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Fiberglass faced polyiso with WRB on the Inside?

airfix | Posted in General Questions on

New construction climate zone 6B.  I plan to have my OSB walls with 3″ polyiso on the outside.  My original plan, because of outtie windows, was to have the WRB on the outside of the polyiso. However I’m now thinking about putting the WRB on the inside of the polyiso to help with integration with an EIFS layer and stone cladding layer.

I managed to come by 3″ of fiberglass faced roofing polyiso as a factory over run for cheap.  I think it is the GAF HD stuff.

This is normally for a roof covered by a waterproof layer.  If this is used behind my rain screen gap do you think I need to protect it from moisture absorption by having my WRB on the outside of the foam?


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  1. Expert Member
    Dana Dorsett | | #1

    Tape the seams, but it should be fine if it's facing an vented air gap, with minimal direct wetting.

    I doubt it's GAF HD , which is only sold in thin sheets and is over 4 lbs per cubic foot density. Most fiberglass faced roofing foam is ~2lbs density.

  2. airfix | | #2

    The same questions but this time with the Polyiso behind a layer of stone veneer.

    If I put the tyvek behind the polyiso on the siding locations like I mentioned above then it would make sense to also put it behind the polyiso by the stone veneer. That seems more of a problem to me because of how the stone and lathe picks up water. See my edited picture of my plan.

    I am planning an air gap between the lath and the polyiso. I was originally thinking that Sure Cavity 5mm mat but I think it will likely be a dimple mat.

    Do you guys see any issues?

    PS Dana you were right it's not GAF HD, it is 2lb density reported as R6 per inch.

  3. Expert Member
    Dana Dorsett | | #3

    It should be fine. It's not clear what would be gained using housewrap if there's a waterproof dimple mat, which would also serve as a WRB. (The housewrap is would still be needed if using a mesh type rainscreen underlayment though.)

  4. airfix | | #4

    Got you. I never thought of the dimple mat as a WRB. I'd have to tie the Tyvek into the WRB though. I imagine we would just use tyvek tape and tape the tyvek to the top of the dimple mat?


  5. Expert Member
    Peter Engle | | #5

    In theory, current code requires two layers of WRB behind adhered stone and stucco. The inner one is the actual WRB and your flashings, tapes and everything are detailed to that layer. The outer one acts as a slip sheet to prevent the stucco from bonding to and/or clogging the primary WRB. IN your stackup, the Tyvek would be the WRB, and the dimple mat would be the other WRB. Even though the dimple mat is pretty waterproof, it only acts as the second layer, not the primary layer.

    Of course, if the surface of the foam is waterproof and can be taped and sealed, that surface could be your primary WRB. Then you would flash and seal everything to the surface of the foam, and the dimple mat provides a cavity and the second WRB.

    1. airfix | | #6

      The foam is going to be fiberglass faced polyiso. I'm not sure if that's deemed waterproof or not even taping the seams. I always thought that it is important to keep water away from polyiso because it absorbs water. I understand behind my siding it should dry out but I'm worried about being behind the rock veneer.

      The EIFS process from BASF calls out for EPS foam (I'm using 3.5") and uses their paint on WRB. I don't think the foam seems are taped so that's probably not considered a WRB.


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