The best exhaust fans for a bathroom with no window

A bathroom with an openable window has two ways to get rid of moisture. One without has a fan, and that raises the stakes on which fan.

In a bathroom with a window, the extract fan handles the shower and the window handles everything the fan missed. Forget to switch it on and the room still dries out eventually.

In a windowless bathroom there is no second route. Every gram of water vapor that leaves the room does so through that duct, and anything that does not leave condenses on the coldest surface available.

Three consequences follow:

• Quiet matters more than airflow. A fan you can hear is a fan that gets switched off, and in this room that is the failure.
• Continuous beats burst. A low background rate running all day removes more total moisture than a high rate for twenty minutes.
• The duct route matters. There is no backup if it is blocked, crushed or terminating in a loft.

This is the analysis that makes the continuous-low strategy financially obvious rather than merely sensible.

Across 351 certified multi-speed bathroom and utility fans, 71.8% move air more efficiently below full speed. The median gain is 23.9% and the maximum we found is 81.4%.

So running a multi-speed fan on high is usually the least efficient way to use it — you get more air, and you get proportionally less air per watt.

The Panasonic is the pattern: peak efficacy at the middle speed, which is exactly the setting a windowless bathroom wants to run at all day.

The Broan is the exception that proves checking matters — its efficacy falls with speed. It is still a reasonable buy for a different reason: its 40 CFM low is the gentlest continuous rate here.

The instinctive objection to leaving a fan on is the bill. At these efficacies the objection does not survive the arithmetic.

A certified fan at 18.4 CFM per watt moving 80 CFM draws about 4.3 W. Running continuously that is roughly 1.9¢ a day, or about 57¢ a month at 18.34¢/kWh.

For comparison, a certified dehumidifier treating the same moisture problem runs 283 kWh a year and up, and it condenses the water back into a tank in the same room rather than removing it from the building.

Extraction at source is the cheapest moisture control available, and in a windowless bathroom it is also the only one.

A perfect fan on a bad duct moves very little air, and in this room there is nothing to compensate.

• It must terminate outdoors. A duct ending in a loft or a wall cavity moves the moisture from a room where you can see it to a structure where you cannot.
• Short and straight. Every bend and every foot of flexible duct costs airflow. The certified figures are measured at 0.25 inches of water gauge; a poor route exceeds that.
• Rigid beats flexible where the route allows, and properly stretched beats concertinaed where it does not.
• Insulate a duct running through cold space, or the moist air condenses inside it and runs back.
• Check the exterior flap opens. The same check as a dryer vent, and the same common failure.
• Match the duct size to the fan. Reducing a 6-inch outlet to 4-inch duct throttles it.

• A timer keeps the fan running after the light goes off, which is when most of the moisture is still in the room. The single most useful control.
• A humidistat runs the fan on rising humidity regardless of the light, which suits a windowless room well.
• A continuous trickle with a boost is the strongest arrangement: low all day, high while the shower runs. It is what the multi-speed finding above is built for.
• Not a plain light switch alone. Tying the fan to the light means it stops the moment the room empties, which is the moment it was most needed.