What is the minimum overlap of a vapor barrier in an unvented crawl space?

What is the minimum overlap of a vapor barrier in an unvented crawl space?

• A. 8 inches
• B. 6 inches
• C. 4 inches
• D. 2 inches

Answer: (B)

Seam integrity matters when a vapor barrier is used in an unvented crawl space because moisture can move through soil and bypass gaps if seams aren’t sealed properly. The barrier is laid on the ground and the seams are overlapped so moisture cannot slip through between sheets. The minimum overlap for these seams is six inches, with the seam sealed with tape or an approved sealant. This width accounts for installation tolerance and slight shifting, helping keep the barrier continuous. Overlaps smaller than six inches can allow moisture to migrate, while a larger overlap is fine but not necessary as a minimum.

Seam integrity matters when a vapor barrier is used in an unvented crawl space because moisture can move through soil and bypass gaps if seams aren’t sealed properly. The barrier is laid on the ground and the seams are overlapped so moisture cannot slip through between sheets. The minimum overlap for these seams is six inches, with the seam sealed with tape or an approved sealant. This width accounts for installation tolerance and slight shifting, helping keep the barrier continuous. Overlaps smaller than six inches can allow moisture to migrate, while a larger overlap is fine but not necessary as a minimum.