Bathroom ventilation: why a new shower wants a working fan
A new walk-in shower puts the same water into the air that the old tub did, and often more, because people use a shower they like. What removes that moisture is the exhaust fan, and in a great many bathrooms the fan is decades old, undersized, vents into the attic, or does not exist. This page is what a fan is for, how big it should be, where its air has to go, and how one is added during a conversion.
Last updated September 2026 · No prices on this page; the Estimate Builder carries those · Part of the knowledge base
What the moisture does
Paint that peels at the ceiling. A ceiling that sags. A door and trim that swell and stick. Mildew along the caulk and the grout no matter how often they are cleaned. Condensation running down the window in winter. None of that is caused by the shower; it is caused by the air the shower leaves behind, staying in the room. A fan that moves that air outdoors is what protects a new bathroom, and it is the cheapest part of the room to get right.
How big a fan
Fans are rated in cubic feet per minute, CFM, on the box. The usual rule of thumb is one CFM for each square foot of floor and never less than fifty. A five-by-eight bathroom, which is forty square feet, wants at least a fifty CFM fan. A larger bathroom is sized by adding fifty for each toilet, shower or tub and a hundred for a jetted tub. The new exhaust fan offered as a priced option in our Estimate Builder is a quiet fan rated at a hundred and ten CFM, which covers most bathrooms with room to spare, provided the duct lets it breathe.
Where the air has to go
Outdoors, through a wall cap or a roof cap with a damper, and never into the attic. A fan blowing into an attic puts the shower's moisture into the insulation and the underside of the roof, where it does the same damage it was meant to prevent, one floor up and out of sight. The duct wants to be as short, straight and smooth as the house allows, in the size the fan asks for. A long, crushed, flexible duct can halve what a fan actually moves, which is how a fan with a good rating ends up with a wet ceiling under it.
How long to run it
Through the shower and for twenty to thirty minutes after it, which is how long it takes to move the moisture out of the room and off the surfaces. A timer switch does that without anybody remembering. A humidity-sensing switch turns the fan on when the air gets damp and off when it dries, which is the better answer in a bathroom used by several people, and it is the one we would put in if asked. Leaving the fan on all day removes heated or cooled air from the house for no benefit.
Why a window is not a substitute
A window vents a bathroom in June and does nothing in January, when nobody opens it and the moisture stays in the room. Some building codes accept an openable window in place of a fan and others require the fan regardless; either way it is the fan that dries a bathroom in winter, which is when the damage is done. If your bathroom has a window and no fan, treat the fan as the thing to add during a remodel rather than the thing the window excuses you from.
Sound
Fan noise is rated in sones on the box. A modern quiet fan is around one sone or less; many older fans run at three or four, which is loud enough that people switch them off and leave the moisture in the room. A fan that rattles usually has a loose housing or a duct pushing against it. A fan that roars but moves little air usually has a long or crushed duct behind it. Replacing a loud fan with a quiet one of the right size is the single change that most reliably gets a fan used.
Adding a fan during a conversion
A new exhaust fan is a priced option in the Estimate Builder, and it is priced in the written proposal rather than found on the day. It needs a duct to the outdoors and a circuit, so where the fan goes and where the duct can run depends on the ceiling and the attic above it. In some jurisdictions changing the ventilation brings a permit and an inspection into a job that would not otherwise need one, which the permit page explains without pretending to answer for your address. If your bathroom has no fan, or a fan that has not moved air in years, say so when you send photos, and include the ceiling in them. Which Ohio rule lets a fan stand in for a window, and the circuit and ground-fault rules the wiring answers to, are on the bathroom electrical page.
A wet room needs more of everything here. When the whole bathroom is the shower, the vanity, the door and the trim all live in a room that gets damp, and extraction stops being an afterthought. The wet room page covers what that changes.
Common questions
Do I need an exhaust fan if my bathroom has a window?
In practice, yes. A window vents a bathroom in June and does nothing in January, when nobody opens it and the moisture stays in the room, and some building codes accept an openable window in place of a fan while others require the fan regardless. A fan ducted to the outdoors dries the bathroom in every month, which is what protects the paint, the ceiling, the trim and the caulk. If your bathroom has a window and no fan, treat the fan as the thing to add during a remodel rather than the thing the window excuses you from.
How big a fan does a bathroom need?
The usual rule of thumb is one cubic foot per minute of airflow for each square foot of floor, and never less than fifty. A five by eight bathroom, which is forty square feet, wants at least a fifty CFM fan; a larger bathroom is sized by adding fifty for each toilet, shower or tub, and a hundred for a jetted tub. Fans are rated in CFM on the box. The new exhaust fan offered as a priced option in our Estimate Builder is rated at a hundred and ten, which covers most bathrooms with room to spare, provided the duct lets it breathe.
How long should the bathroom fan run after a shower?
Through the shower and for twenty to thirty minutes after it, which is how long it takes to move the moisture out of the room and off the surfaces. A timer switch does that without anybody remembering, and a humidity-sensing switch turns the fan on when the air gets damp and off when it dries, which is the better answer in a bathroom used by several people. Leaving the fan on all day removes heated or cooled air from the house for no benefit.
Where should a bathroom fan vent to?
Outdoors, through a wall cap or a roof cap with a damper, and never into the attic. A fan blowing into an attic puts the shower's moisture into the insulation and the underside of the roof, where it does the same damage it was meant to prevent, one floor up and out of sight. The duct wants to be as short, straight and smooth as the house allows, in the size the fan asks for, because a long crushed flexible duct can halve what a fan actually moves.
Can a new exhaust fan be added during a tub-to-shower conversion?
Yes. A new exhaust fan is a priced option in the Estimate Builder, and it is priced in the written proposal rather than found on the day. It needs a duct to the outdoors and a circuit, so where the fan goes and where the duct can run depends on the ceiling and the attic above it, and in some jurisdictions changing the ventilation brings a permit and an inspection into a job that would not otherwise need one. If your bathroom has no fan or a dead one, say so when you send photos and include the ceiling in them.
Why is my bathroom fan so loud?
Because it is old, or undersized and straining, or ducted badly. Fan noise is rated in sones on the box: a modern quiet fan is around one sone or less, and many older fans run at three or four, which is loud enough that people switch them off and leave the moisture in the room. A fan that rattles usually has a loose housing or a duct pushing against it, and a fan that roars but moves little air usually has a long or crushed duct. Replacing a loud fan with a quiet one of the right size is the single change that most reliably gets a fan used.
See your actual price in about two minutes
Pick your shower, choose real products with real model numbers, and watch the number move. No phone number required, and nobody will call unless you ask.