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The role of the 6-mil poly under the slab, and maybe footings?

Sal_123 | Posted in Energy Efficiency and Durability on

The 6 mil poly acts a capillary break between the EPS (pink, blue or white) – if you insulating your slab – and the ground moisture, right? Therefore does the moisture performance of the insulating product matter that much? Going from exterior to interior, its: ground – gravel – poly – insulation – concrete. Assuming yes, the 6mil poly is present, and in that order, any benefits to upping it, to say a 12mil poly to insure less minor tears and better moisture protection? Maybe even an EPDM membrane? While we’re at it, any benefit to laying it under the footings and overlapping it on the interior side with the slab poly, and on the exterior side sealing it or some other way make it continuous with the outer foundation exterior waterproofing, so in essence, place the house in a “poly sock”, to keep ground moisture from wicking into all exposed concrete, footings, slab, and foundation walls creeping into the home, appearing as effluorescence on the walls and humidity in the basement air. And such as in my case, eventually into the dehumidifier that runs 24/7?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Does the moisture performance of the insulating product matter that much?"

    A. Yes, it matters a great deal. That's why we don't use cellulose to insulate under a slab. The environment under a slab will be damp. One layer of poly -- or even a layer of EPDM -- isn't going to change that.

    The purpose of the poly is to act as a vapor retarder. It doesn't have to be perfect to perform well. If the poly has holes in 2% of its area, it will still be 98% effective in interrupting vapor flow.

    The way we handle liquid water is with good grading, crushed stone, and perforated drainage pipe leading to daylight or a distant drywell or sump.

    It's always a good idea to include a capillary break under the footings (or between the footings and the foundation wall). This can be polyethylene or an asphaltic compound.

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