The shower has been off a good while and the mirror still has not cleared. The fan is running — you can hear it from the hallway — the room is warm, the wall behind the door is damp to the back of your hand, and a bead of water is sitting on the paint above the casing. When a fan you can hear leaves a room in that condition, the motor is almost never the broken part.
Separate the mirror from the room first. Glass is the coldest surface in a bathroom, and a mirror on an exterior wall is colder again, so it fogs first and clears last even in a room that ventilates properly. If the mirror is hazy but the walls are dry and the ceiling paint is sound, you have a cold sheet of glass rather than a ventilation problem, and where a mirror sits is a separate decision. If the whole room stays wet, the air is not leaving, and there are three reasons it is not.
The duct can throttle the fan, so it moves a fraction of what the box promised. The duct can end somewhere useless, so the air leaves the room and comes back into the house. Or the room has no make-up air, so the fan has nothing to pull and hums. They feel identical from the doorway. They separate in an afternoon, and the checks run in an order, cheapest first.
Check one: the grille, the wheel and the damper
Everyone knows the tissue test. Hold a square of toilet paper to the grille and if it sticks, the fan is pulling. That proves one thing: there is suction at the grille. It says nothing about where the air ends up, and a fan dumping into the attic holds the tissue as well as one ducted to a roof cap.
Take the grille off instead. Most drop on spring clips — squeeze them inward and the cover comes away. Kill the breaker before your fingers go past it.
What you are looking for is felt. Bathroom air carries lint, skin and hair, and it lands on the leading edge of every blade of the blower wheel. A wheel with a gray coat on it is stirring air, not moving it, and it sounds busy doing it. Vacuum the housing, wipe the wheel, clear the grille mesh — that clogs from the room side and nobody looks at it.
Then find the damper. A light plastic flap sits at the fan's outlet collar, and a second at the outside termination. Lift the collar flap with a finger. It should swing freely and fall shut on its own. Lint and paint glue those flaps in place, and one stuck closed turns the fan into a motor spinning against a shut door. One stuck open is quieter about it: the fan works during the shower, then cold attic air drifts back down the duct and the room re-condenses what it lost.
Reassemble, run a normal shower, watch the mirror. This is a Saturday morning and the only one of these fixes that costs nothing. If it works and then fades again within a season, the cleaning was a symptom rather than the fix — a run reloading with lint that fast is long, sagging or unsealed at the housing, which puts you in check three.
Check two: prop the door open and run it again
Every cubic foot of air the fan pushes outside is replaced by a cubic foot pulled in from somewhere. In a room with a weatherstripped door, thick pad and carpet sealing the bottom, that somewhere can be nowhere. The fan runs against a closed room, moves a fraction of its rating, and the only report you get is the noise.
The test takes one shower. Prop the door properly open and run the room the way you normally would. If it clears with the door open and never clears with the door shut, you have found it — and it is the cheapest of the three.
The fix is unglamorous. The door needs an undercut, or the wall above it needs a transfer grille. Undercutting a hollow-core slab has a limit — the bottom rail is a narrow strip of wood and past it is void — so a solid-core door gives you more to work with. Where the door cannot give up more, a grille or a jump duct does the same job without a visible gap, the same trick that fixes a basement room that feels stuffy with the door shut.
Look at what else pulls on the house at the same time. A range hood, a dryer and a bathroom fan running together in a tight house compete for the same replacement air, and the smallest one loses.
Where this touches things we do not test for
A fan is a depressurizing appliance. The house makes up what it removes wherever it can — the rim joist, the attic hatch, a chimney, and through the slab. That last path matters here. Hamilton County is mapped by the EPA as Radon Zone 1, the highest of three, meaning a predicted average indoor screening level greater than 4 pCi/L; the EPA's action level is 4 pCi/L, and it suggests considering a fix between 2 and 4 pCi/L. None of that argues for under-ventilating a bathroom. It argues for testing rather than assuming, and the Indiana State radon office is on 1-800-272-9723. We do not test, diagnose or mitigate radon; the same is true downstairs, where radon has to be accounted for before anything gets framed.
The other one is combustion. If an atmospherically vented gas water heater or furnace shares air with the living space, strong exhaust and no make-up air is a question for an HVAC contractor — not for us, and not for a blog. Working carbon monoxide alarms are the baseline either way.
Check three: follow the duct
Start outside, because it is free. Have someone run the fan while you walk the exterior and find the termination — a hood on a gable wall, a cap on the roof, a vent low on the siding. The flap should lift, and warm damp air should land on your hand. If it does not move, the air is not reaching it. If there is no termination anywhere on the outside of the house, that has answered it: the bathroom vents into the attic.
Then get above it, if you safely can. An attic is joists, blown insulation and heat, and if you cannot reach the area over the bathroom standing on something solid, this part is not yours. What we look for up there:
- A reducer at the fan collar, joining a fan built around one duct size to whatever pipe was already in the bay. Resistance climbs steeply as diameter drops, so the fan runs choked at its own outlet.
- Flex hanging in loops between joists, kinked over a truss chord, or crushed under a drift of blown insulation. Every sag is a low point, and every low point holds condensate.
- A joint that let go. Foil tape releases in attic heat, and the last stretch of flex ends up lying on the insulation, blowing into the space.
- A termination in a soffit, a short reach from the intake vents that feed the attic, so the attic breathes the same wet air straight back in.
- A cap whose flap no longer closes, painted open or propped by lint.
The evidence shows without the attic, too. A ceiling stain in a room with no plumbing above it is usually condensate pooling in a low spot of a duct. Insulation matted flat over the bathroom has been rained on from inside. Frost on the underside of the roof sheathing is the same story told louder, and one of the more expensive things freeze-thaw weather does to a house here.
What size a duct should be, how it is pitched and where it is allowed to end is a whole assembly on its own, written up in the fan matters more than the tile. Read that before buying anything.
Which of these is a Saturday, and which is not
Yours, with ordinary tools:
- Cleaning the grille, the housing and the blower wheel.
- Freeing or replacing a damper flap at the collar.
- Undercutting a door, or fitting a transfer grille above it.
- Re-taping and clamping a joint you can reach from a floored section of attic.
Not yours, and not because we want the work:
- Re-routing a run across an unfloored attic in summer heat. Bad place to be alone.
- Cutting a new roof penetration. That is flashing, and a roofer should own it. A gable or sidewall termination is a different, easier job.
- Growing the fan. A bigger unit is a bigger box, and whether it fits the joist bay is decided at the ceiling, not on the shelf.
- Anything where the bathroom sits under another floor and the duct runs in a joist bay you cannot open from above.
When it has to be opened up to know
Some rooms will not tell you. A vaulted ceiling with no attic over it, a bathroom with living space above, a duct that disappears into a chase and reappears nowhere — in those, the run has to be exposed before anyone can say what it does. A borescope through the fan opening buys a little information. Not enough to price a repair.
We would rather say that than guess. It is the same position we take on what is behind the wall in any bathroom: we look, then we quote.
What people replace first, and why it does not fix it
A new fan on the old duct. The most common attempt, and it fails because the duct was the problem. A higher-rated fan on a necked-down, sagging run buys a louder room and the same wet ceiling.
New caulk. When a room never dries the joints go dark first, and the caulk gets blamed for reporting it. Cutting it out buys a clean season — the pattern laid out in black mold in the shower caulk and in the joints that fail first.
Repainting the ceiling. Chalky, peeling paint above a shower is a moisture report, not a paint defect, and the finishes that survive a wet room still need the air to leave. We do not test for, identify or remediate mold, and neither does a can of paint. Where growth goes past surface mildew on a caulk line, that is lab and remediation work — the EPA publishes homeowner guidance on mold and moisture, and the Indiana Department of Health is the state authority.
The short version
Run the checks in order. Pull the grille, look for felt on the blower wheel, free the damper. Run a shower with the door propped open and see whether the room clears. Then stand outside while the fan runs and find out whether anything leaves the house at all. Two of those three cost nothing but an evening.
If the answer is in the attic, or there is no attic to look in, the work is a duct route and a real termination — not a new fan. It is cheapest while a bathroom is already open and the ceiling is accessible, which is why venting gets settled before any tile goes on in the rooms on recent projects.
If your mirror has stopped clearing and you would rather someone else got in the attic, ask for an estimate and we will follow the pipe to wherever it ends.
