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Community and Q&A

Foil-Faced Polyisocyanurate for Foundation Walls and Below Grade

oldbungalow | 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?


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  1. Expert Member
    BILL WICHERS | | #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
        BILL WICHERS | | #3

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

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


    2. oldbungalow | | #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
        BILL WICHERS | | #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).

  3. kramttocs | | #7

    I put this in a similar thread but this one is likely a more appropriate one for it.

    Just today I've talked to two different polyiso manufacture's tech support.
    One is a foiled faced 1.5" and the other a fiberglass faced 1.75".

    The foil one is marketed for a lot of stuff including being explicitly marketed for exterior/ground-contact/below grade like around the perimeter of a foundation wall.
    The fiberglass faced is the commercial roof insulation that if often sold used.

    The fiberglass tech told me that he has seen some get wet and just fall apart after absorbing the water and didn't recommend it for ground contact.
    The foil tech said it wouldn't be a problem though taping any exposed edges would be a good idea.

    How can one make sense of this? I understand the facers behaving differently when wet but I wouldn't expect the polyiso core to.

    For my specific application I plan to apply(paint roller) a water proofing adhesive material like regular roof asphalt to the both the foundation and insulation, press the insulation to the foundation while still wet and then support until dry. Once dried I will apply another layer of asphalt to the external face of the insulation and press Nudo groundbreaker to the top 12" and support it. Once dried, backfill.
    The insulation will be in strips of 16" and 2', both with around 6" exposed but protected by the Nudo. Because of the cost of the asphalt, getting the fiberglass polyiso used has a lot of appeal.

  4. Expert Member
    BILL WICHERS | | #8

    The problem isn't the material used for the facer, the problem is that the polyiso itself -- the foam part BETWEEN the facers -- will absorb and hold water. There's no getting around that, since it's an inherent property of the polyiso foam material. Foil faced polyiso still has exposed edges, and any little hole, cut, tear, scrape, or gouge will let in water over time. Tape is just as at risk as the facer itself in this regard. Once the foam is saturated with water, it's not really an insulating material anymore.

    The simple answer is don't bury polyiso underground, and don't use it where it will be likely to encounter bulk water (such as the lower edge in a puddle in a basement).

    If you want to insulate the EXTERIOR of your foundation, where the material will be in contact with moist earth, just use EPS or XPS which are acceptable for this application. I would use XPS myself, and try to get one of the newer/greener formulations that are now becoming available.


    1. kramttocs | | #9

      Thanks Bill!
      Not meaning to argue at all - just figure out what I am missing.
      Attached is the big thing that is causing my issues. Big fan of working with xps so and it was only seeing used polyiso prices that made me consider it.
      I just can't understand why they are marketing polyiso for this.

      Side question and I can call Dupont about it but think the roof asphalt would work just as well on the xps to adhere it to the foundation? Wanting a fully adhered option vs running beads of adhesive. I know some things can eat the foam but haven't researched asphalt's impact.
      Edit- scratch that idea since it's solvent based. Will just go the pl foam board in a tube route.

      1. Expert Member
        BILL WICHERS | | #10

        Unless they are somehow making this material differently, what they say there contradicts everything I know about polyiso and water absorption that I've read or heard for years. Most manufacturers even warn you about storing polyiso outdoors for too long for risk of it taking on water if exposed to the weather.

        I don't think the Rmax people are shady in any way, but I'd want to know more from them about their product before I'd consider using it in applications where polyiso has been restricted from use for years.


  5. Mikethewaterproofer | | #11

    I'm new here, we install EPS and XPS board below grade every day. The asphalt roofing will probably be ok because we install asphalt waterproofing all the time with EPS and XPS over It.
    I would rethink the roofing though, if your going to the trouble to do that jut put peel and stick
    waterproofing on it and PUT A FRENCH DRAIN IN!
    All of the jobs we take off today that has insulation board above grade show protection.
    You have to protect it somehow.
    We're working on a cost effective solution for this issue right now. Actually that's how I came across this post.

  6. kramttocs | | #12

    Thanks Bill and Mike.

    Just going to forget the polyiso based on historical opinions over marketing. Not worth the risk.

    We already have a french drain in the back as we used to get a flooded crawlspace but it stays nice and dry now. Just going to use the PL300 Foam to adhere the xps to the bare concrete and call it good. I am using Nudo groundbreaker for the top 12" to protect it and for aesthetics.

  7. Mikethewaterproofer | | #13

    Have you purchased the Nudo? I was told serval weeks ago by my suppliers its no longer available.

  8. kramttocs | | #14

    Yeah, I've got it. Looks like it's still available at Menards. Thanks for the heads up though.

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