Bathroom Exhaust Fans: Is Yours Actually Venting Outside?

A bathroom fan is supposed to take the steam from your shower and send it outside. A surprising number of them send it into the attic instead, or into the space between the ceiling and the roof, or nowhere at all. It's one of the most common things a home inspector writes up, and in winter it can do real damage.

01Why it matters

A hot shower puts a lot of moisture into the air. If the fan dumps it into a cold attic, that moisture condenses on the underside of the roof boards, freezes into frost on the nails, and then melts and drips. Over a winter or two you get stained insulation, moldy roof sheathing and rotting wood. Building codes require bathroom exhaust to go outdoors, not into an attic, soffit or crawl space.

If the fan doesn't work at all, the moisture stays in the bathroom instead: peeling paint, mildew on the ceiling, and moldy grout.

02Is it working?

• The tissue test. Turn the fan on and hold a single square of toilet paper up to the grille. It should be pulled against the cover and stay there. If it falls off or barely moves, the fan isn't moving much air.
• The mirror test. After a hot shower with the fan running, the mirror should clear within several minutes after you turn the water off.
• Listen. A loud rattle or a fan that hums without spinning needs cleaning or replacing.

Pull the grille off (they usually pop down on spring clips) and vacuum the dust off the fan blades and housing. A clogged fan moves a fraction of the air it should.

03Where does it go?

From the attic (carefully, stepping only on the joists): find the fan housing above the bathroom and follow the duct.

• Good: a duct that runs to a dedicated vent cap on the roof or a side wall, with a flap.
• Not good: a duct that ends in the middle of the attic, points at a soffit, or just lies on the insulation. A duct aimed at a roof vent or ridge vent without connecting to it doesn't count either; most of the moisture stays in the attic.
• Look for frost, water stains or dark mold on the roof boards around the fan area. That's the moisture telling on itself.

From outside: look for a vent cap on the roof or wall near the bathroom. Turn the fan on and see if the flap opens.

04What the duct should be

• Insulated flexible duct or rigid duct, as short and straight as possible, connected to the fan and the vent cap with clamps or foil tape.
• Insulated, where it runs through a cold attic, so the moisture doesn't condense inside the duct and run back down into the fan.
• Sloped slightly toward the outside, where possible, so any condensation drains out instead of back.

05How long to run it

Run it during the shower and for about 20 minutes after. A timer switch or a humidity-sensing switch makes this automatic and is a cheap upgrade.

06Fixing it

Extending the duct to a proper roof or wall vent is a straightforward job for a handyman or a roofer. If there's already mold on the roof boards, get it looked at by someone who handles mold and ventilation. Our guides to attic insulation and ventilation, ice dams, and mold vs. mildew explain how it all ties together.

Sources (2)

Roofing Magazine: Is Moisture Being Properly Exhausted to the Outdoors? · Liberty Mutual: 5 Reasons Not to Vent Bathroom Fans to the Attic

Related: A Comprehensive Guide to DIY Home Inspection: Safeguarding Your Home Inside and Out