How to Vent a Bathroom Fan

In this video, This Old House general contractor Tom Silva explains how to vent a bathroom fan.

Steps:

1. Drive nail through house wall from attic; this will serve as a reference point for positioning the wall cap.
2. From outside, use 4¼-inch hole saw to cut hole for wall cap.
3. Apply caulk around the backside of the wall cap.
4. Slide wall cap and connector duct into hole; secure cap with stainless steel screws.
5. Wipe away excess caulk.
6. In attic, seal holes in fan housing with foil duct tape.
7. Use foil duct tape to attach flexible duct to connector protruding from the wall cap.
8. Tape 90-degree elbow to opposite end of flexible duct.
9. Screw elbow to outlet port on side of exhaust fan.
10. Cut fiberglass insulation to fit tightly around fan housing.
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Pro Tip: This Old House general contractor Tom Silva prefers venting through a gable end wall rather than through the roof. As discussed in a This Old House episode, venting through the roof creates a possible leak point. Running the exhaust duct across the attic and out a gable end is a more reliable approach that avoids roof penetrations.

For optimum performance, position the vent fan on the bathroom ceiling between the shower and the toilet. Use an extra-long, 3/8-inch-diameter spade bit to bore a reference hole through the ceiling and into the attic. In the attic, clear away insulation from around the hole and try to position the fan directly between two joists, taking into account any nearby pipes or obstructions.

When sliding the wall cap into place, check for gaps where the cap doesn’t sit flush against the siding. Cut a foam-rubber gasket to fit into any spaces where the wall cap doesn’t contact the siding, then apply a bead of silicone adhesive to the siding and gasket around the hole before pressing the cap firmly into place. Also install a grill on the exterior vent to keep animals from getting inside the house.

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