The best exhaust fan venting is through smooth, rigid ducts with taped joints and screwed to a special vent hood. Although this isn't always possible in attic crawl spaces, you should always insulate the duct to prevent condensation problems. You can find 4-in. duct already wrapped in insulation at home centers.
If you’re tempted to vent your exhaust fan through an existing roof vent, or even vent it into the attic, don’t do it. First, you’ll partially block your roof vent with the piping, reducing the flow of cooling air through your attic. Second, during cold winters, you’ll be blowing warm, moist air onto a cold surface (the roof vent and roof plywood). The water will condense and drip into the insulation below and perhaps into the house. Special bathroom fan roof vents with an internal damper that opens only when the fan is blowing will send moist air outdoors and keep cold air out of the house.
Installing a Vent Hood on the Roof
Start in the attic and drill a hole through the roof in the desired vent location. Try to keep it close to the fan location. Leave the drill bit sticking through the roof so you can find the hole. From up on the roof, use a jigsaw or reciprocating saw to cut a 4-in. round hole. Next, measure out a square slightly larger than the protruding part of the vent. Remove the asphalt shingles with a hook blade fitted into a utility knife. Gently pry up the shingles around the hole, making room for the vent to slide under the first course. Apply a bead of asphalt roof cement on the bottom of the vent. Slide the vent under the shingles so they cover the top half of the vent flange. The lower half of the flange sits on top of the shingles. Nail the lower corners with roofing nails and tar the heads.