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Fiberglass-clad sheetrock as vapor retarder?

original_ToolMonger | Posted in GBA Pro Help on

I live in IRC zone 5A Pennsylvania. I’m insulating the walls of my kitchen; it was gutted to redo the wiring and the plumbing. There are two exterior walls a north and south side an uninsulated garage on the east side and the main house on the west side.
The exterior walls are clad with vinyl siding over house wrap. The corners of the kitchen are sheathed with ¾ inch plywood –the rest is clad with ¾ polyisocyanurate (PIR). The framing is 2×6, 16 inches OC.
I plan to install 6 in rock wool batts in the stud bays after caulking junctions where the sheathing and studs meet and filling the wire and pipe penetrations with foam. Finally I was planning to cover the wall with fiberglass clad sheetrock. The sheetrock would be later be painted with latex paint. My question, do I need a vapor barrier or vapor retarder other than the painted sheetrock?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Do I need a vapor barrier or vapor retarder other than the painted sheetrock?"

    A. No. For more information, see Do I Need a Vapor Retarder?

  2. Expert Member
    Dana Dorsett | | #2

    Where the stackup is:

    vinyl siding | housewrap | 3/4" ply | 5.5" rock wool | fiber faced gypsum

    ...the back ventilated characteristic of the vinyl siding lets you get by without interior vapor retarder.

    Where the stackup is:

    vinyl siding | housewrap | 3/4" PIR (foil faced?) | 5.5" rock wool | fiber faced gypsum

    ... you don't have sufficient R in the PIR layer- you would need a minimum of R7.5 to meet code in zone 5A.. If the PIR is foil faced a poly vapor barrier would create a moisture trap, but you can use a "smart" vapor retarder such as Certainteed MemBrain or Intello Plus, which are class-II vapor retarders when the air humidity is low, but class-III when humid- the assembly can dry.

    Having only R4.5 or so of PIR leaves the interior face of the foam cold enough that you would have liquid condensation dripping down the foil facers over the course of the winter, since there would be zero drying to the exterior. If the PIR were half-perm (or higher) fiber-faced PIR it might dry adequately to the air behind the vinyl, but it would also collect some amount of moisture in the PIR itself, which could then be damaged in freeze/thaw cycles.


  3. GBA Editor
    Martin Holladay | | #3

    Good catch! I agree. The thin, vapor-impermeable foam is a problem.

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