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Polyisocyanurate (thermax) below grade: any moisture issues?

K T | Posted in Energy Efficiency and Durability on

Hi I’ve read conflicting information about polyisocyanurate and just want to clarify. Some sites say polyiso, unlike XPS/EPS, absorbs moisture from concrete. Is this an issue? I’m interested in putting foil-faced polyisocyanurate against a foundation wall, for potentially unfinished basement space.

If the insulation is properly installed and air-sealed against indoor air, are there any moisture concerns for a) polyiso foam contact against wall and b) polyiso foam contact with slab? I’m less concerned about condensation of indoor air, but outside moisture absorption.

Finally, is there a code reference for what meets code (fire) for foam insulation, i.e. thermax vs EPS with wallboard etc?



  1. Expert Member
    Zephyr7 | | #1

    Polyiso = polyisocyanurate. You can’t use polyiso below grade since it will absorb moisture. You most certainly CAN use it on the inside of a foundation wall. If you’re using the usual foil-faced polyiso, the water absorption issue is only a concern around the perimeter of the panel: the foil facing is a vapor barrier and will prevent any moisture absorption on the faces of the polyiso.

    It’s a good idea to leave an inch or two gap between the bottom of a sheet of polyiso and the floor slab of the basement to make sure the polyiso is never sitting in a puddle. The water absorption issue is with liquid water, not vapor in the air.

    I’m not aware of a comprehensive list of insulation fire ratings. I know Dow’s Thermax polyiso is rated to be left exposed while most polyiso needs thermal or ignition barriers depending on the application. You should ask your local building department people if this will be an issue in your particular application.


    1. Jon R | | #2

      > You can’t use polyiso below grade

      I'd use the term "ground contact".

      1. Expert Member
        Zephyr7 | | #3

        Yeah, “ground contact” is a better term here.

        Polyiso can’t be directly exposed to liquid water, so no direct burial.


    2. K T | | #4

      Thanks, a little confused, you mention below grade (no) and basement (yes). so a below-grade basement is a no? I was thinking more in terms of contact against the foundation wall, the non-foil faced side is the same as the perimeter edges. Would moisture wicking in through concrete be an issue then and is XPS/EPS preferred?

      I see basement thermax installations on the web so not sure how that works.

      1. Expert Member
        Zephyr7 | | #5

        Not what I meant. As Jon pointed out, “below grade” and “ground contact” are not the same thing and I apologize for causing a little confusion there.

        What I meant was you can’t bury polyiso in the ground, so it can’t be used on the exterior of a below-grade foundation wall where the ground is against the wall, and it can’t be used under a slab (XPS and EPS are ok to use in either of these places).

        Polyiso CAN be used on the INSIDE of a below-grade wall, because the polyiso will NOT be in contact with the moist earth on the inside face of a foundation wall.

        Polyiso typically has two foil facers, one on either side. There is polyiso that has permeable kraft facers (and fiberglass facers), but it’s not as commonly seen (mostly it’s used in commercial roof assemblies). If you’re using the common foil-faced polyiso, it will be fine against the inside of a basement foundation wall. This is what I put in in my own house.

        Note that polyiso is perfect for this application too, since you get the best R per inch of rigid foam, but the foundation wall doesn’t get cold enough to get into the R value derating issues that you have with some types of polyiso.


  2. Jon R | | #6

    The damp air gap formed by a vapor barrier (like foil) against concrete creates some (??) risk of mold growth and some (??) of the odor will make it into the interior. Permeable insulation (like unfaced EPS) results in measurably dryer walls (ie lower mold/odor risk).

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