What is the required lap distance for joints in a 6-mil polyethylene vapor retarder installed under a slab?

What is the required lap distance for joints in a 6-mil polyethylene vapor retarder installed under a slab?

• A. 4 inches
• B. 6 inches
• C. 8 inches
• D. 10 inches

Answer: (B)

Ensuring a continuous vapor barrier under a slab is about getting the seams to stay tight so moisture can’t sneak through. For a 6-mil polyethylene vapor retarder, the standard practice is to have joints overlap by six inches. That overlap provides enough coverage for the seam during concrete placement and any minor shifts in the slab, especially when the barrier is taped or sealed to keep the joint closed. A smaller overlap, like four inches, risks gaps where vapor could pass. Larger overlaps (eight or ten inches) aren’t typically necessary for this thickness and add extra labor and material without added benefit in normal installations.

Ensuring a continuous vapor barrier under a slab is about getting the seams to stay tight so moisture can’t sneak through. For a 6-mil polyethylene vapor retarder, the standard practice is to have joints overlap by six inches. That overlap provides enough coverage for the seam during concrete placement and any minor shifts in the slab, especially when the barrier is taped or sealed to keep the joint closed. A smaller overlap, like four inches, risks gaps where vapor could pass. Larger overlaps (eight or ten inches) aren’t typically necessary for this thickness and add extra labor and material without added benefit in normal installations.