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Community and Q&A

Using Two Types of Recycled Polyiso Foam Panels

mikeysp | Posted in General Questions on

Hi. I am in zone 4a building my shop.

I am using 3.5″ (2″ + 1.5″) of recycled polyiso foam between sheathing and siding.

Pictures of foam attached.

I have two types of polyiso and am unsure which should inboard (closest to housewrap/OSB) and which should be outboard (closest to rainscreen/siding)?

The 2″ appears to have some sort of white composite fiberglass skin.
The 1.5″ has a black felt skin.

I will not have any insulation in walls.

Thanks for any clarity.


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  1. Expert Member
    BILL WICHERS | | #1

    The black one looks like fiberglass mat facing, the white one is hard to tell. kraft/felt/fiberglass mat are all vapor permeable materials, so there isn’t a lot of difference between them in terms of vapor permanence. With 3.5 total inches of foam, the entire assembly is going to be pretty resistant to vapor migration anyway.

    I would probably put the thicker polyiso on the exterior side to help to keep the entire thing flatter. Thinner pieces are more likely to bow out, so putting the thicker stuff on the exterior helps to keep the thinner ones pressed flat resulting in a flatter finished assembly.


  2. GBA Editor
    Kiley Jacques | | #2

    I realize you aren't entirely sure of the types of rigid foam you have but since Bill brought up vapor permeance, I thought I'd share some information on it:

    One inch of unfaced EPS is relatively permeable; it has a permeance of 2.0 to 5.8 perms. If you are looking for vapor-permeable EPS, though, be careful: if the EPS is faced with polyethylene or foil, it will have a very low vapor permeance.

    One inch of XPS has a permeance of 1.1, while a 2-inch-thick sample of XPS has a permeance of 0.55. XPS is classified as a semi-impermeable material.

    One inch of foil-faced polyiso has a very low permeance of 0.03 perm. Polyiso with a glass-mat facing or a felted-paper facing has a permeance ranging from 1 to 10 perms, depending on the type of facing used.

    To read the full article, go to: Choosing Rigid Foam.

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