Cooling a bathroom

A bathroom after a shower is one of the least comfortable rooms in a house, and almost none of that is temperature. It is the only room you routinely fill with water vapor on purpose.

Your body sheds heat by evaporating sweat. That process needs air with capacity to absorb water. A bathroom immediately after a shower is close to saturated, so there is very little capacity left and evaporation nearly stops.

The result is that the room feels considerably hotter than a thermometer says, and lowering the air temperature helps far less than you would expect — because the limiting factor was never the temperature.

There is a second reason extraction beats cooling here, and it is about the building rather than you. Moisture that is not removed condenses on the coldest surfaces — windows, tile grout, the back of a cool exterior wall — and sustained condensation is what causes mold, peeling paint and failing plaster.

The widely used residential rule is straightforward, and unlike many cooling rules it has a real basis in air-change rates.

For context on what the market actually offers: across the 898 certified bathroom and utility fans, published airflow runs from 35 to 359 CFM with a median of 92.2. Only 56 are below 50 CFM, so the sizing rule is comfortably servable at every room size.

Two adjustments worth making. A ceiling higher than 8 feet increases the volume for the same floor area, so size up. And a fan at the end of a long or convoluted duct run delivers less than its rating — the certified CFM is measured at a standard static pressure, not at the end of thirty feet of flexible duct with three bends in it.

Bathroom fans are the one fan category with a certified noise figure. Sones are measured to a shared procedure, which makes them comparable across brands — unlike the dBA numbers on portable fans, and unlike ceiling fans, which publish no noise rating at all.

The distribution is less flattering than the marketing suggests. Of the 338 certified records that publish a sones figure:

• The median is 1.4 sones, which is audible and unremarkable.
• The range runs 0.1 to 6.8 sones — a factor of 68 between the quietest and the loudest.
• Only 7 of 338 are at or below 0.3 sones, and exactly one reaches 0.1.

Sones are also linear where decibels are logarithmic, so 1.4 sones is genuinely twice as loud as 0.7. If quiet matters — an ensuite off a bedroom, a fan you want to leave running — that 7-of-338 group is the whole shortlist, and it overlaps heavily with the efficiency leaders.

The commonest mistake is switching the fan off as you leave. The air is at its wettest at that moment, and the surfaces are still streaming.

1. Switch it on before the shower, so it is already moving air when the steam starts.
2. Leave it running for about 20 minutes afterwards. This is the part that does the work. A timer switch or a humidity sensor makes it automatic and removes the decision.
3. Close the door while it runs. An open door lets the fan pull dry air from the landing instead of pulling wet air from the bathroom, which sounds counterintuitive and is not — the fan extracts whatever is easiest to reach.
4. Leave the shower door or curtain open afterwards, so the enclosure itself dries rather than holding water.

Note that 456 of 1,076 certified ventilating fans have two or more speeds. If yours does, the low setting is the one to leave running for the twenty-minute tail — full speed during the shower, low afterwards.

An internal bathroom has no passive route out at all, which makes mechanical extraction the only mechanism rather than a helpful addition.

It also means make-up air has to come from the rest of the house, so the door undercut matters — a door sealed tight to the floor will starve the fan and it will move far less than its rating. Roughly three-quarters of an inch of gap under the door is the usual allowance.

Windowless bathrooms are the strongest case for a humidity-sensing fan that runs on its own judgement, because there is no backup if someone forgets — extraction for a bathroom with no window.

Some bathrooms really are hot rooms — a west-facing one with a large window, or an upstairs one under an uninsulated roof. Once extraction is sorted, the remaining options are the ordinary ones, in this order:

• Shade the window from outside if there is one. Solar gain into a small room is disproportionate — shading and film.
• Deal with the roof above it if it is a top-floor bathroom. That is usually the same job as fixing the rest of the floor — why upstairs stays hot.
• Add a small wall-mounted fan, well away from water, for the ten minutes you are in there.
• Use the extraction fan as air movement even when the room is dry. It is already installed and it already moves air.

What is rarely worth it is dedicated cooling equipment. A bathroom is occupied briefly, has no space, and has water everywhere — three reasons that an air conditioner belongs in the room next door.

Comfort is the reason people search for this. Building damage is the reason it matters.

Sustained high humidity on cold surfaces produces condensation, and condensation produces mold, which is a health issue as well as a decorating one. A bathroom that is consistently extracted after use does not develop that problem; one that is not, does, regardless of how often it is cleaned.

If the damp has spread beyond the bathroom, or if the whole floor feels humid, the extraction fan is not the whole answer and a dehumidifier is doing a different job — dehumidifiers compared and what to set one to.