Bath fan exit with 2×4 walls and bird cut rafters. TIGHT!
As with my own home (1960 2×4 walls with 2×8 rafters with hip roof), I’ve been asked to add bathroom fans for others. The roof sheathing is only 3.5″ above the top wall plate so I usually go from 4″ rigid round duct (don’t want to lose cfm) to 10″ wide rectangle, extension to get it over wall plate then back to round. I use a butterfly style backdraft damper near the fan so it’s under the insulation. I try to seal it before pushing it through the narrow soffit opening. Not easy when you have zero head room. I need to keep it very narrow where it becomes rectangular because I like to seal above the top plate with 1″ XPS and foam and lay the rectangular duct on top. Usually this is squashed against the underside of the roof sheathing. I did find a manufactured rectangular duct with double angles but it has a backdraft damper which ends up on the cold end which means it will most likely fail when temps are below zero. Anyone out there that has a better idea? Is it just better to exit the roof?
Thanks, PK
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Replies
Are you trying to exhaust the fans under the soffit? This can introduce other problems if it’s a vented soffit since the humid exhaust air is likely to get drawn in to the attic through the soffit vents.
Here is an article explaining bathroom fan venting:
https://www.finehomebuilding.com/2018/03/01/bath-fan-venting-cold-climates
Bill
Thanks! Old plywood covered soffit (with retro aluminum placed over wood), next vent is over 10' away so no issue there.
I usually vent up out the roof away from street side using 4" PVC pipe. I can keep the run short (again, we're talking about a 4:1 pitch hip roof) and use blanket insulation around the pipe. I use a large 90 degree elbow and 45 for a downslope exit and I've never seen condensation drip back down inside. I use the best butterfly spring damper close to the fan so it's buried under lots of insulation.
We had a similar situation with leaky bathroom exhaust duct causing ice dams and roof leaks. I threw that system away and put in a through the wall Panasonic unit. Very happy with it.