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Poly Sheeting

Polyethylene plastic film used under concrete slabs as a vapor barrier to block ground moisture

Poly sheeting (polyethylene sheeting) is a plastic film used under concrete slabs as a vapor barrier to block ground moisture from migrating upward through the concrete. Available in thicknesses from 4-mil to 20-mil, it's the most common and cost-effective vapor barrier material in residential construction.

Why It Matters

Ground moisture constantly moves upward through capillary action and vapor diffusion. Without a poly barrier, this moisture causes flooring adhesive failures, mold growth, efflorescence, and corrosion of embedded metals. For any slab receiving flooring (tile, vinyl, hardwood, epoxy), poly sheeting is essential—most flooring manufacturers require it for warranty coverage.

At $0.10-0.60 per square foot installed, poly sheeting is one of the cheapest components in a slab but prevents some of the most expensive problems. Moisture damage to flooring can cost $5-15 per square foot to remediate.

Technical Details

Thickness grades:

  • 6-mil: Minimum for basic residential slabs without flooring. Class C vapor retarder. Punctures easily during construction.
  • 10-mil: Recommended standard for residential slabs with flooring. Class B vapor retarder. More durable during placement.
  • 15-mil: Premium grade, cross-laminated with reinforcement. Class A vapor retarder. Required by some commercial flooring systems.
  • 20-mil: Heavy-duty commercial applications. Maximum durability and lowest permeance.

Installation:

  1. Prepare sub-base (compacted gravel, smooth, free of sharp objects)
  2. Unroll poly sheeting over entire slab area
  3. Overlap seams 6-12 inches minimum
  4. Seal seams with poly tape or mastic
  5. Turn up edges 6 inches at forms
  6. Seal around all penetrations (pipes, conduits)
  7. Protect from punctures during rebar/mesh placement

Performance ratings:

  • Permeance measured in perms (lower = better)
  • Class A: ≤ 0.1 perms
  • Class B: 0.1 - 0.01 perms
  • Class C: > 0.1 perms
  • ASTM E1745 is the standard specification for vapor retarders under concrete slabs
  • Vapor Barrier - General term for moisture barriers including poly sheeting
  • Slab-on-Grade - Slab type requiring poly sheeting underneath
  • Moisture - What poly sheeting prevents from entering concrete

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