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Polyiso panels alone for insulating walls and ceiling of building?

mikeysp | Posted in General Questions on

I am in zone 4a and have a large pile of recycled polyiso panels I wanted to use in 3-4 layers for walls and ceiling of my post frame shop and am unsure if this will cause any problems?

I notice that folks will put foam on outside and then fill cavities with other types of insulation. Since I already have this insulation, I would like to us eit if it is not a bad idea.

My two types of polyiso:
1.5″ panels with grey/black felt covering
2″ panels with a thin white composite (fiberglass?) covering.

Thank you for your advice.


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  1. Expert Member
    BILL WICHERS | | #1

    If you can get the R value you need with just the polyiso you have, then you should be fine insulating as you describe. It sounds like you have roofing type polyiso, which is a bit lower density but still excellent insulation.

    The usual way things are done is to put a layer of rigid foam on the exterior sufficient for dew point control, then fill the stud cavities to get more total R value. There is no reason you can't just put ALL the insulation on the exterior, it's just usually more expensive. In your case, since you already have all that rigid foam, you really don't have any downside to putting everything on the exterior aside from the usual issue of getting fasteners through thick exterior foam.


    1. mikeysp | | #2

      Bill, It is indeed recycled roofing foam. Did you note that all the 2" polyiso has a composite that appears to be a thin layer of fierglass? Does this create any type of problem with trapping moisture? Does it need to only go on a certain location like the exterior layer?


      1. Expert Member
        BILL WICHERS | | #3

        Many/most types of rigid foam are vapor barriers with thicker layers like you're proposing. If you have no fluffy insulation, there is no "ratio" to worry about, so you end up being pretty safe. By putting ALL the insulation on the EXTERIOR, you keep the interior parts of the wall warm, so little if any moisture risk, and you still have interior drying for the entire wall this way. You'll end up with a pretty safe assembly, it would just not be cost effective for most to build that way new.


  2. walta100 | | #4

    If I remember correctly dew point should not be a problem in zone 4 so you can put the insulation inside or out.

    What will be the exterior surface? Often the steel siding is part of the structure that keeps the walls square or does your frame have diagonal bracing? Without the bracing you will want the siding attached directly to the framing no insulation in between.

    Is the frame steel? If so you will want the insulation on the outside.

    If it is a post and beam frame you will need to plan your air sealing details very cautiously.


    1. Expert Member
      Dana Dorsett | | #5

      >"If I remember correctly dew point should not be a problem in zone 4 so you can put the insulation inside or out."

      That's right- it's either all exterior or all interior but not both, which would create a moisture trap.

      From a design point of view derate used polyiso to R5/inch due to age- even though it will probably do better than that. Assuming some sort of wood sheathing and interior finish wallboard it takes 3" of continuous used polyiso to hit IRC 2018 code minimum performance (U0.60, or R16.7 "whole wall") without cavity insulation. A double layer of the 1.5" goods would be enough on the walls. But it takes a continuous 7-8" to hit code minimum performance (U0.026, or R38.5 "whole assembly") for the ceiling.

  3. walta100 | | #6

    Just to be 100% clear we agree he could but the foam on the inside or outside but not foam in both.

    He could have foam outside and fluffy stuff inside as long as all the foam is in direct contact and the only vapor impermeable layer in the wall is the foam.


    1. Expert Member
      BILL WICHERS | | #7

      Foam on the outside and fluffy stuff inside also means you need the ratio of outside to inside R values correct for the climate zone. Everything in one place or the other avoids that issue.

      I agree foam on the two sides and fluffy between is a problem and should be avoided. This is the situation that can trap moisture within the wall.


      1. Expert Member
        Dana Dorsett | | #8

        >"Foam on the outside and fluffy stuff inside also means you need the ratio of outside to inside R values correct for the climate zone. "

        In zone 4A there is no minimum ratio.

        With the fluff on the interior side of the assembly the finish materials on the interior can have Class II or Class III vapor retardency (latex paint on wallboard is good) to allow drying toward the interior but not Class 1 (vinyl or foil wallpapers, etc.) which would create a moisture trap.

  4. BCinVT | | #9

    You could use the polyiso both inside and outside of a sheathing layer. Just be sure to leave permeable coverings on both interior and exterior so any moisture can dry. I've done this with reclaimed polyiso roofing. It's just a cut and fit process on the interior which most folks don't wish to tackle. But I would rather do that then use the long screws needed for a thick layer outside. I don't like the idea of all the exterior finish depending on that insulation and screws for support. Since you won't get a good press fit using the board between studs it is a good idea to shoot spray foam in the open seams.

  5. mikeysp | | #10

    Ok, I know it has been a month (we got COVID) but I want to get some clarification on conflicting information:

    Opinion 1: put recycled polyiso roof board outside or inside, but not both.

    Opnion 2: putting it inside and outside is not a problem as long as wall can dry in both directions.

    A little clarification...please.

    Thank you. -Mike

    1. Expert Member
      BILL WICHERS | | #11

      If you have vapor permeable (kraft faced) polyiso, you could put some on both sides, but "some" means not enough for the polyiso itself (and not just the facer) to become enough of a vapor retarder to prevent drying.

      It's safest to put all of the polyiso on either the inside or the outside, with the assumption that the "other" side of the wall is the side that then allows for drying. The most common example would be all the polyiso on the exterior, then painted drywall on the interior. The painted drywall isn't a vapor barrier, so you have drying to the inside in this assembly. Since you have drying to the inside, you can put as much polyiso on the exterior as you want and not worry about moisture (as long as you have at least the minimum amount for your climate zone).


      1. mikeysp | | #12

        OK, I will place all foam on inside to avoid any question of problems. Easier with a post frame assembly.

        I am using metal roofing panels for my exterior siding, with a 3/8"x3 pine rain screen over Typar and OSB (stuff I have on hand).

        Question: While metal siding (not moisture permeable) does not allow drying to outside, will the 3/8" rain-screen allow drying to outside through the air channel between typar and metal?

        Thank you. -Mike

  6. BCinVT | | #13

    That is the theory of a rain screen, and 3/8" should be enough. I would do the same on my house, but my wife does not like the aesthetics of metal. Make sure you include some manner of insect/rodent barrier along the top and bottom of the rainscreen.

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