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The bathroom exhaust fan that stops mold

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1Mold has a shopping list, and soap scum is on it

Soap scum and skin cells collect on shower surfaces and are literally food for mold and mildew; a bathroom supplies the other two requirements, warmth and moisture, on its own every time someone showers.

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2Run it during and after, not just during

Running the exhaust fan during a shower and for a few minutes afterward clears the moisture that would otherwise sit on every surface in the room. A timer switch takes the guesswork out of remembering to leave it on.

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3There's a simple test for whether it's working

A working fan pulls enough air that you can feel a draw under the bathroom door while it runs; no draw means the fan isn't moving enough air to matter.

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4A shower makes a measurable amount of water

A shower puts about half a pint of water vapor into the air for every five minutes it runs. A ten-minute shower is a full pint of water hanging in the room, and without a fan it settles onto the floor, walls and ceiling where mold and mildew can use it.

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5Five minutes is the floor; an hour is what the energy lab suggests

The extension rule of a few minutes after the shower is the minimum. The Department of Energy's building research center goes further and recommends leaving the fan on for an hour after a shower for real humidity control, with a timer switch set to shut it off. The same guidance gives the fan size behind the code: at least 50 cubic feet per minute for a fan you switch on and off, or 20 if it runs all the time.

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