What is the minimum overlap for joints in the polyethylene film moisture barrier applied over the exterior of a below-grade foundation wall?

What is the minimum overlap for joints in the polyethylene film moisture barrier applied over the exterior of a below-grade foundation wall?

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

Answer: (C)

The key idea is that joints in the polyethylene moisture barrier must have a generous overlap to keep water from finding a path through seams. Six inches is the minimum overlap because it creates a wide, continuous barrier at every joint, which helps prevent water from migrating along the seam when the wall is backfilled and under hydrostatic pressure. This overlap also provides enough surface area for sealing with tape or adhesive so the joint stays watertight. Smaller overlaps offer less protection; they can pinch or gap under backfill movement, increasing the chance of seepage. An eight-inch overlap isn’t typically required by standard practice, and two or four inches aren’t sufficient for reliable protection.

The key idea is that joints in the polyethylene moisture barrier must have a generous overlap to keep water from finding a path through seams. Six inches is the minimum overlap because it creates a wide, continuous barrier at every joint, which helps prevent water from migrating along the seam when the wall is backfilled and under hydrostatic pressure. This overlap also provides enough surface area for sealing with tape or adhesive so the joint stays watertight.

Smaller overlaps offer less protection; they can pinch or gap under backfill movement, increasing the chance of seepage. An eight-inch overlap isn’t typically required by standard practice, and two or four inches aren’t sufficient for reliable protection.