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)

Seams in a polyethylene moisture barrier must be overlapped enough to create a continuous, watertight shield against moisture moving through the wall and soil. For a barrier applied to the exterior of a below-grade foundation wall, the minimum overlap is six inches. This size helps the seam stay watertight under hydrostatic pressure and accommodates minor movement or settlement without letting moisture path through. When installing, lay the sheets so the upper panel covers the seam by at least six inches and seal the joint with appropriate tape or sealant per manufacturers’ instructions. A smaller overlap increases the risk of leaks, while a larger overlap isn’t typically required unless specified by the project.