I have a bathroom fan that died recently. It is a basic Broan from the 90's, 50CFM and 2.0 sones.
I want a quieter fan for sure, but I'm not sure on the sizing. As shown in the picture, our bathroom (shower, tub, toilet) area (green) is 50 square feet, but our vanities are in an adjacent room (blue) that is 30 square feet. Both have standard 8' ceilings. Our master bedroom is next to that. We almost never close any of the pocket doors between the rooms, unless we are getting ready when the other of us is asleep.
So, do I need to account for the additional area when calculating CFM? Accounting for the short duct to the roof cap (~35' EDL, I think we have 4" duct) that puts me at either 60 or 90 CFM according to this chart:
So, I guess the question is, does it matter that much? 50, 60, or 90 CFM? (or 110??). Our bathroom tends to stay humid for a while currently, but I'm not sure our fan has been running at full capacity either.
Best Answer
You shouldn't have to count the room with the sinks, but you may want to. If you find that the mirrors in that area tend to fog up, during normal use of just the sinks. Then you might want to include the area in the calculation.
400 ft³ at 40' EDL, means you need at least a 60 cfm fan to meet the minimum air changes per hour required by International Residential Code (and others). You'll also want to make sure you run the fan while you shower, and for 20-30 minutes after you're done. A timer unit is the best way to manage this, but you'll have to make sure you get one that's rated for motor loads.
Every time you exhaust conditioned air out of the house, you're losing some efficiency. However, in the case of steamy bathroom air, it's worth the lose to reduce the chance of mold and mildew issues. You want to exhaust the moist air, but you want to exhaust as little conditioned air as possible. So it becomes a balancing act between wasting conditioned air, and preventing mold/mildew.