How a working bathroom fan prevents mold

To keep mold out of a bathroom, you need a fan that's big enough for the room, a duct that goes all the way outside, and about 20 minutes of run time after every shower. Mold grows when wet air hangs around long enough to condense on the ceiling, the grout and the caulk. If the mirror stays fogged long after you're out, the ceiling paint is peeling, or black spots keep coming back in the same corner, the fan is too small, vented badly, or shut off too soon.

Where the water goes

A hot shower dumps a lot of water vapor into a small room. When that air hits something cooler, like the mirror, an outside wall, the window, or the ceiling under a cold attic, it turns back into droplets. Give mold moisture, a surface and time, and it grows. A bathroom that dries out within half an hour almost never has a mold problem. One that stays damp for hours a day almost always does.

Capital Region winters make it worse. Outside walls and attic ceilings are cold, so condensation forms faster and in more places, and the window that sat cracked open all summer stays shut.

Is your fan actually working?

If the mirror is still fogged 10 or 15 minutes after the water's off, the fan isn't keeping up.

Then try the toilet paper test. Turn the fan on and hold a single square of toilet paper up to the grille. A working fan pulls it flat and holds it there. If it flutters or drops, the airflow is weak, no matter how loud the fan sounds.

Go outside and find where the duct ends. With the fan running you should see the damper flap open or feel air coming out. If you can't find a vent cap at all, the duct may well stop in the attic.

Look at the ceiling over the shower for peeling or bubbled paint, and at the caulk and grout for mildew that comes back a week after you clean it. And if water drips out of the fan grille in cold weather, warm wet air is condensing inside a duct that runs through a cold attic and draining back down.

How big a fan you need

Fans are rated for airflow in cubic feet per minute (CFM) and for noise in sones. The residential code sets a minimum airflow for bathroom exhaust, and the Home Ventilating Institute gives a simple rule for sizing.

For a bathroom up to about 100 square feet, figure about 1 CFM per square foot of floor, with 50 CFM as the floor. An 8-by-10 bathroom calls for about 80 CFM. For a bigger room, size by fixture instead: roughly 50 CFM each for the toilet, the shower and the tub, and 100 CFM for a jetted tub.

Pay attention to the sone rating too. Around 1 sone or less is quiet. Quiet fans get left on. Loud ones get switched off the minute someone leaves the room.

The CFM on the box assumes a short, smooth duct. Long runs, tight elbows, and flex duct that sags between the trusses all cut the real airflow, sometimes by a lot.

The duct is where it goes wrong

Most bad installs are bad in the duct, not the fan.

It has to end outdoors, at a roof or wall cap with a damper. Not in the attic. Not blowing at a soffit, where the soffit vents just pull it back in. Wet air dumped in an attic turns to frost on the underside of the roof sheathing in winter, soaks the insulation, and grows mold on the attic side of your ceiling.

In an unheated attic the duct needs insulation. A bare duct gets cold enough that moisture condenses inside it and runs back down into the fan housing. That's the dripping grille.

Keep the run short and smooth. Rigid or semi-rigid duct with as few bends as you can manage moves the most air. If it has to be flex, pull it tight and support it so it doesn't sag into a belly that holds water. Match the duct to the fan's outlet size or go bigger, never smaller.

Damp air in the attic also feeds ice dams and frost damage on the roof. We get into that in ice dam damage repair.

Run it long enough

Turn it on when the shower starts and leave it for about 20 minutes after. Most people flip it off on their way out, which is too soon. A timer switch fixes that: set it and walk away. A humidity-sensing switch, or a fan with a humidity sensor built in, is better still, since it comes on by itself and runs until the room dries.

The fan also needs air coming in to replace what it pushes out. If the bathroom door seals tight against the floor, the fan struggles. A small gap under the door takes care of it.

Swapping an old fan for a new one in the same hole is often simple. Adding a fan where there wasn't one, running a new circuit, or putting a fan over a tub or shower means electrical work and the maker's rules for wet locations. Have a licensed electrician do that part.

Builder-grade fans in Wilton

A lot of houses in Wilton's newer neighborhoods, and in subdivisions all over Saratoga County, came with the builder's cheapest bath fan. They're noisy, on the small side, and sometimes ducted to a soffit or through 15 feet of flex in the attic. They meet the minimum and fall behind in a busy family bathroom. Putting in a quiet fan sized for the room, with a short insulated duct to a real exterior cap, is one of the cheapest fixes that actually helps in a Wilton bathroom. The paint, the caulk and the grout all last longer for it.

Cleaning up what's already there

Small patches of mildew on tile, caulk and a painted ceiling can generally be cleaned with a household bathroom cleaner. If the mold is under the caulk, the caulk has to come out and be replaced; see how often to recaulk a tub or shower. Grout that stays dark and damp may need sealing, covered in how often to seal grout.

Bigger areas are a different job. EPA guidance suggests professional help for mold covering more than about 10 square feet, or mold caused by hidden water like a leak inside the wall. If mold keeps coming back in the same spot even with a fan that works, suspect a leak, and read the signs of a leaking shower behind tile.

Frequently asked questions

How long should a bathroom fan run after a shower?

About 20 minutes is the common advice. A timer or humidity switch makes it happen, since hardly anyone waits that long to flip a manual switch.

Can a bathroom fan vent into the attic?

No. It leaves warm, wet air where it condenses on cold roof sheathing and insulation, and that leads to mold and rot. Run the duct to a roof or wall cap with a damper.

Why does my bathroom fan drip water in winter?

Most often, moist air is condensing inside a duct that passes through a cold attic. The fix is generally insulated duct, a shorter run, a proper exterior cap, and running the fan long enough to clear the duct after each shower.

When we remodel a bathroom in Wilton or elsewhere in Saratoga County, the fan and duct get done along with the tile, fixtures and waterproofing, since the ceiling's already open. Our bathrooms page has some of the finished rooms.