Bathroom Ventilation: Humidity, Mold, and Texas Summers

Sizing the fan

For larger bathrooms, size by fixtures rather than area: allow roughly 50 CFM each for a toilet, shower, and tub, and about 100 CFM for a jetted tub. A separate enclosed water closet needs its own fan.

Also check the sone rating. A loud fan does not get run long enough, which defeats the purpose. Anything at 1.0 sones or below is quiet enough that people leave it on.

Where the duct goes matters more than the fan

This is the most common failure we find in North Texas homes: a fan that exhausts into the attic rather than to the outside. Every shower then dumps warm moist air into an attic space, where it condenses on the roof deck and framing. The result is mold on the sheathing and, in bad cases, deteriorated decking.

The duct must terminate at a roof cap or a wall cap with a damper, outside the building envelope. It should be as short and straight as practical, insulated where it crosses an unconditioned attic so the moist air inside it does not condense before it exits, and sloped slightly toward the exterior so any condensate drains out rather than back into the fan.

Flexible duct with sags is a common culprit — water pools in the low spots and airflow drops.

Run time is the Texas-specific part

Turning the fan off when you leave the bathroom removes maybe half the moisture. Surfaces are still wet, and the room is still saturated. In a humid North Texas summer, the outdoor air is not doing you any favors either.

The fix is a timer switch set for 20 to 30 minutes after use, or a humidity-sensing fan that runs until the room reaches a set level. Either costs very little and is the single most effective anti-mildew measure in a bathroom.

Make-up air

A fan can only exhaust what it can replace. In a tight bathroom with a well-sealed door, an exhaust fan starves and moves far less air than its rating. A three-quarter-inch undercut on the door, or a transfer grille, gives it somewhere to pull from.

Where ventilation matters most

Windowless bathrooms, wet rooms and curbless showers where the whole floor gets wet, steam showers, and any bathroom on an exterior wall where cool surfaces meet humid air. Those are the rooms where undersized ventilation shows up as mildew on the ceiling within a year.