Bathroom exhaust fan guide

What a bathroom exhaust fan does

A bathroom exhaust fan pulls humid air out of the room and sends it outside. That is the whole job. It also carries away some odor, but moisture is what does the damage. A hot shower puts a lot of water into the air, and that water has to go somewhere. If the fan does not move it out, it lands on cool surfaces: the ceiling, the exterior wall, the back of the door, the mirror.

Over months, that shows up as peeling paint, dark spots on the ceiling, grout that stays damp, and a musty smell. In a bad case it reaches the drywall and the framing behind it. A weak fan, a fan nobody uses, or a fan that is not actually connected to the outside are the usual causes.

Sizing a bathroom exhaust fan

Fans are rated by how much air they move, in cubic feet per minute (CFM). A bigger room, a higher ceiling and more showerheads all call for a fan that moves more air. A small powder room needs far less than a large bathroom with a walk-in shower.

Two points are worth knowing before you shop.

• The rating on the box is measured with no duct restriction. Once you add a long run, elbows and a wall cap, real airflow drops.
• A separate toilet room or a shower enclosed with a door often needs its own ventilation, because the air in that space does not circulate with the rest of the bathroom.

The sizing method in the building code is based on room size and fixtures. Your city enforces a current ventilation requirement for bathrooms, so confirm it with the building department when you plan the work. We size the fan to the room and then check it against the duct we can actually build.

The duct matters more than the fan

The best fan on the market performs badly on a poor duct. Here is what a good installation looks like.

1. The duct is rigid or smooth-walled, not a crushed flexible tube with sags in it.
2. The run is as short and as straight as the house allows, with few elbows.
3. The duct is sloped or sealed so that condensation cannot run back into the fan.
4. In an attic, the duct is insulated so warm air does not condense inside it.
5. The duct ends at a cap on the roof or an exterior wall, with a damper that opens when the fan runs.

The most common failure we see is a duct that ends in the attic. The installer saved a few feet of pipe and the homeowner pays for it later. All the moisture goes into the insulation and roof framing. Venting into a soffit is nearly as bad, since the air can drift right back into the attic through the soffit vents.

Another common problem is a duct that has come loose at the fan or the cap. The fan sounds like it is working, but the air goes nowhere. If your fan seems weak, look at the duct before you buy a new fan.

Noise, and why people stop using the fan

A loud fan does not get used. People shower, hear the roar, and turn it off. Then the moisture stays. Fans carry a noise rating called sones, and a lower number is quieter. In a bathroom where you will actually use the fan, pick a quiet one, and make sure the housing is fastened firmly to the framing so it does not rattle.

Noise often comes from airflow fighting a bad duct. A fan on a short, smooth duct runs quieter than the same fan on a long, kinked one. The remodel is the time to get the duct route right.

Controls: switch, timer or humidity sensor

The control is where most fans fall down in daily life. A fan on a plain switch gets turned off too early, or never turned on. Better options include:

• A timer, so the fan keeps running after you leave.
• A humidity-sensing control that starts the fan when the room gets damp and stops it when it dries out.
• A fan wired to the light switch, which works only if you do not mind the fan running whenever the light is on.

Pick the one the household will really use. A timer is simple and hard to get wrong. Fan and light combinations and extra features add wiring, which is a job for our electrical team or a licensed electrician on the project.

Installing or replacing a fan during a remodel

Swapping a fan of the same size in the same spot is a small job. Adding a fan where there was none, enlarging a fan, or moving it over the shower is a bigger one. It means a new ceiling opening, a power feed, a duct route through the attic or wall, and a new cap on the outside of the house. Walls and ceilings need patching after, and the paint has to match.

That is why a bathroom remodel is the natural moment to fix ventilation. The ceiling is often open or being finished anyway. If you are comparing the fan with other upgrades, our bathroom budget guide shows where ventilation sits among the other line items, and the plumbing upgrade guide covers the work behind the walls.

When a fan is not the whole answer

If the ceiling already has stains, soft drywall or mold, a new fan only stops the cause. The damage still needs to be repaired. Our guide to the signs your bathroom needs a remodel explains how to tell a surface problem from a deeper one. In East Bay homes, a duct left bare in the attic or ending without a proper cap is the first thing we check.

Bathroom exhaust fan questions