Replace bath exhaust fan details
20 year old builder poorly installed bath fan gave out so must be replaced. The fan is to the 2nd level master bath and the attic above is accessible unconditioned vented attic. I’ve read the GBA article on bath fans. https://www.greenbuildingadvisor.com/blogs/dept/musings/bathroom-exhaust-fans
I’ve sized the fan properly. 400 cf room + 12′ of duct with no elbows (I think; see below) + 30′ to account for wall outlet = 60 cfm fan, per https://www.efi.org/factoids/bathroom_sizing.html. So I bought an 80 cfm fan. (Existing appears to be 40 cfm).
Fan I bought has a humidity sensor and can operate manually on full speed mode. It is energy star rated, 0.8 sones, and has 3 year warranty. It has 11 watts input power and 0.1 static pressure, per http://www.hvi.org/proddirectory/CPD_Reports/section_1/index.cfm.
I would appreciate some guidance regarding the below:
1. Duct: The existing 3″ flex duct is in there like an octopus. I will replace it with rigid duct. Two questions
a. The GBA article and the instructions with the new fan indicate the duct is to be installed so it is sloped to the exterior. My project cannot include relocating or installing a new exterior vent, and the vent is approx 18″ higher than the fan outlet. I think the best thing to do is install new rigid duct with a slope in a straight line from the new fan outlet to the exterior outlet. Is this correct?
b. New fan has a 4″ outlet. Existing exterior wall cap outlet is 3″. At which point should I transition from 4″ to 3″? at the fan, halfway along the 12′ of new rigid duct, or at the exterior wall outlet?
2. Fan install. GBA article indicates I should install the new fan so it’s lower lip rests on the top (attic) side of the gyp (image 3 of 5). Instructions that come with the fan say the bottom lip should be even with the bottom edge of gyp. Does it matter in a retrofit install?
3. Sealing fan opening in gyp. GBA article indicates sealing between fan and gyp with caulk. Is it ok to seal between them with spray foam? Either way, what specific product is preferred? The yellow expanding foam or the white window/door foam, or other?
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