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Community and Q&A

Bathroom fans and a service cavity

Pascalli2 | Posted in Green Building Techniques on


I am planning to have a service cavity beneath the air barrier of the ceiling of my home that I am building. This will consist of taped plywood attached to the underside of the roof trusses to form the air barrier, 2×4 furring to create a service cavity, and drywall attached to the furring.

Now that it is coming up, I am wondering what to do about my two bathroom fans? I am planning Panasonic Whisper fans installed in the ceiling, and it looks like the duct goes out the side from them. The ducts are either 4″ or 6″, but either way, they will be too tall for a 4 inch service cavity (especially since 4″ is not really 4″ in lumber). How do I do this without carving a big trench all the way from the fan to the wall?

Is it possible to vent the fans up instead of sideways?


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  1. GBA Editor
    Martin Holladay | | #1

    Before you design a service cavity, you need to figure out what ducts you intend to put there. If your service cavity is too shallow, I guess you goofed.

    You can build a box around your Panasonic fan -- a kind of frame that will put the fan's grille lower than the plane of the ceiling. You can then build a soffit to hide the exhaust ducts. People build duct soffits all the time.

  2. Expert Member

    If you do as Martin suggests try and think of the bulkhead as an architectural element which may add to the space, rather than merely some way to hide a duct.

    You can choose take the duct straight up and either run it to a gable or through the roof.

    i'd suggest re-thinking the service cavity. What is it there to accommodate? If it isn't ducts, probably not plumbing, then it's almost certainly not worth doing for a few wire penetrations or electrical boxes. Service cavities for both walls and ceilings have a design appeal that is often at odds with the reality of house construction.

  3. Pascalli2 | | #3

    Thanks guys. Yeah - I didn't really think out how the bath fans would fit when planning to do a service cavity. It would take a pretty big cavity to house the units, though, since they are about 11" tall. There are only two up there, so it might be feasible to ether embrace the duct-box, or turn the ducts up at the earliest possible point and extend the plywood air-barrier down and around it for that section.

    I like the service cavity for two reasons - it puts a space between my air barrier and my ceiling, and leaves me the option to do things like install recessed lighting without wrecking my envelope, and it give me a place to stick some more insulation (part of the attic will have a floor on it, limiting how high we can pile insulation there).

  4. Chaubenee | | #4

    Stephen, I planned this, but in the end on my 2,000 SF ceiling upstairs it added about six thousand dollars worth of materials and carpentry to the plans as the reality is that I needed a 12" tall cavity for recessed lights, ducts, etc. so my ceiling height was greater. I needed about 70 sheets of OSB, then I needed the framing and labor to drop, then to install the cellulose to desired depth I needed two foot deep energy heels on the trusses, plus the increased wall sheathing and cement board on the outside of the building. So at the end of the day I am exploring foaming the underside of my roof deck and bringing the entire building into the envelope including the attic. I won't have to vent my soffits and that means I save on that labor and material and relieve myself of bee's nest etc in the attic. So now I am examining the strategy by which to seal it up with open cell foam. Let us know how you make out.

  5. brp_nh | | #5

    This might be too extreme for you, but we dealt with our ceiling air barrier (drywall) by not compromising it except for two holes (plumbing vent and solar conduit). All upstairs lights are on walls.

    For our upstairs bath fan, we built a box and ducted short and straight out the wall with 4" duct. Attached file shows the box frame with fan installed and then when trimmed with wide pine boards.

  6. Pascalli2 | | #6

    I think I would follow your lead on that, Brian, but there's no way my wife is going for it. Now I am thinking about just having the ceiling in the bathrooms, or a strategic area of the bathrooms, be 4" taller than the rest, and putting the whole unit above the air barrier (except for where the grill comes through). It's not ideal, but might be an acceptable compromise.

  7. GBA Editor
    Martin Holladay | | #7

    Nice job. It's also possible to finish this kind of soffit with drywall instead of boards.

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