PSA: Your bathroom fan is probably undersized by 50 CFM or more

I was redoing my master bath and thought a 50 CFM fan would be fine like the old one. Then I actually looked up the HVI rating standard and found out you need at least 1 CFM per square foot of floor area. My bathroom is 80 square feet so I should have an 80 CFM fan minimum. Found this on the Home Ventilating Institute website after I already bought the cheap one at Home Depot. Who else has been running too-small fans and wondering why their mirror fogs up?

3 comments

3 Comments

scott.alex9d ago

I rechecked mine after reading this and realized I'm running a 50 CFM fan in a 90 square foot bathroom, so I'm basically one of those people wondering why my mirror looks like a steamed up car window every morning. My old fan probably hasn't moved real air since the 90s, and I just assumed that's how bathrooms work. You're telling me I could actually see myself shaving without wiping the mirror first? That's crazy talk. Guess I'll be swapping it out this weekend, but I'm already mentally preparing to curse halfway through the project. At least my towels will smell less like a wet dog after.

zaranelson9d ago

Wait, if your fan is that old and weak, have you checked whether the ductwork is actually clear all the way to the outside? I swapped mine last year and found a bird's nest and about four inches of lint blocking the vent cap, so my brand new 110 CFM fan was basically humming into a wall of debris. Did you snake the line or just assume the old duct is fine? Because that first morning after your upgrade might still be a foggy mess if the path is clogged.

nathan_kim9d ago

See, I gotta push back a little here ... I've run a 50 CFM fan in a similar sized bathroom for years and it works fine for me, maybe y'all just need to take shorter showers or crack a window... @zaranelson's point about checking the duct is solid though, that's probably the real issue with your setup.