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Bath fan alternatives

JonathanBeers | Posted in Mechanicals on

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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  1. davidmeiland | | #1

    Perforated soffit, meaning the attic vented and the soffit is the means of air entry? I'd be a little cautious about terminating a vent there but if the attic is large and well vented it might be OK. At the very least you should cut a round hole for the pipe, rather than asking the fan to push air thru the perforations.

    If you left me alone to do the job, I'd replace your existing fan with an 80CFM Panasonic, install a timer switch, throw out the existing pipe and replace it with new rigid and elbows, and insulate the pipe. Depending on the particulars I would try to exit thru a gable end wall but might go thru the roof or the soffit. Particulars include type of siding, type of roofing, distance to gable, etc.

    That perlite looks like no fun.

  2. GBA Editor
    Martin Holladay | | #2

    How are you doing? It's been a while since we last corresponded. Still working for the local utility?

    I don't generally recommend soffit terminations. But in some houses, you have few choices. If you're interested, Duraflo makes a soffit termination fitting for bath exhaust fans:

    I have no experience with the Renewaire fan, but the Panasonic fans are excellent, and very quiet.

    I think that 50 cfm is plenty.

    Go ahead and foam over the ductwork.

    Don't foam over your exhaust fan. Protect the fan with a thin fiberglass batt, and foam over that. Or else protect the fan with a cheap white polystyrene beer cooler, foamed to the drywall.

  3. Riversong | | #3

    I would use an 80cfm fan. I'm also unfamiliar with Renewaire, but their 80cfm has the same very low noise rating as the 50. What will make a quiet fan noisier, however, is to restrict airflow with too small a duct and forcing it to exit through a 75% net free area restriction. I would never terminate an exhaust duct at a venting soffit, as some of that moisture will simply return into the attic (there isn't now much evidence of moisture accumulation, but with such an undersized fan there may not be).

    Install an adequate, efficient, quiet fan and use the proper rigid 4" metal or plastic duct, properly sealed at joints with butyl-foil tape and properly insulated, and terminate through gable or roof. A gable termination is preferable to allow drainage of condensate if duct is sloped down and out.

  4. JonathanBeers | | #4

    Unfortunately, there's not enough room for a proper soffit vent termination (and it's 33' to the only gable, plus that's the front of the house, so appearance matters.) I guess I'll just bite the bullet and put a roof cap in the fairly new architectural asphalt shingles. I was hoping to stick w/ current venting and only change if it didn't work. Thanks for the replies.

    Here's a picture of the soffit in question:

  5. Riversong | | #5

    Does "bungalow" mean someone bungled the roof design?

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