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Polyiso HD cover board to insulate a basement floor

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.

Asked by Brian Winke
Posted Nov 14, 2012 12:38 AM ET
Edited Nov 14, 2012 12:52 AM ET

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6 Answers

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1.
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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.

Answered by Martin Holladay, GBA Advisor
Posted Nov 14, 2012 6:17 AM ET

2.
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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/

Answered by Brian Winke
Posted Nov 14, 2012 7:53 AM ET

3.
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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.

Answered by Martin Holladay, GBA Advisor
Posted Nov 14, 2012 8:36 AM ET

4.
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Fair enough. Thanks.

Answered by Brian Winke
Posted Nov 14, 2012 8:41 AM ET

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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.)

Answered by Dana Dorsett
Posted Nov 14, 2012 12:34 PM ET

6.
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Conceeded and appreciated. Thanks!

Answered by Brian Winke
Posted Nov 16, 2012 1:39 AM ET

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