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Product recommendations/ details for air-sealing subfloor under T&G fir finish flooring

DavidDrake | Posted in Green Building Techniques on

Hello all. Trying to wrap up construction this summer on a two story ADU, 600 SF apartment above heated 250 SF studio and unheated 350 SF single car garage. Climate zone 5, North Idaho. Frost protected slab on grade, R-20 under slab and slab perimeter, R-40 walls, R-60 vented roof.

Joists between ground level and second story are 9.5″ TJI. Plan to insulate joist bays with fiberglass or mineral wool batts. Subfloor is 3/4″ square-edged ply (T&G not available due to supply chain issues), glued and nailed with ring shank nails.

Any suggestion for a product to air seal gaps in the subfloor? I’ll be putting down 3/4″ x 4.5″ T&G fir flooring as the finished floor, blind nailed and sanded in place. I’ve installed a number of wood floors, and usually like to put a layer of kraft paper between subfloor and finish floor. Will this be adequate? Would rosin paper be a better choice? Or some other product?

Aiming for ≤1.0 ach, at least for the second story apt. I’ll be doing an initial blower door test as soon as the second floor ceiling is drywalled.

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  1. Expert Member


    You can tape the seams with one of the air-sealing tapes talked about here:

    However your best bet is probably to not use the use the sub-floor as your primary air-barrier. Either use the drywall below or attach a variable perm membrane. Then you don't have to worry about sealing joints or penetrations.

    Either kraft or rosin paper work well as slip sheets.

    1. user-2310254 | | #4

      Isn't 5/8-inch fire-rated drywall usually a code requirement for garages attached to occupied space?

      1. DavidDrake | | #5

        Sure is in my area, Steve.

  2. DavidDrake | | #2

    Thanks, Malcolm. Now that you point it out, using the drywall below as primary air barrier seems like an obvious choice.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      Especially with vehicles and their attendant pollutants below, it's nice to use an easily sealed continuous surface like the ceiling as the air barrier. If you can keep lights or other outlets off it, even better.

  3. DavidDrake | | #6

    Thats a good point, Malcolm.

    Biggest mistake I've made on the project—so far—was not adding a strip of membrane connecting sheathing (primary air barrier) to top plate during construction, and leaving excess to connect to drywall ceiling air barriers. Obviously too late now. I suppose I'll know better next time.

    Seems like from what I read on other posts and articles here, my best option is to do the best job I can sealing the rim joist to top plate connection, and the ceiling drywall to top plate connection.

    There's four single-gang receptacle boxes in the garage ceiling, and one round box. Boxes are not designed to be air tight. I assume caulking the romex entry points and caulking the joint where box penetrates drywall is a good start.

    I saw you post on some other answers that you mostly use Big Stretch. Still the case?

    1. Expert Member
      MALCOLM TAYLOR | | #7


      Big Stretch for things that need bit of movement, but for boxes pretty much anything will do.

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