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Vapor Retarder for Wall with Hybrid Insulation

narnal | Posted in General Questions on

Hi there. I have the interior walls open in the front entry of my 1940 FL home and would like to add insulation and air sealing. The wall assembly is as follows (from exterior to interior):

hardie panel
tyvek house wrap
diagonal board sheathing
air sealing (?)
mineral wool insulation
drywall / plaster

I was thinking I could use 1/2″ XPS foam + the cut and cobble method to air seal and then fill the rest of the cavity with mineral wool (for sound absorption, it faces a busy road). My only concern is the permeability of the XPS foam (0.2 perms). Is it OK to have a class 2 vapor retarder in this wall assembly? Or can I air seal with something more vapor open?

Thanks
Nick

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Replies

  1. narnal | | #1

    Refreshing the post

  2. Expert Member
    Akos | | #2

    In your mild climate, your cut and cobble assembly will work great, no issues with the permeability of the foam. The thing you want to avoid is a low perm surface on the interior such as vinyl wallpaper.

    I think the best bet in your case is dense packing the walls. This will seal up the walls and also provide the best sound insulation. This will also keep the whole assembly permeable.

    For extra sound, I would recommend going with 5/8" drywall.

    There are also other ways to cut down on sound but these are only worth if you are upgrading the windows. Without the window upgrade, a high STC wall assembly won't do much.

  3. narnal | | #3

    Thanks for the response Akos. I didn't think of dense pack and wasn't aware that it would form an air barrier - seems like a great solution.

    So is it fair to say that although low perm is not an issue, fully permeable is best? Could you elaborate a little more on that?

  4. Expert Member
    PETER Engle | | #4

    The best walls dry to both the outside and inside, since vapor drive reverses in winter/summer. In FL, the majority of the vapor drive is inward and there is risk of condensation in walls that have a combination of low permeability and chilled interior surfaces. The classic case is vinyl wallpaper, which has caused untold millions worth of mold damage in the south. In your proposal, the low-permeability XPS is also insulating and the exterior side of the XPS will probably always stay above the dewpoint of the exterior air, so no condensation. Also, unless the XPS is faced, the permeability is probably higher than you think. 1" of XPS generally has a permeability of slightly greater than 1, so 1/2" will probably be around 1.5-2. That is considered vapor semi-permeable, and perfectly safe for your climate (without vinyl wallpaper or other interior VB of course).

    1. narnal | | #5

      Thanks Peter. That makes sense.

      Turns out the permeance is indeed 1.5. I was accidentally looking at the datasheet for the foil faced version.

      For sake of DIY install, I think I will move forward with the 1/2" XPS + Roxul Safe and Sound

      Thanks for all the help

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