Use a ten to fifteen mil polyethylene sheet as the practical minimum vapor retarder under slab on grade for most house construction.

For slabs that will receive moisture sensitive floor coverings, place a continuous vapor retarder directly on the compacted base before placing concrete, ensuring seams are overlapped a minimum of six inches and sealed with compatible tape to prevent vapor migration. Position the polyethylene so it extends over the edge of the slab and up the stem walls or the interior face of perimeter foundation walls by at least four inches to maintain continuity at critical transitions and to allow a proper seal to floor finishes. For higher performance where site conditions or installed finishes are especially vulnerable use a sixty mil sheet or an engineered vapor membrane and follow the manufacturer instructions for priming and seam treatment. Avoid placing the membrane on top of a moisture prone base such as uncompacted or saturated fill; compact and compact to specification and provide a thin layer of clean sand or a capillary break before the membrane to reduce punctures and ensure a smooth bearing surface. When installing in cold or windy conditions keep material warm and flat, and tape and seal seams as you go so wind does not lift edges and create tears that compromise performance. After pouring keep heavy traffic and sharp tools off the curing slab until curing is sufficient to avoid membrane displacement and patch any punctures with the same material and tape.

Common mistakes include using nominally thin film such as under ten mil polyethylene that tears during placement and allows moisture paths; the fix is to upgrade to at least ten to fifteen mil for ordinary installations or to sixty mil where the floor finish or site conditions demand more robustness. Another frequent issue is poor seam detailing where inadequate overlap or improper tape choice leaves gaps; always overlap by at least six inches and use a tape rated for polyethylene to create a durable vapor tight seam. A third concern is failing to protect the membrane at wall and slab edges which creates discontinuities; extend the membrane up walls or onto stem walls by at least four inches and fasten or seal to the wall substrate so the membrane remains continuous with the floor finish layer.

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