Yes. A bathroom fan is one of the best tools against bathroom mold, if it vents outdoors and you leave it on after a shower.
A fan cannot fix a leak, though. It only deals with the moisture from steam, so a drip under the sink or behind the tile needs its own fix.
How does a bathroom fan help prevent mold?
Mold needs moisture to grow. A hot shower fills a small room with steam, and that water lands on the ceiling, the walls, the grout and the mirror.
The fan pulls that damp air out of the room before it soaks into those surfaces. The EPA’s guide to mold and moisture says to use the bathroom fan or open a window when you shower.
The EPA also says to aim for indoor humidity below 60 percent, ideally between 30 and 50 percent. A closed bathroom blows past that with one long shower.
How long should you leave the bathroom fan on?
The Home Ventilating Institute recommends leaving the fan on for 20 minutes after you use the bathroom. HVI suggests a timer switch, so the fan shuts off by itself at the right time.
Switching the fan off as you walk out leaves most of the steam behind. That wet air then settles on the coldest surfaces in the room, usually the ceiling and the outside wall.
What size bathroom fan do you need?
Fans are rated in CFM, the cubic feet of air they move each minute. HVI’s rule of thumb is one CFM for each square foot of floor. A bathroom of 50 square feet or less should have at least 50 CFM.
That means a bathroom that measures 8 feet by 10 feet, or 80 square feet, needs a fan of about 80 CFM. You can usually find the rating on a label inside the fan housing, behind the grille.
Where should a bathroom fan vent?
It should vent outdoors. The International Residential Code, in section M1501.1, says exhaust air has to go outdoors. It bars exhaust into an attic, a soffit, a ridge vent or a crawl space.
A fan that dumps into the attic only moves the moisture up one floor. In winter, that warm, wet air hits cold roof boards, turns to frost, and then melts back into water.
To check yours, go outside while the fan is on and find where the duct ends. You should see a flap open on the wall or the roof cap, or feel air coming out.
Why do Colorado bathrooms get moldy even with a fan?
Front Range air is dry, but a hot shower fills a closed bathroom with steam even on the driest day. Here is what our team sees behind the moldy bathrooms we get called to:
- Winters with the house shut tight. From fall to spring, windows are closed and little fresh air comes in, so the steam has nowhere to go but the fan.
- Ducts through a cold attic. Warm, damp air cools inside an uninsulated duct, turns back to water and can drip back down to the fan. The EPA recommends insulating cold surfaces, and insulated duct does that job here.
- Leaks no fan can reach. A dripping line under the vanity, a worn wax ring at the toilet or cracked grout at the tub lets water in.
How can you tell if your bathroom fan works?
- Try the tissue test. With the fan on, put a single square of toilet paper against the grille. A working fan pulls it tight against the cover.
- Watch the mirror. If it fogs up and does not clear while the fan is on, the fan is not moving enough air.
- Look for dust. A grille caked with dust and lint cannot move much air, so clean it once or twice a year.
- Listen. A fan that hums but barely pulls may have a stuck flap or a crushed duct.
When is it time to call about bathroom mold?
A fan helps prevent new growth. It will not remove mold that is already in the grout, the caulk or the drywall behind them.
Get help if you see any of these:
- Black spots that come back within a few weeks of cleaning.
- A soft floor around the toilet or the tub.
- Stains or peeling paint on the ceiling above the shower.
- A patch bigger than about 10 square feet, the size the EPA gives for most do-it-yourself cleanups.
If you are unsure whether those dark spots are mold at all, read about the difference between mold and mildew first.
Damaged drywall or flooring sometimes has to come out. The EPA notes that absorbent materials may have to be replaced once they grow mold.
Send us photos of the spots and tell us roughly how big the room is. We send back a number, usually the same business day. Our bathroom mold removal page covers where bathroom mold hides and how we quote it.
Call (720) 443-4222 in the Denver area or (970) 469-9030 in Northern Colorado.