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Polyiso with thin fiberglass on both sides

markgh1 | Posted in Energy Efficiency and Durability on

I am helping a disabled friend upgrade his 1964 ranch home.  This house leaks like a sieve and costs a small fortune every month to heat and cool it.  I decided that exterior insulation would be the best way to do that with the least amount of deconstruction and time.  The current insulation in the wall is R11 and poorly installed, where we had to do interior drywall repair work.

We went through the calcs to make sure the exterior foam had more insulation value than in the wall.   I used a value of 5.8 per inch of polyiso since there is no stop declaring an r value.  I derated the foam by 2o percent and ended up at r11.5 and the wall at R11.

Hears the problem, the 2.5 iso boards are faced with a thin layer of fiberglass on both sides.  If placed against the plywood on the wall, would it not act as a sponge if any moisture comes out of the wall?  I can place the moisture barrier over the polyiso after application of it to the the wall.  I thought about putting two moisture barriers on, one in front of the polyiso and one after the polyiso is secure to the wall.

I scratched that idea and thought that maybe I have to remove the fiberglass layer from the polyiso on one side that will be placed against the plywood.  If so, is there a method to removing that layer other than scraping it off with a putty knife?  Thanks in advance for your help. Mark

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Replies

  1. gstan | | #1

    Polyiso is also commonly available with foil faces - generally no
    difference in price - essentially a really good vapor barrier. your
    estimate of 11.5 is pretty close to real for the polyiso but a 2x4 wall
    with r-11 fiberglass is actually around r-7 when you adjust for the wood
    so you don't need 2.5 inch polyiso - you could get by just fine with
    1.5 inch and it's both cheaper and far easier to work with when
    reinstalling the siding and trim. Don't get too wound up with the calcs
    they are only approximations at best - you might want to check the
    rules with your local building department before committing.

  2. Expert Member
    Akos | | #2

    The fiber facer on the plyiso is not an issue. Generally when I use it, I put the WRB outside the foam and flash to there so any bulk water is channeled outside the foam. The fiber faced stuff does hold up surprisingly well, I've had a piece I left outside exposed over a winter and looked great, so the wrb outside the foam is not a must if protected by cladding.

    The issue with your retrofit is you need to air seal. With the thousands of nail holes in the sheathing, the best would be a peel and stick WRB but this does add cost and now it is on the wrong side of the foam and would still want to install regular house wrap over the foam.

    Overall, I think the better option as suggested above is go for foil faced polyiso. The bonus is you can tape the seams which would let you detail it as the air barrier instead of needing to install a peel and stick under the foam.

  3. gusfhb | | #3

    I found some leftover roofing iso on the ground under a pile of scrap wood.
    10+ years on it looked ok
    Ground contact is not the recommended install, but it was still insulation

    I found the scrim iso significantly cheaper through a roofing company than foil faced but that was a while back. ABC roofing vs big box. IT does grow and curl when exposed to moisture, but not a real problem installed

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