Ventilation is the cheapest part of a bathroom and the one most often got wrong, usually because the fan was chosen on its rated airflow and nothing else. A powerful fan on undersized flexible duct with a long run and a poorly chosen termination moves considerably less air than its box claims, and the room behaves as though it has no fan at all.
Two numbers matter and they are frequently conflated. There is a code minimum, which is what an inspector checks. And there is a sizing rule of thumb, which is what actually makes a room work. They are not the same thing, and meeting the first does not guarantee the second.
The code minimum and the sizing rule are different things
California follows ASHRAE 62.2, incorporated through Title 24, which sets local exhaust for a bathroom at **50 CFM intermittent** — operated by a demand-controlled switch — or **20 CFM operating continuously**. That is what a jurisdiction checks, and either path complies.
Separately, the long-standing sizing guidance used in the trade is roughly **1 CFM per square foot of floor area**, which for a typical bathroom of fifty to eighty square feet lands somewhere above the 50 CFM code floor. For rooms with a separate enclosed toilet, a large shower or a jetted tub, the practical figure goes higher still.
It is worth being clear about which is which, because "it meets code" and "it clears the room" are different claims. Code compliance is the floor. Whether the mirror clears in a reasonable time is the test that actually matters to you, and it depends at least as much on the ductwork as on the fan.
The number on the box is a laboratory figure
A fan rated at 110 CFM is rated in near-ideal conditions. Installed on undersized flexible duct, with several bends and a long horizontal run to a restrictive termination, real delivered airflow can be a fraction of that. This is why we would rather specify a modest fan on good ductwork than a powerful one on poor ductwork — the second combination is both more expensive and worse.
What actually determines airflow
In roughly descending order of how often each one is the problem.
- Duct diameter
- Undersizing the duct relative to what the fan expects is the single most common cause of poor real-world performance. A fan connected to a smaller duct than it was designed for is throttled from the moment it is installed.
- Rigid versus flexible duct
- Flexible duct has considerably more internal resistance than smooth rigid duct, and it gets worse if it is compressed, sagging or run with slack. Where flexible duct is used it should be pulled taut and supported. Rigid is better wherever the route allows it.
- Run length and number of bends
- Every elbow costs airflow, and a long horizontal run costs more. The shortest sensible route to the outside beats a tidy-looking route that takes twice the distance.
- Where it terminates
- Outside the building, through a wall or the roof, with a proper hood and a working damper. A termination with a restrictive screen or a stuck damper undoes good ductwork.
- Whether it terminates outside at all
- Venting into an attic or a soffit is a real and common fault in older houses. It moves the moisture from the bathroom into the roof space, where it condenses on cold surfaces and causes a considerably more expensive problem.
- Make-up air
- Air removed has to be replaced. A tightly sealed bathroom with a heavy door and no undercut starves the fan. A gap under the door is usually sufficient and is easy to overlook.
When the fan runs matters as much as its size
Most of the moisture leaves the room after the shower stops, not during it. A fan switched off as you step out has done a fraction of its job — the room is still saturated, and the water condenses on cool surfaces as the air settles.
The habit that works is running the fan during the shower and for a further fifteen to twenty minutes afterwards. Because nobody reliably remembers to come back and switch it off, the practical fix is a timer switch: a countdown dial or a simple electronic timer that runs on and stops by itself. It is an inexpensive addition during a remodel and one of the highest-value ones for the long-term condition of the room.
Humidity-sensing switches take this a step further by running until the room reaches a set level. They work well; the only caution is that in a very humid house the set point needs to be reachable, or the fan runs indefinitely.
What good ventilation prevents
Mould on ceilings and in corners is the visible consequence, and it is the one people try to solve with cleaning. If mould returns quickly after being cleaned, the cleaning was never the problem — the room is not drying, and it will keep happening until the air changes.
Less visible, and more expensive: sustained humidity degrades paint and finishes, swells the bottom edges of cabinetry, and works on the sealant joints in the wet area. In a bathroom with any existing waterproofing weakness, persistent moisture accelerates it.
A steam shower changes the calculation entirely. Steam produces far more moisture than a normal shower and needs a sealed enclosure, a sloped ceiling to shed condensate, and vapour management the rest of the bathroom never requires. Specifying steam without addressing that is how a spa bathroom becomes a maintenance problem.
What to ask, and what a good answer sounds like
| Question | Good answer | Concerning answer |
|---|---|---|
| What CFM is the fan, and how was that chosen? | A figure, plus the reasoning — floor area, room shape, whether the toilet is enclosed. | "It meets code." That is the floor, not the design. |
| What duct size and material? | A specific diameter matched to the fan, rigid where the route allows. | Vagueness, or flexible duct with no mention of diameter. |
| Where does it terminate? | Through a wall or roof to the outside, with a hood and damper. | Anything involving the word "attic". |
| Is there a timer or humidity switch? | Yes, or an offer to add one — it is inexpensive during a remodel. | Not considered. |
| How does air get into the room? | A door undercut or another intentional path. | No answer. Sealed rooms starve their fans. |
None of this is exotic. It is a short list of unglamorous details that decide whether the fan you paid for does anything, and it is worth two minutes of conversation.
Questions we are asked about this
What size fan does California actually require?
California follows ASHRAE 62.2 through Title 24, which sets bathroom local exhaust at 50 CFM intermittent on a demand-controlled switch, or 20 CFM running continuously. Either path complies. Treat that as the floor rather than the design target — the common sizing rule of thumb of roughly 1 CFM per square foot of floor area usually lands higher, and for a large shower or an enclosed toilet higher again.
My fan is powerful but the mirror still takes forever to clear. Why?
Almost always the ductwork rather than the fan. Rated airflow is measured in near-ideal conditions; an undersized flexible duct, several bends, a long run and a restrictive termination can leave real delivered airflow at a fraction of the number on the box. A modest fan on good ductwork routinely outperforms a powerful one on poor ductwork.
Can the fan vent into the attic?
No, and it is a genuinely damaging shortcut rather than a technicality. Venting into an attic or a soffit moves warm moist air into a roof space where it condenses on cold surfaces. You have relocated the problem from a room you can see into one you cannot, and the eventual repair is far more expensive than the correct termination would have been. It is a common fault in older houses and worth checking.
How long should the fan run after a shower?
Fifteen to twenty minutes. Most of the drying happens after the shower stops, when the room is still saturated. Since nobody reliably remembers to come back and switch it off, a timer switch is the practical answer — inexpensive to add during a remodel and one of the better long-term investments in the condition of the room.
We keep getting mould even though we clean regularly. What is wrong?
Almost certainly ventilation rather than cleaning. If mould returns quickly after being removed, the room is not drying between uses. Check three things in order: that the fan actually moves air — hold a tissue up to it — that it runs long enough after the shower, and that it terminates outside rather than into a roof space. Cleaning treats the symptom; airflow addresses the cause.
Does a steam shower need different ventilation?
Yes, and substantially. Steam produces far more moisture than an ordinary shower and requires a sealed enclosure, a ceiling sloped to shed condensate back into the shower rather than dripping, and vapour management the rest of the bathroom never needs. It is a different engineering problem rather than a bigger version of the same one, and it should be designed in from the start rather than added to a standard bathroom.
Where to check this yourself
- California Building Standards Commission(opens in a new tab)
Title 24, which incorporates ASHRAE 62.2 and sets the 50 CFM intermittent / 20 CFM continuous requirement for bathroom local exhaust.
- ASHRAE(opens in a new tab)
Publishes Standard 62.2, the residential ventilation standard California adopts.
- Home Ventilating Institute(opens in a new tab)
Independent certification of fan airflow and sound ratings, and the source of the sizing guidance used in the trade.
- Placer County Building Permits(opens in a new tab)
Bathroom ventilation is checked at inspection, including that the duct terminates outside the building.