The mirror stays fogged long after the shower ends, the paint over the tub is starting to bubble, and a dark line in the caulk keeps coming back. Those are moisture problems, and in most bathrooms they trace back to ventilation. A fan that is undersized, poorly ducted, or never left running can’t keep up with daily showers.

Why a bathroom needs mechanical ventilation

A hot shower puts a lot of water vapor into a small room. If that moisture isn’t removed, it condenses on the coolest surfaces: mirrors, windows, exterior walls, and the ceiling. Over time, repeated condensation leads to peeling paint, swollen trim and vanity edges, and mildew in grout and caulk.

Opening a window helps, but it depends on the weather and on someone remembering. Few people want a window open on a 100-degree Fresno afternoon, and in winter, cooler walls and glass give steam even more places to condense. An exhaust fan ducted outdoors works in every season.

Sizing the fan

Fans are rated in cubic feet per minute (CFM). The Home Ventilating Institute (HVI), which certifies fan performance ratings, recommends these intermittent rates:

• Bathrooms up to 100 square feet: about 1 CFM for every square foot of floor area, and HVI’s ventilation guide lists 50 CFM as the minimum.

• Larger bathrooms: add up the fixtures, at 50 CFM each for a toilet, a shower, and a bathtub, and 100 CFM for a jetted tub.

• An enclosed toilet room should have its own fan.

Location matters as much as size. HVI recommends a fan rated for wet locations over the shower or tub when possible. It also calls for at least three-quarters of an inch of clearance under the bathroom door so replacement air can get in. Ratings are measured under test conditions and real airflow drops with long or twisting ducts, so a little extra capacity is reasonable.

Noise, controls, and run time

A fan nobody runs doesn’t help, and loud fans don’t get run. Sound is rated in sones, and lower numbers are quieter. For new homes, California’s Energy Code calls for bathroom fans that run on demand to be rated at 3.0 sones or less, and fans that run continuously at 1.0 sone or less. Controls make the difference between a fan that works and one that only exists:

• Timers keep the fan running after you leave. HVI recommends a control that keeps ventilation going for at least 20 minutes after each use.

• Humidity sensors switch the fan on and off based on moisture in the air, so nobody has to remember.

• Continuous low-speed fans run all the time at a low rate. HVI says continuous ventilation of at least 20 CFM can stand in for an intermittent 50 CFM fan.

If you choose a fan with a built-in light, the Energy Code requires the light to be controlled separately from the fan. Replacing an existing fan is fixture-level work, while adding a fan where none existed or running new wiring for controls may involve licensed electrical trades, which is evaluated during the in-home consultation.

Is your current fan doing its job?

Before you replace a fan, check whether it moves air at all. Hold a single square of toilet tissue up to the grille with the fan running; if the fan can’t hold it against the grille, airflow is weak. Weak flow usually comes from a clogged grille, a crushed or disconnected duct, a stuck damper at the outside cap, or a fan that was undersized from the start.

Noise is another clue. A fan that rattles or hums loudly while moving little air is often fighting a restrictive duct, and it tends to get switched off early. Dust on the grille and housing also cuts airflow, so a careful cleaning with the power off is worth trying first.

Ducting it outside, the right way

A fan is only as good as its duct. California’s green building code, CALGreen, requires the bathroom fans it covers to be ducted to terminate outside the building. A fan that dumps moist air into the attic just moves the problem somewhere you can’t see it. Good ducting practice is straightforward:

• Keep the run as short and straight as practical, with as few turns as possible.

• Use smooth duct of the size the fan calls for, not a smaller one.

• Insulate duct that passes through an unconditioned attic, so moisture doesn’t condense inside it.

• End at a proper exterior termination with a damper, never in open attic space.

Where the duct leaves the house, through a wall or the roof, is part of the plan and is evaluated during the in-home consultation. An existing fan that vents into the attic is worth correcting while the ceiling is open.

What California code requires

Two parts of California’s building standards address bathroom exhaust. The 2025 Energy Code, which applies to permit applications on or after January 1, 2026, requires new homes to have bathroom exhaust vented outdoors, with at least 50 CFM for a fan that runs on demand. CALGreen adds that in new construction, and in additions or alterations that increase a home’s area or volume, bathroom fans must be ENERGY STAR compliant, ducted outside, and controlled by a humidity control unless the fan is part of a whole-house ventilation system.

For a remodel inside an existing footprint, which rules apply depends on the scope of work and your building department. Permit needs are confirmed during the in-home consultation, and our remodeling permits section links to guides for local building departments. Even where it isn’t required, the new-construction standard is a sensible target, because it describes a fan that actually keeps a bathroom dry.

Moisture control beyond the fan

Ventilation handles vapor in the air. The rest of a dry bathroom comes from how it is built and used:

• A continuous waterproofing layer behind shower tile, with the shower floor sloped to the drain

• Flexible sealant where tile meets the tub, the shower pan, and inside corners

• Moisture-resistant backer board and drywall in wet zones

• Paint with a finish that tolerates humidity and wiping

• A habit of running the fan and leaving the door open after showers

Our shower tile and waterproofing page explains the layers behind the tile. In a small bathroom, where steam has less room to spread, a properly sized fan matters even more. Open walk-in showers let steam reach the whole room, so fan placement deserves attention there too, and the bathroom estimator shows a planning range for your project.