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Community and Q&A

Vapor Control With LP SmartSide Panels

atmanseer | Posted in General Questions on

I have a large shed which will be converted to a shop studio. It is sided with primed and painted thin (approx 3/8″) LP smartside  panels as siding and sheathing. I can’t get a straight answer as to if the panels are considered impermeable and I will have to plan for drying to the interior or if they will dry exteriorly. There is a permeable house wrap between the panels and the studs. Can I use rigid foam for insulation between the studs using “cut and cobble ” technique? what type of rigid foam is permeable enough to will allow drying to the interior. Or do I need to go with fiberglass batts or cellulose to allow drying in that direction?thanks

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  1. GBA Editor
    Kiley Jacques | | #1

    What is your climate zone?

    "Wood, vinyl, and composite siding aren’t reservoir claddings, making inward vapor drive less important, but a drainage space or material prevents moisture accumulation behind the siding and helps protect the wall, even if not required by code. Note that some sidings may not qualify as a vented cladding without a back-vented air space." --Take a look at the attached resource.

  2. atmanseer | | #2

    Thank you for your response and the article. I'm in Climate Zone 6. Don't want to create a double vapor barrier, so wondering if the smartsiding panels are impermeable enough to act in that way if combined with rigid air-sealed foam used in the stud cavities as insulation. If the panels are more permeable then I have less apprehension about using the foam.

  3. user-1147813359 | | #3

    Atmanseer, what did you end up doing?

    I have the same question, know really nothing about insulation principles. My shed is also built with LP SmartSide and 2x4 framing, but was not wrapped with Tyvek between siding and studs. Looking at the resource Kiley Jacques posted, it almost seems like I should put a layer of Tyvek on from the inside (only option), then a layer of foam insulation, and then another layer of Class II "interior vapor retarder" (am in the Seattle, WA, USA area), and then the sheetrock. Could I just use vapor retarder paint on the living space side of the sheetrock, or do I need something between exterior face of sheetrock and the insulation? Also, since my insulation is 2" thick and the 2x3 stud bay is 3 1/2 inches deep, should I leave the 1 1/2" air gap between the foam and siding or between the drywall and foam?

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