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Off-label use for open cell Iso foam?

user-892612 | Posted in Green Products and Materials on

GC is proposing to use Iso open cell foam as bulk fill in lieu of compacted gravel / sand fill over existing residential interior concrete slab on grade, to provide a base for a new interior concrete slab. Requirement is to bring a 15′ X 19′ X 12″ deep “ashtray” up to level with surrounding floors and finish with tile to match adjacent. Application is strictly structural; insulation value is not a benefit. Existing recessed slab is 4″ above base flood elevation including freeboard, existing plumbing drain pipes are below the existing slab. Gutted frame structure under roof allows ample drying opportunity. So – Bad idea? Good idea? Any cautionary suggestions?

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  1. Expert Member
    Dana Dorsett | | #1

    Polyisocyanurate foam isn't open cell- it's a closed cell product with some amorphous aspects that allow it to absorb water (slowly). The compressive strength of polyiso is adequate for supporting a new slab, but it can become waterlogged and once is does it can take forever to dry if it's under a slab & finish floor.

    EPS would be a better choice, since it has better defined closed cell structure and even though the interstitial spaces between the macroscopic bead structures take on water quickly in a rising tide, it leaves just as quickly. It would be cheaper per inch than polyiso too.

  2. wjrobinson | | #2

    insane imo

  3. charlie_sullivan | | #3

    I agree with Dana--EPS is better for this and cheaper.

  4. user-892612 | | #4

    Thank you all. My sense is that there is no advantage of any kind of foam (and moisture disadvantage with Iso) over conventional sand fill with vapor barrier under new slab. Core drill some drain holes through depressed slab, use dry sand blown or wheelbarrowed in, dowel new slab to adjacent.

  5. dickrussell | | #5

    With vapor barrier right under the new slab, you should be ok, but be sure to read this:

    That sand layer will become a sponge if it gets wet, with no way to dry out. That moisture MUST be kept away from the new slab.

    But why do you say that there is no benefit from having insulation under the new slab? In what climate zone is this located?

  6. Expert Member

    Check the compression specs for whatever foam you are considering. If it is matching up with existing floors, something like 5% would cause real problems.

  7. user-892612 | | #7

    Thanks all - this is climate zone 8, warm and humid, excessively well drained sand soil, soil temperature year round is stable within a few degrees of conditioned interior air temp.

  8. Dana1 | | #8

    DOE climate zone numbers are are different from USDA climate zones. DOE zones are what's used in building codes, and zone 8 is something of the opposite of warm & humid (unless you think of Fairbanks AK as warm & humid :-) ).

    No matter what climate zone, if there's a vapor barrier between the sand or foam and the new slab it doesn't much matter if what's under it gets wet. But foam insulation is pretty expensive back-fill, and will in some warm humid climates increase energy use by isolating the house from the thermal mass of the soil. In DOE climate zones 1 & 2 or even some parts of zone 3 that would be the case, depending on the deep subsoil temperatures and the soil types.

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