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Polyiso HD cover board to insulate a basement floor

Brian Winke | Posted in Energy Efficiency and Durability on

I need every inch of height on my basement ceiling, so I was wondering if either 1 or 2 layers of roofing type polyiso HD cover board would work as both the insulation and sub-tile floor? A couple of companies make such cover board and both claim an R value of 2.5 for only 1/2″ thickness. All input is appreciated.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Brian,
    My guess is that the foam will be too squishy to work as a substrate for tile.

    My advice: (a) use XPS, not polyiso, (b) include a subfloor of plywood or cement backerboard above the XPS, and (c) choose a more flexible type of flooring -- not tile.

  2. Brian Winke | | #2

    Thanks Martin. I was thinking that cover board was inherently not squishy. To confirm we're talking about the same thing, I'm attaching the following link. There is at least one other manufacturer with a similar cover board, but this will give you the idea: http://www.firestonebpco.com/roofing/insulation/coverboards/isogardhd/

  3. GBA Editor
    Martin Holladay | | #3

    Brian,
    Just because an insulation board is stiff and rigid enough for roofing applications, doesn't mean that it's stiff and rigid enough to use under ceramic tile.

  4. Brian Winke | | #4

    Fair enough. Thanks.

  5. Expert Member
    Dana Dorsett | | #5

    There's also the issue that in an over-slab application it's likely to take on groundwater over time, reducing it's effective R-value. Iso is oftenOK to use on basement walls, slabs, not so much (no matter how rigid the subflooring is for distributing the mechanical loads.)

  6. Brian Winke | | #6

    Conceeded and appreciated. Thanks!

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