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Community and Q&A

Double Foil Faced Polyiso (Enerfoil)

marks77 | Posted in General Questions on

In my search for rigid polyiso insulation for my basement I ran across Enerfoil. It seems to have the aluminum foil on both sides and I’m wondering if that is an issue for below grade. My plan is concrete->2 inch polyiso with foil on the warm side taped and sealed->wood framing->1/2 inch drywall. Does anyone have any experience with this product vs the Dow and Johns Mansville polyiso carried by HD and Lowes? Thanks.

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  1. Expert Member
    Michael Maines | | #1

    Mark, I don't think I've used that specific product but all polyiso is expanded against two face layers, either foil or glass-fiber. Polyiso should not be used below grade on the exterior because it can become saturated and lose its insulating value, but it's great on the interior. I like to hold the bottom a few inches off the floor so it won't wick water in the event of a minor flood. Dow's Thermax brand polyiso is rated to be left exposed and there are minor differences in insulated value between brands but I don't see any reason why Enerfoil wouldn't work for you.

  2. Jon_R | | #2

    There is some debate about an interior side vapor impermeable barrier keeping the concrete wetter, mold growing in the gap and then the odor leaking into the room (because no air sealing is perfect). More data on this is needed.

    Personally, I'd reduce the risk by using unfaced EPS. The data is clear that this leaves the concrete drier. So less mold risk.

  3. Expert Member
    RICHARD EVANS | | #3


    I've worked with Enerfoil and really like it. ( I have it in my own basement.) I don't believe that the foil layer is fire rated like Thermax however.

  4. Expert Member
    BILL WICHERS | | #4

    It's not the facer that is a problem below grade, it's the nature of the polyisocyanurate insulation material itself: it likes to suck up water. The exposed edges are an issue, as are any punctures or tears that might get into the facer during install time.

    I don't see what the big deal is with the dual foil layers with this particular product. The most common polyiso I see is foil faced, and has foil facers on both sides.


  5. marks77 | | #5

    Thank you everyone for the knowledgeable replies. I did some more googling and saw IKO rep say Enerfoil is not to be used in basements on website (screenshot below). I reached out to IKO and their reps say it's fine for basement use (including adhesives install) and the comment on is wrong.

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