What is the best roof vent for a bathroom exhaust fan?
Short answer. The best roof vent for a bath fan is a dedicated roof cap with a built-in backdraft damper, a collar that matches the duct size (usually 4 inch), a hooded outlet with a bird guard, and a metal base flange wide enough to weave properly into the shingles. Never vent a bath fan into a ridge vent, soffit or the attic.
What to look for in the cap
- Backdraft damper: a flap that closes when the fan is off, keeping out wind, cold air, insects and nesting birds.
- Matching collar: most bath fans use 4 inch duct, some 3 or 6 inch. Reducing the duct size at the cap cuts airflow.
- Hood and guard: a hooded outlet sheds rain, and a guard stops birds without a fine screen that clogs with lint.
- Metal construction: steel or aluminum caps stand up to hail and sun better than thin plastic.
- A deep flange: so the upper part tucks well under the shingles above and the lower part laps over the shingles below.
Where it goes on the roof
- As close to the fan as practical, so the duct run is short.
- On a plane of roof, away from valleys where water concentrates.
- Separate from the ridge vent and any intake vents, so moist air is not pulled back into the attic.
The duct matters as much as the cap
Use insulated duct through the attic, sealed at both ends, and avoid sags where condensation can pool and drip back into the fan. In Georgia winters, an uninsulated duct can condense enough water to drip from the grille. If the fan is close to an exterior wall, a wall cap avoids a roof penetration entirely. Adding a cap is easiest during a reroof, when it can be flashed into fresh shingles. See roof flashing explained and roof replacement day.