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Community and Q&A

insulation under slab for garage

tsloss | Posted in General Questions on

Looking to insulate under a slab for a garage in NW PA (zone 6 I think). Plan is to install pex for radiant heating the slab, and the slab is to be 5″ thick. Temp will likely be set to heat around 60F. What R value is recommended/required, and what type of insulation is recommended. I know EPS seems more economical than XPS, but not sure about that, or much else. I’ve had sellers tell me polyiso is better, but I’m pretty sure that’s wrong (moisture wicking?). Is 10 psi adequate, or to “brittle” or compressive for installation? I had someone tell me use 25 psi around perimeter, and 10 psi everywhere else? Can’t seem to find any reclaimed stuff near me. Any input is appreciated. Thanks all.

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  1. Expert Member
    Dana Dorsett | | #1

    Either EPS or XPS is fine for sub-slab insulation, but polyiso would get saturated with ground water.

    XPS is something of a climate-warming disaster due to the HFC-soup blowing agents used. EPS is blown with a far more benign hydrocarbon (isopentane), not hydro fluorocarbons, all of which are extremely potent greenhouse gases. As it's blowing agents diffuse out over a few decades the performance of XPS drops to that of EPS of similar density. The labeled R5/inch is only warranteed to R4.5/inch at 20 years (read the fine print.) From a lifecycle global warming perspective XPS does 8-10x the damage of EPS at any given R-value, making it by far the LEAST green insulation in common use in North America:

    For sub-slab insulation for a radiant slab in climate zone 6 something between R15-R20 would make sense. (R10-R12 would make sense in zone 6 even without the radiant.) That would be 4" of EPS or XPS.

    Under a 5" reinforced slab the psi rating is meaningless- 10 psi is fine. You could park your fully armored Humvee or even a Caterpillar D10 bulldozer on a 5" slab without any risk cracking due to compression of the foam. The issue with foam density less than 1lbs per cubic foot ("Type-I") has more to do with taking on moisture if it isn't adequately drained, usually not a problem with slabs.

    There is often surplus or used Type VIII (1.25 lbs density) roofing available for cheap at foam reclaimers, which is just fine for slabs (but not for under footings or grade beams). With used foam priced at 1/3 or 1/4 the price of virgin stock there would be a pretty easy rationale for 5-6" of Type -VIII (R21-R25) EPS under the radiant slab.

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