Your Bathroom Fan Is Probably Doing Nothing: The 30-Second Tissue Test

Here is a test that takes thirty seconds and costs nothing. Turn on your bathroom fan, tear off a single square of toilet paper, and hold it flat against the grille. If the fan holds it there, it is moving air. If the tissue sags, flutters or falls, your bathroom fan is making noise and doing almost nothing else — and every shower is dumping its moisture into your house instead of outside it. In an Alberta summer, with the windows shut against wildfire smoke, that is exactly how a bathroom grows black speckles on the ceiling and a homeowner ends up wondering where the smell came from.

The Tissue Test, And What Failing It Means

Bathroom fans are the most neglected mechanical equipment in a house, and it is easy to see why: the fan makes noise, so it must be working. Noise is not airflow. A fan with a seized bearing, a lint-packed blower wheel or a crushed duct can roar impressively while moving a fraction of the air it did when it was new. The tissue test cuts through that.

If yours fails, resist the urge to buy a new fan. The motor is often fine — the airflow is being strangled somewhere between the grille and the roof. Start at the grille and work outward, because the cheapest problems are also the most common ones.

• The grille itself. Pull the cover down — most pop off with a squeeze of two spring clips — and look at the slots from the back. Years of dust and lint felt over them, and that felt is a filter nobody asked for. Wash it, dry it fully, put it back.
• The blower wheel. With the cover off and the breaker for that circuit turned off, look at the little squirrel-cage wheel inside. If the blades are furred with grey dust, the wheel is out of balance and moving far less air than it should. A vacuum crevice tool and an old toothbrush recover most of it.
• The damper. Just past the housing there is usually a small flap that opens when the fan runs. Stuck shut with lint or paint, the fan is pushing against a closed door. Stuck open, cold outdoor air pours back in all winter.
• The duct. The big one, and it gets its own section below.

Where Does Your Fan Actually Vent? Go Look.

This question catches a startling number of homeowners. A bathroom fan is supposed to carry moist air all the way outside — through the roof, a gable wall, or a dedicated wall cap. What it often does instead is end somewhere convenient: a duct that stops a metre into the attic, one that was never connected to the roof cap, or one aimed at a soffit vent so the moisture gets pulled straight back under the roof.

The consequences are quiet and expensive. Warm, wet shower air released into a cold attic condenses on the underside of the roof sheathing. Over a few Alberta winters that shows up as dark staining on the plywood, damp compressed insulation that no longer insulates, and in bad cases rot in the roof deck. It is one of the most common causes of attic mould that home inspectors find, and the homeowner almost never suspects the bathroom, because the symptom is two floors from the cause. Canadian building codes require exhaust ducts to discharge to the outdoors precisely to prevent this — but plenty of houses were finished in a hurry, and plenty of attic ducts have shaken loose since.

If you can safely get into the attic, follow the duct leaving the fan housing: it should run to a cap penetrating the roof or an exterior wall, firmly attached at both ends. If you cannot, go outside on a cold morning, run the fan, and look for the plume. No plume anywhere is a strong hint the moisture is staying inside.

The Duct Is Usually The Problem, Not The Fan

Even a correctly terminated duct can throttle a good fan. Bathroom exhaust is typically run in flexible duct, and flex duct is unforgiving: every sag, kink and sharp turn adds resistance, and resistance is what turns a 100 CFM fan into a 40 CFM fan. A duct that droops between joists collects condensate in the low spot, and a duct holding standing water is barely passing air at all. Long horizontal runs make it worse; uninsulated duct in a cold attic makes it worse again, because the moist air condenses on the way out instead of leaving as vapour.

What good looks like is boring: the shortest practical route, pulled taut with no dips, as few bends as possible, insulated across unheated space, sloped very slightly toward the outside so condensate drains out, and sealed at every joint. If your duct is a sagging accordion looping twice across the attic, no replacement fan will fix that. Fix the duct. And while you are up there, check whether two bathrooms share a single line — that shortcut lets one fan push moist air backwards into the other room, which is often why an ensuite smells faintly of the main bath.

Is The Fan Even Big Enough?

Sizing is where builders economize. The Home Ventilating Institute’s rule for bathrooms up to 100 square feet is one CFM per square foot of floor area — roughly eight air changes an hour — and ASHRAE Standard 62.2 sets 50 CFM as the floor for an intermittent bathroom fan regardless of how small the room is. A typical 5-by-8-foot main bathroom needs about 40 CFM by area, which that 50 CFM minimum rounds up. Larger ensuites are sized by fixture instead: HVI counts 50 CFM each for the toilet, shower and tub, and 100 CFM for a jetted tub, which is how a big bathroom legitimately lands at 150 CFM or more.

Two caveats. The CFM stamped on the box is measured on a test bench with almost no duct attached, so real installed airflow is always lower — another argument for a short, straight, sealed run. And bigger is not automatically better: a powerful exhaust fan in a tightly sealed house with nowhere for makeup air to come in can pull air backwards down a chimney or flue. If you are jumping from 50 to 150 CFM in a newer airtight home, ask an HVAC contractor about makeup air first.

Run It Far Longer Than You Think

Most people run the fan during the shower and snap it off on the way out, which is roughly the worst possible timing. The bathroom is at its most humid the moment you finish, and the moisture is not only in the air — it is on the walls, the grout and the towels, and it keeps evaporating long after you leave.

HVI’s guidance is to leave the fan running 20 minutes after the bathroom is used. Nobody remembers to go back and do that, which is why the best upgrade in this whole article is not a fan at all: it is a countdown timer switch in place of the light switch. They cost less than a decent dinner, install like an ordinary switch, and turn a good habit into an automatic one. Humidity-sensing fans that run until the room dries out work well too, and earn their premium in a bathroom with no window.

Why does the run-on time matter so much? Because mould does not need a flood, only time and dampness. The US EPA’s guidance is that materials dried within 24 to 48 hours usually will not grow mould — and a bathroom that never fully dries between showers never starts that clock. Health Canada likewise advises keeping indoor relative humidity below 50 per cent in summer, which a room that stays fogged for hours makes impossible for the whole floor, not just the one bathroom.

The Warning Signs, In Order Of Urgency

Bathroom moisture problems announce themselves long before they become repairs. The trouble is that the early signs look cosmetic, so they get wiped away rather than investigated.

• The mirror stays fogged more than a few minutes after you finish. The mildest sign and the most useful one. A well-ventilated bathroom clears within a couple of minutes.
• Peeling paint or a bubbling ceiling above the shower. Moisture is getting into the drywall. Repainting without fixing the ventilation buys a season at most.
• Black or grey speckles on the ceiling and in the corners. Surface mould. A small patch on a hard surface can be cleaned, but it returns until the room dries between uses.
• A musty smell that survives cleaning. Usually growth somewhere you cannot see — behind drywall, inside the duct, or up in the attic where the fan has been venting.
• Damp insulation or dark staining on the attic sheathing. The serious one. This is structural moisture, not housekeeping, and it gets more expensive every season it is left.

What To Do This Week

Run the tissue test in every bathroom, including the powder room and the basement bath nobody uses. Take the covers off, wash them, vacuum the blower wheels. Then get eyes on where each duct terminates — that single check is most likely to turn up something genuinely worth fixing. And whatever you decide about the hardware, put a timer switch on the wall. It is the cheapest thing in this article and it fixes the habit that causes most of the damage.

Bathroom Exhaust Fans: Quick Answers

Links open official industry and government pages in a new tab. Home Pros Group isn’t affiliated with these organizations — we cite them so you can verify the facts yourself.

This article is general home-maintenance information, not a repair manual for your specific equipment. Turn off the breaker before opening any fan housing, and take proper precautions before entering an attic. Electrical work, roof penetrations and mould remediation beyond a small surface patch should be handled by qualified professionals. If you find extensive mould or structural moisture damage, have it assessed before disturbing it.