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Community and Q&A

Vapor Condensation With Cut-and-Cobble Insulation

Andrewsteiner2014 | Posted in GBA Pro Help on

Cut and cobble 1 inch polypro eps between 2×4 studs with gap between poly and osb and gap between poly and drywall with no other insulation do you think I’ll have vapor drive or condensation issues .I’m in east Tennessee. Any input is appreciated.

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  1. Expert Member
    Zephyr7 | | #1

    Yes, you would seem to have two vapor barriers -- poly against the OSB and another layer of poly under the drywall. Why all the poly?

    You'd be much better off with no poly (unless you have a local code requirement, in which case you only want poly on ONE side of the wall, which is the "warm in winter side", so the interior side in most of the US), regular batts, and a smart vapor retarder on the interior.

    You do NOT want a double vapor barrier with TWO layers of poly as you describe, regardless of the type of insulation you end up using.


  2. Expert Member
    Dana Dorsett | | #2

    (Bill- I think by "poly" he is referring to the PolyPro sheet EPS, but I cood b rong, offen am.)

    If the EPS has a plastic or aluminum facer(s) a sheet of interior side polyethylene becomes a problem, otherwise not. (Are all PolyPro products aluminum clad?)

    What's on the outside of the OSB? If brick, stone, or stucco foil facers on the EPS are a problem unless the either the gap between the siding & OSB or the gap between the EPS is vented to the exterior, both top & bottom. If it's vinyl siding or rainscreened wood/cement/other siding it's not a problem.

  3. Andrewsteiner2014 | | #3

    Its 1 sheet of poly foam board sorry for confusion and its a t 111 type osb siding that is painted

  4. Jon R | | #4

    The number one cause of condensation problems is lack of air sealing. What you going doing to verify it is well sealed?

  5. Andrewsteiner2014 | | #5

    I sealed it very heavy with spray foam in the can

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