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Community and Q&A

Ventilation for Small Space

washxhouse | Posted in General Questions on

I’m currently finalizing the contract with my GC for finishing a currently-unfinished in-law space over a 2-car garage (SE PA, border 4A/5A)

It’s about 400 sq ft with vaulted ceilings, and will have a 400 cfm hood for a hot plate and a bathroom vent fan. We’ve already installed a 1-ton Hyper Heat split unit with ceiling cassette.

My original thinking was to spray closed-cell foam on the walls, ceiling, and under the floor (the ceiling of the first floor of the garage) in order to ensure a good air barrier, but my GC is suggesting doing batts in the floor because they’re concerned we’ll end up so tight that we’ll need MUA for the hood and bathroom fan.

They raise a good point, and now I’m also concerned about IAQ if we go the full-spray route. With that said, it feels off to deliberately leave the place leaky rather than be specific about where and how our fresh air makeup is taking place.

I wonder if we should be thinking about, rather than an intermittent-use ventilation fan in the bathroom, having some kind of always-on (whole-house ventilation) fan paired with MUA or even just doing a (very small) ERV somewhere.

According to this article
… we’re looking at << 20 CFM for always-on ventilation, which seems quite tiny and might only be available in a few specialized SKUs I can’t find right now.

Or should we put batts in the floor and air-seal decently well and assume we’ll be leaky enough to not worry about MUA? GBA anathema I know but bears asking.

Any thoughts on this? Products I should be looking at?

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  1. Expert Member


    Deliberately having poor air sealing is a really bad idea - doing it between a garage and a living space is straight up nuts.

    My advice would be to select building assemblies and air-sealing details that don't rely on spray foam. It's not a binary choice - the vast majority of very tight buildings don't rely on spray foam. Include balanced ventilation - with heat recovery or not - and use an operable window if necessary to balance the vent hood or bathroom fan. You can mitigate the need for make-up air by downsizing the range hood, which if properly located doesn't need to be anything near 400cfm.

    1. washxhouse | | #3

      Hey Malcom thanks — point taken about spray foam not being the only way to achieve good air sealing. The building is extant and the only task we're undertaking is finishing so it'd be down to the details rather than assemblies but it's probably doable with my getting all up in the GC's business about it. Spray foam felt more sure-fire in that I'd know that the thing was well-sealed even without getting involved in details if that was the specification.

      Regarding the hood - the literal mother-in-law who will be using this space will be doing some legit wok cooking and from an IAQ standpoint will be producing a lot of combustion byproduct that needs to get out, so I'm reluctant to downsize below that.

      Regarding MUA / HRV / ERV - as little as I want to get into the complexity of the ERV thing, I'm not really sure MUA will get the job done for the all-the-time situation where we need fresh air for CO2 reasons rather than just to make up what the hood is moving out.

      1. Expert Member
        MALCOLM TAYLOR | | #5


        Fair enough. You know your MIL's cooking habits better than I do. With a range hood that size, both practicality and by code you will need a dedicated make-up air source. I'd suggest the inestimable Martin Holladay's blog:

        One big reason I'd suggest avoiding anything but judicious use of spray foam in small problematic air-sealing areas, like penetrations in the sub-floor, is the potential for things to go wrong.

        1. washxhouse | | #6

          I have read some horror stories so I do get your point re: spray foam. Will discuss with my GC.

          Also revisiting the 400CFM requirement in light of your advice as well - maybe we can get under that and obviate the need for the MUA, subsisting only on a tiny ERV.

  2. Patrick_OSullivan | | #2

    > we’re looking at << 20 CFM for always-on ventilation, which seems quite tiny and might only be available in a few specialized SKUs I can’t find right now.

    Panasonic FV-04VE1. Readily available. Reasonably priced. 10-40 CFM.

    1. washxhouse | | #4

      Excellent I did not see this. Thank you!

  3. kurtgranroth | | #7

    The Lunos e2 HRV (not quite an ERV) can handle 20 CFM with minimal effort. It also can act as passive air intake for high powered exhaust fans by simply turning itself off when it detects enough pressure and since it's just a 6" (filtered) hole in the wall at that point, it should be relatively effective at that.

    It is more than twice the cost of the Panasonic FV-04VE1 but it's notably smaller, too, and uses very little energy. It's also an HRV instead of an ERV... but I'm not sure how much of a practical difference that makes for very low CFM

    1. washxhouse | | #8

      I like these units for retrofit but since I have the whole place open I think I'm going to go with something simpler. Thanks for the rec, though.

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