Bath fan alternatives
Prior to insulating the attic of my 1938 bungalow, I’m replacing a bath fan. (It tested at about 30 CFM). It currently vents into a perforated soffit, about a 2 foot run w/ one elbow, however, the vent is only 3″ diameter, due to the extremely narrow soffit. The bathroom is 320 cubic feet (40 sq. ft. w/ 8′ ceiling). I don’t want to install a wall fan due to stone siding. Here’s a link to two pictures of existing fan (note HVI sticker but no add’l specs anywhere on fan):
The flex duct in the picture is very rigid and quite smooth – maybe some kind of muffler pipe? It’s simply taped to the fan – no reducer as far as I can see. (And yes, I am aware of potential hazards of vermiculite and took precautions).
I plan to replace the fan but initially keep the existing venting, reducing from 4″ at the fan to 3″. If it’s too noisy or doesn’t test at enough CFM, I figure I can always put in a roof cap and hook the fan up to 4″ insulated duct. Questions: 1) Should I try to cut a little circle out of the perforated aluminum soffit where the exhaust duct rests? (This should reduce static pressure, although I’d want to put some kind of insect screening or grill on the exhaust opening. Wasps tend to nest in the oddest places).
Questions: 2) Rather than a Panasonic fan, do you think I’d be OK w/ one of the new Renewaire fans? Renewaire is here in Madison and offers great factory support, so I’d like to use their fan. Their specs on noise and efficiency look good, but they don’t offer the ECM motor that automatically adjusts to static pressure. Here’s a link:
3) Is a fan rated at 50 CFM OK? The charts say probably it is, but I’m not sure what static pressure the existing venting produces. An 80 CFM fan might be too noisy, although perhaps I could put in a Panasonic variable speed ECM model if I want to spend more.
4) I plan on using 2-part foam on the perimeter floor to get the highest R-value possible (ice dams last winter). Is it OK to foam over the existing duct? If I abandon the duct in favor of a roof cap it shouldn’t matter, but is there a scenario where I’d regret having foamed over the duct?
5) Any recommendations for the best way to insulate over the new fan? I’ll be using cellulose to re-insulate the rest of the attic (except the areas I’ll maintain for storage).
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