Bathroom fans and humidity in Dallas homes: sizing, ducting and control

The four reasons a fan does not work

Most humidity problems in a bathroom trace back to one of four things.

• The fan is too small for the room.
• The duct is long, crushed or full of tight bends, so the fan moves little air.
• The duct ends in the attic instead of outside.
• The fan is switched off as soon as the shower ends.

Sizing a fan

The common rule for a standard bathroom is about one CFM of airflow per square foot of floor, so a 5 by 8 bathroom needs about 40 CFM and a 10 by 10 master about 100. Larger or taller rooms, and rooms with a separate enclosed shower or toilet, need more.

Noise matters too, because a loud fan gets switched off early. Look at the sone rating as well as the airflow; quieter fans get used longer, which is when they do their work.

Why the duct matters more than the fan

A fan's rating assumes a short, smooth duct. A long flexible duct with tight bends can cut its real airflow dramatically. The best runs are short and straight, in rigid or smooth insulated duct, to a roof jack or wall cap with a damper.

In an attic, uninsulated duct lets warm, wet air condense inside it and drip back into the fan. Insulated duct prevents that.

The attic problem

In older and builder-grade homes a fan duct sometimes stops in the attic insulation or was never connected at all. That puts moist air onto the roof deck and insulation instead of outside. If you can safely look in the attic, follow the duct from the fan; it should end at a cap on the roof or an outside wall.

Rerouting it outside is usually a straightforward job and often the single change that fixes a fogged mirror and peeling paint.