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I am insulating a shed as a wine cooler. I have house wrap on the outside. I will install unfaced batt insulation between studs. Can I then install foam insulation board under the drywall? I am concerned if the vapor barrier of the Insulation board may cause a double vapor barrier problem?

I am in Northern CA, and cooling the inside. So I believe the vapor barrier on the outside is correct. I just do not understand if the addition on the insulation board with foil will cause an issue.

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That foil is probably a vapor barrier, if the foamboard isn't already

Most foamboard is made from what is called closed cell foam; this provides it with adequate rigidity, but also makes it largely impermeable to moisture, atop being an effective air barrier when taped properly at the seams. Even if your foamboard itself wasn't a vapor barrier, the foil facing on it certainly is!

I would take off the housewrap and put the foam board on the outside instead

Generally, board-type continuous insulations are put on the outside of the studs instead of using a "housewrap" type vapor barrier in order to prevent condensation inside the cavity; this also means they are between the sheathing and the cladding, which helps keep them away from being an interior-space fire hazard.

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  • Will a vapor barrier on both sides of the stud cavity cause (or significantly increase the likelihood of) condensation in the stud cavity? Is the foam board on the inside (with the foil facing inside - at least in my head) a significant fire hazard? While I agree 100% that it's usually done the way you're suggesting, your comments aren't clear enough to me that OPs proposed method is actually detrimental as opposed to just unusual. – FreeMan Jun 26 '19 at 12:12
  • @FreeMan -- it will definitely increase the condensation risk (to the point where the termites might beat the mold to the party, even!) – ThreePhaseEel Jun 26 '19 at 22:07

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