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Community and Q&A

Use of polyiso below a slab

user-4435615 | Posted in Energy Efficiency and Durability on

In CZ-4, I will be building a Net Zero slab-on-grade ranch, on a flat lot, on top of a ridge. All water drains away from the site. The slab itself will be raised about 2 feet above the current grade level. Under the slab will be a sheet of poly, 2″ of sub-slab insulation, and about about 18″ of gravel, all above the current grade level. There will also be 3½’ Overhangs. Foundation drains will drain to air.

My question relates to the sub-slab insulation. The consensus at GBA seems to be to use only XPS as sub-slab insulation as other products can absorb water or suffer other adverse effects. I see no chance that the area within my foundation walls will ever get wet, but should any water ever get there, there will be 18″ of gravel below the insulation.

I have access to enough reclaimed PolyIso to provide for all my sub-slab insulation needs. In this instance, would PolyIso be satisfactory ?
Would it have the requisite resistance to compression to place under a slab ?

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  1. GBA Editor
    Martin Holladay | | #1

    Almost all experts agree that polyiso shouldn't be used under a slab. It absorbs water too readily.

    This insulation layer is important. Don't choose the wrong material just to save a few dollars -- you can never fix this error.

    What you want is EPS. You may be able to buy reclaimed (recycled) EPS for a fraction of the cost of new EPS.

  2. Expert Member
    Dana Dorsett | | #2

    What Martin said- polyiso under slabs can load up with water semi-permanently. EPS and XPS won't.

    Since vertical space is as cheap as removed-dirt, there is no point to paying the premium for XPS. The higher R/inch of XPS isn't particularly relevant here, and it's temporary. Within a reasonable lifetime of a slab, as the HFC blowing agents slowly dissipate, the performance of XPS will eventually drop to that of EPS of similar density. Owens Corning only guarantees R4.5/inch for their XPS at age 20, down from an LTTR of R5.0/inch guaranteed at age 5. With EPS the hydrocarbon blowing agent is already gone by the time it's reached the distributor's loading dock, and the R4.2/inch performance of value of 1.5lbs density EPS is stable for decades.

    Both EPS and XPS are available from reclaimers in my area at 1/4-1/3 the cost of virgin stock. The greenest possible insulating foam are goods that has been removed from the disposal stream and re-used as insulation, independent of the blowing agents used. I have ozone-depleting high global warming CHFC blown polyiso insulating the walls of my basement, reclaimed goods pulled from large scale commercial roof demolition. The environment cost has already been taken, but re-use extends the benefit.

    The compression resistance only becomes relevant if it's going to be under a grade beam or footing. If it's a floating slab with stem walls supporting the house, any 1.5lb density or higher foam is going to have plenty of load capacity.

  3. Chaubenee | | #3

    EPS all,the way. It works. Polyiso is not for ground contact. It is simply never going to be dry under there no matter what.


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