Sizing bathroom exhaust vent

I'm remodeling our large (>200 sq. ft.) master bathroom. The bathroom is currently not vented, but does have three windows (which are almost never open). It seems like venting is a good idea, but I'm trying to keep that part of the project simple. The rule of thumb seems to be 1 cfm per square foot of bathroom, but the largest single vent I can find is 150 cfm. Can anyone suggest an approach to venting a bath of this size? I could go with a remove in line fan and two ceiling registers or simply pick a reasonable size vent (>100 cfm) and I'll be better off than I was before. Is this rule of thumb for sizing valid or very conservative for a dry climate (San Jose, CA).

Rick T

The CFM of an exhaust is based on how many air changes per hour you want. If your ceilings are 8', you have 1600 cuft, so your rule of thumb 1CFM per sq. foot would get about 7 air changes per hour. I don't know if any codes address this, but I believe numbers between 5 and 8 changes per hour are tossed around, so you are in the ball park.

A single fan of 200 CFM, or even 150, will be noisy. Panasonic makes probalby the quietest; I think they have a 0.8 sone 160CFm unit. It is pricey; around $260 last I looked.

I would consider a remote inline duct fan, or even 2.


In most areas, having windows in the bathroom meets the code ventilation requirement. But your experience is typical: the windows are never opened!

Thanks for the tip on panasonic. They have a 240 cfm remote unit that might fit the bill. Panasonic also provides the duct calculation (pressure drop) method on their website. List is about $250 so the total of fan, ducting and ceiling fixtures won't be cheap, but I'm determined to do this thing right.

Rick T

i've got a good size bathroom with vaulted ceilings. used a panasonic wall mount above the shower. it has a 6" exhaust duct to move more air plus is very quiet. being above the shower the room never gets condensation.

Good job on running with more research. I like your plan.

I personally believe in overkill on bathroom venting. Mold is just hovering there waiting for a little spot with just enougn moisture and not enough fresh air!