms where humidity and moisture levels are high. However, sometimes they can come with problems of their own. If you feel cold air sometimes dropping from the area where your exhaust fan is located, then you may have an insulation problem to deal with.
Go into your attic in the space above the room where the fan is located if possible. If you can access the space above the fan, you will be able to do much more insulating.
Get the right type of duct tape to insulate the exhaust fan. You need professional duct tape that can be purchased through retailers that sell ductwork and home heating and cooling systems.
Seal up any joints in the metal piping going from the exhaust fan to your attic using the professional duct tape. You may be able to access some of these joints from the exhaust fan housing, but more can be accessed from the attic.
Spray down all metal pipes you see around the exhaust fan and leading away from it with foam insulation. Again, you can access some of these pipes from below, but more can be insulated if you can get to the spaces above the exhaust fan through the attic.
Check the fitting of the internal flapper damper on the exhaust fan. Sometimes, these can become loose, which leads to poor insulation. Tighten it or replace it with a roof cap.