Basement slab insulation - how to use polyiso
Last Post 16 Sep 2018 12:18 AM by Dilettante. 1 Replies.
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Jesse_BoyerUser is Offline
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15 Sep 2018 05:36 PM
Background: planning a new home located in SE South Dakota. Preliminary excavation indicated sandy soil, but I would expect some clay as well. Home will be a ranch with full basement and all exterior walls will be ICF; roughly 20' of ICF wall, 15 courses of ICF block. Basement floor needs to be insulated as I will be all radiant heat. I'm looking for the best way to use reclaimed polyiso sheets of insulation as I have access to quite a lot. It has a fiberglass paper on both sides of the 2" thick sheets and should have an aged R-value of about 5.5 per inch. I know polyiso tends to absorb available moisture, but I have 350+ sheets of this stuff. If it's not possible to use, that's fine as well. Just hoping to give it a shot with research now. After footings are complete, can I use plastic over the compacted soil, lay the network of insulated sheets, another layer of plastic to prevent the concrete from potentially floating the insulated sheets while pumping concrete? Additionally, the home will have interior drain tile installed and I will have a radon fan drawing air from this space. That would help in the drying of the subgrade, insulation, etc, albeit slightly.
DilettanteUser is Offline
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16 Sep 2018 12:18 AM
If you're throwing the insulation below the concrete DO NOT USE POLYISO! If it gets wet, you lose your insulation value. And, trapped beneath the slab, it's going to be nearly impossible to dry.

Even above-slab, I'd recommend going with EPS if there's ever any possibility of water infiltration or standing water on a horizontal surface.
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