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Vapor retarder/barrier in an insulated above-garage floor

lance_p | Posted in Energy Efficiency and Durability on

CZ6A, Ottawa ON

Our build will have a bedroom that extends out above the un-insulated garage. The floor will be 14″ deep I joists and will be dense packed with cellulose (R50-ish). I’m planning to use Intello on the interior walls as a vapor retarder.

Question: What should I do for a vapor retarder/barrier above the garage?

Do I use a vapor retarder on top (between the floor sheething and joists), underneath (between the joists and the garage ceiling drywall), or is a totally different approach necessary? Should I be using a vapor barrier instead, like 6 mil poly?

I’m detailing the framing in this area and want to have a clearly defined path laid out for my vapor and air control layers.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Lance,

    The code treats the walls and floors between attached garages and living spaces as though they were exterior ones. So you could look at the floor as being similar to one that cantilevers over a foundation. Those floors don't typically have a vapour barrier on the interior, just the subfloor.

    Your main concern should be air-sealing, just as it would be for the walls. Not, as is usually the case, to prevent moisture problems, but to keep exhaust fumes for getting into the house.

  2. GBA Editor
    Martin Holladay | | #2

    Lance,
    In a floor assembly, the subfloor (usually OSB or plywood) is the vapor retarder. Both materials are "smart" vapor retarders.

    When dry, plywood has a vapor permeance in the range of 0.5 perm to 2 perms. Vapor permeance rises to 20 perms if the plywood gets wet.

    When dry, OSB has a vapor permeance of about 0.7 perm. Vapor permeance rises to 12 perms if the OSB get wet.

  3. lance_p | | #3

    Malcolm and Martin, thanks for the information and apologies for the late reply.

    So if I understand correctly, I should be bringing my Intello up the first floor walls past the joists and sealing it to the subfloor sheathing that covers the floor area that goes over the garage, correct? Then sealing my 2nd floor wall Intello to that sheathing at the base of the walls. This seems simple enough.

    Treating my garage ceiling in this area like my exterior walls (per Malcolm's advice), my exterior air barrier, likely Zip sheething, should extend through this garage ceiling area as well and be taped off to the exterior walls? If this is the case, these details will be easier to execute than I thought!

    Malcolm, absolutely, I plan to seal the heck out of the walls that separate the house and garage, as well as find doors that seal well but don't break the bank. Part of my plan is to use an ERV in the garage, and if I'm able, make sure the garage is always at a slightly lower pressure than the house. We'll see if that's practical once I start the mechanical work, but I'd like to plan it that way from the start.

    QUESTION: If the subfloor over the garage is intended to be my interior vapor retarder, should I be sealing the T&G joints and between the sheets with tape or sealant? I would think so in order to keep moist air exfiltration through the floor to a minimum. Is there some generic advice applicable here, or perhaps an article or web page that talks to this approach? I want to set out planing for maximum airtightness, both in my exterior air barrier as well as my interior vapor retarder layer.

  4. Expert Member
    MALCOLM TAYLOR | | #4

    Lance,
    You only need to seal air-barriers. You only need to seal vapour-barriers, or vapour retarders, when they are performing a dual function as air-barriers (As is typically the case with interior poly in most Canadian construction). As long as you air-seal the Zip you are fine.

  5. lance_p | | #5

    Thanks for your advice, Malcolm!

  6. GBA Editor
    Martin Holladay | | #6

    Lance,
    Malcolm is correct. Vapor retarders work just fine, even if seams aren't sealed, at their intended purpose (limiting vapor diffusion).

    You're overthinking the question of vapor diffusion through your floor assembly. Vapor diffusion problems in floor assemblies are basically unheard of.

    As with any building assembly, airtightness matters.

  7. lance_p | | #7

    Thanks Martin. I have been known to overthink things now and then!

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