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Community and Q&A

Should I tape vapor barrier to foam board?

TX Tim | Posted in General Questions on

Good day. I’m remodeling a tub surround on an exterior wall and want to air seal properly. Should I tape closed the “pockets” of black plastic, which I believe are the laps of vapor barrier where they come through the foam board and then down on the outside?

Tim in TX

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  1. GBA Editor
    Martin Holladay | | #1

    Tim in Texas,
    It looks like the black polyethylene is being used as Z-flashing at the horizontal joints in the rigid foam. I'm guessing that the rigid foam is being used as your wall's water-resistive barrier (WRB).

    Yes, it won't hurt to tape both sides of the visible bands of black polyethylene with high-quality tape. The reason you want to tape these seams is to create a better air barrier.

    If you end up installing fiberglass batts between the studs, remember that you need another air barrier on the interior side of the fiberglass before you install your tub/shower unit.

  2. TX Tim | | #2

    Thank you, Martin!

    I'll be installing mineral wool batts between the studs. From the top of the tub flange up will be sealed with a liquid waterproofing membrane on CBU. If the space behind this upper area is isolated from the lower area (underneath and directly adjacent to the tub), is anything else recommended for the lower area, besides the wool batts?

    Would aluminum tape be appropriate for sealing the black polyethylene to the rigid foam's foil layer? Or would you suggest another tape?

    Kind Regards,
    Tim in TX

  3. GBA Editor
    Martin Holladay | | #3

    Tim in Texas,
    Yes, you definitely need an air barrier below the tab flange (that is, on the walls that will be covered by the tub/shower unit). The traditional material used in this area by builders who care is Thermoply, although other materials will work.

    A photo of a worker installing Thermoply in this location can be found here: Navigating Energy Star’s Thermal Bypass Checklist.

    I recommend housewrap tape for the polyethylene-to-foil facing joint.

  4. TX Tim | | #4

    Much appreciated, Martin.

    Thanks again for your insights and the great website!

    Tim in TX

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