Key takeaways
- An original bath fan rattles because its bearings, blades, damper flap and mounting have worn or worked loose since the house was built.
- Hold a square of toilet paper to the grille with the fan on, and if it drops, the fan isn't pulling enough air to dry the room.
- The Department of Energy says a bath fan should move at least 50 cfm when switched on and vent outdoors, not into the attic.
- Run it at least 20 minutes after every shower, since a bathroom that stays damp keeps the grout wet between showers, and wet grout is where mildew and gray joints start.
You flip the switch in the hall bath and the fan over the tub starts with a clatter, settles into a buzz, and the mirror fogs up anyway. In a 1960s ranch that fan often went in with the house. It's loud, and a loud fan is the one everybody shuts off before the steam clears. That habit matters more than the noise, because a bathroom that never dries out is where gray grout, black caulk and soft board behind the tile get started.
Why does an original bathroom fan rattle?
The noise comes from parts that have worn or worked loose over the decades. In an original fan it is one or more of these:
- The motor bearings. They dry out, the fan wheel starts to wobble, and the hum turns into a grind.
- Dust on the blades. Lint and dust build up on one side of the wheel and throw it off balance.
- The damper flap. That's the small hinged door where the fan meets the duct. It flutters while the fan runs and slaps when the wind blows outside.
- The housing and grille. The mounting screws and grille springs work loose, so the metal box buzzes against the ceiling drywall.
Turn the breaker off before you pull the grille. A vacuum on the blades and a screwdriver on the housing screws quiet a lot of rattles. A grinding motor means the fan is near the end.
Noise and airflow are separate problems. A fan can rattle and still move air, and a quiet one can move almost none. The Department of Energy's Building America guide warns that a fan's rated airflow may not hold up once it's installed on a poor duct run, which describes plenty of original fans in older houses.
The toilet-paper test
Turn the fan on. Hold one square of toilet paper flat against the grille and let go. If the fan holds it there, it's pulling air. If the square flutters to the floor, the fan isn't moving enough air to dry the room after a shower.
It's a rough check, not a measurement. A fan can hold a square and still fall short of its label. Peeling paint or brown spots on the ceiling above the tub tell you the same thing: steam is sitting up there with nowhere to go.
Where does your fan's air go?
Find out before you buy anything. In an older house, the duct may end on top of the attic insulation, stop short of a roof vent, or be missing altogether. The Building America guide says to vent the fan to the outside, not into an attic, crawlspace or the space between floors, and to replace or reroute a fan that does. In a Cincinnati January, warm wet air dumped into a cold attic condenses on the roof boards and nails over your head.
To check yours, go outside with the fan running and look for a wall cap or roof cap with a flap that opens. If you can't find one, take a flashlight into the attic and follow the duct from the fan housing. The same guide prefers smooth rigid duct over flex duct, and says flex duct should be stretched tight so it doesn't sag. A sagging duct with a low spot that holds water chokes the airflow of any fan.
What do cfm and sone ratings mean?
Every fan box has two numbers worth reading. Cfm is cubic feet of air per minute, the amount the fan moves. Sones measure loudness, and a lower number is quieter.
| Rating | What it measures | What to look for | Source |
|---|---|---|---|
| Cfm, fan switched on | Airflow | 50 or more | DOE Building America |
| Cfm, fan runs all day | Airflow | 20 or more | DOE Building America |
| Cfm by room size | Airflow | 1 cfm per square foot, rooms up to 100 square feet | HVI |
| Sones, fan switched on | Loudness | 3 or less | DOE Building America, citing ENERGY STAR |
| Sones, fan runs all day | Loudness | 1 or less | DOE Building America, citing ENERGY STAR |
The Home Ventilating Institute's own example is a 5 by 8 foot bathroom, 40 square feet, which needs at least a 40 cfm fan. A 50 cfm fan meets both numbers in a room that size. A bigger master bath needs more.
Give the sone number as much weight as the cfm. A quieter fan is one you're more likely to leave on until the room is dry, and a fan nobody runs moves zero air.
How long should you run the fan after a shower?
HVI recommends leaving the fan on for 20 minutes after you use the bathroom. The Building America guide goes further and recommends one hour after a shower for humidity control. Treat 20 minutes as the floor and an hour as the target after a long, hot one.
Nobody waits in the hall for an hour to flip a switch, so the same guide recommends a timer switch in place of a standard wall switch. You turn the dial on your way out and the fan shuts itself off.
What a damp bathroom does to the grout and the wall behind it
Grout is cement and sand, full of pores, and it holds water from every shower. If the room stays humid for hours, the joints never dry between showers, and mildew grows in them. That's the gray that comes back a few months after you scrub. Why your shower grout turns gray again explains what's going on inside the joint and gives the cleaning and sealing routine.
The caulk at the tub goes next, with a pink or black line that keeps returning. After that, shower water that gets past worn grout and caulk reaches the board the tile is stuck to, and a humid room keeps it from drying out. A fan in good order, ducted outdoors and run long enough, helps the surfaces dry between showers. It can't stop water that's already getting through. It's worth doing whatever you decide about the shower. An electrician can tell you whether the old housing can take a new motor or whether the box and duct need to come out.
When a new fan won't save the wall
A better fan keeps a sound wall sound. It won't dry board that's already wet behind the tile. Tap the tiles on the tub wall with a knuckle, and press with your heel on the floor along the tub. A hollow sound or a soft spot can mean water has been getting in for a while. Your 20-year-old shower: what fails first goes through those checks in order and shows where repairs stop paying. What's behind the shower tile in a 1960s Cincinnati bathroom shows what the board looks like once it comes out.
If the wall is past saving, a tile contractor can rebuild it with new backer board and tile, and done right that's a good shower. Bulldog builds the other kind: we take the tub or shower area to the studs and put in solid-surface walls with no grout lines, in one day, with your first shower the next morning. The floor, vanity and toilet are a separate, longer job. A non-porous wall has no grout joint to soak up steam, though the ceiling and the paint still need a working fan.
Start with the toilet-paper test tonight, then go find where the duct ends. If the walls pass the tap test, the grout routine and a fan that runs for 20 minutes will keep them that way.
