How do I know if my bathroom fan is working?

Hold one sheet of toilet paper against the grille with the fan running. If it will not hold up on its own, the fan is moving almost nothing. If it holds, some air is moving but not necessarily enough — the practical test is whether the mirror has cleared ten minutes after a shower with the fan running.

Is it bad if my bathroom fan vents into the attic?

Yes, and it is more serious than most people expect. Every shower puts warm wet air into a cold space where it condenses on the underside of the roof. One winter soaks the insulation so it stops insulating; several rot the sheathing and grow mold on the roof deck. It is common — we found it twice in two weeks this October.

How much air should a bathroom exhaust fan move?

Fifty cubic feet a minute is the usual benchmark for a fan you switch on and off, with bigger bathrooms wanting more. Fans are rated on a bench against almost no resistance, so what yours delivers depends on the duct it was connected to — a hundred CFM fan on a long crushed run can deliver a third of its rating.

Should I just replace the fan?

Sometimes, but not before someone has looked at the duct. Fitting a bigger fan to the duct that was strangling the old one buys a louder disappointment. If the unit is genuinely the problem it is replaced like-for-like on the existing circuit, with the fan at cost.

Do you do this in the Lansing area?

Yes — Haslett, Okemos, East Lansing, Lansing, DeWitt, Bath, Williamston, Holt and Mason. It can be done on its own or as part of a full home energy assessment.

Your Bathroom Fan Might Be Venting Into Your Attic

Running is not the same as working

Almost everybody checks a bathroom fan the same way: flip the switch and listen. If it makes a noise, it is working. Noise is unfortunately the one thing a failing fan is still good at. A unit can run, sound busy, and move so little air that the mirror still fogs, the window still runs with water and the paint on the ceiling still lets go in sheets.

What matters is how much air it actually pulls, and nothing about the sound tells you that.

The ten-second test you can do right now

Take one sheet of toilet paper. Switch the fan on and hold the sheet flat against the grille. Let go.

If it falls, the fan is moving almost nothing and everything below is academic. If it holds, that proves some air is moving. It does not prove enough is.

Why a good fan performs badly

Fans are rated on a test bench against almost no resistance. Then they get fitted to twenty feet of flexible duct with four bends in it, half of it sagging between joists and some of it crushed where somebody knelt on it while running a cable.

A fan sold as a hundred cubic feet a minute can easily deliver a third of that once it is in a real ceiling. The usual benchmark for a bathroom is fifty CFM for a fan you switch on and off. The honest version of that rule is simpler: if the mirror is still fogged ten minutes after a shower with the fan running the whole time, the fan is not keeping up, whatever the label on the box said.

The fault that actually costs money

The second half of any real test is not a measurement at all. It is a question: where does the duct go?

It is supposed to end outside the building. A surprising number end in the attic.

A bathroom fan venting into an attic is not a ventilation fault. It is a water leak you switch on every morning. Warm wet air goes up into a cold space and condenses on the first cold surface it meets, which is the underside of your roof. Over one winter that soaks the insulation, and wet insulation stops insulating. Over several it rots sheathing and grows mold on the roof deck, and by then the repair is not a fan, it is a roof.

We found this twice in two weeks this October, in two unconnected houses in the Lansing area. In both cases the owners had no idea, and in both cases the fan had been running faithfully for years, doing harm.

Four things you can check yourself this afternoon

The paper test at the grille. Ten seconds, and it answers the loudest question.
Go outside while somebody runs the fan. Look for a hood on a wall, a soffit or the roof. If you cannot find one, there is a fair chance there is not one.
Put your head in the attic hatch with a torch. A duct lying loose, or ending in a pile of damp gray insulation, settles it on the spot.
Look at the ceiling around the fan and the roof deck above it. Staining, rusted nail points, or frost in winter all say the same thing.

What a proper test involves

Airflow measured at the grille, so there is a number rather than an impression. The duct followed from the fan housing to wherever it really terminates. The outside damper checked for whether it opens when the fan runs and closes when it stops, because a stuck damper is both a ventilation problem and an open door for insects. The duct run itself looked at for crushing, sags holding water and disconnected joints. And photographs of whatever is found in the attic, so the decision afterward is made on evidence rather than on somebody’s verdict.

Why we look at the duct before selling you a fan

Replacing the unit is the easy job and the profitable one. It is also, often, the wrong one. Fitting a bigger fan to the duct that was strangling the old one buys you a louder disappointment and the same fogged mirror.

So the duct gets looked at first. If the fan genuinely is the problem, it is replaced like-for-like on the existing circuit with the unit supplied at cost.

Where our work stops

A bathroom fan, its duct and its termination are squarely our trade. Furnace and air conditioning work is Michigan mechanical contractor territory and is not something we take on. A new circuit or switch position is licensed electrical work and is referred out; swapping a fan on the wiring already there is not. And where a duct genuinely has to go through a shingled roof, a gable or an eave is usually the better answer anyway — where it really must go through the roof covering, that part goes to a roofer.

If there is visible mold from years of a fan venting into the attic, you get told so and shown the photographs. Remediation is a licensed specialist, not us.

If your bathroom will not dry out

Do the paper test, look for the vent outside, and look in the attic. If any of those three answers worries you, we test bathroom exhaust fans across Greater Lansing — and if you already know the duct ends in the attic, say so when you call and we will come ready to put it right on the same visit.