Foundation Air and Vapor Strategy

Insulation is critical, but a continuous air barrier is just as important.

Foundation Air and Vapor Strategy 1

A few years ago, we began using 15 mil-thick plastic as part of our under slab air and vapor strategy. Plastic that thick may seem like overkill to some, but I’ve remodeled multiple houses where the scope of the project required removing concrete slabs and have seen thin plastic used for a vapor barrier so cracked, brittle, or dissolved that it wasn’t accomplishing anything more than taking up space. Since we are relying on the plastic as part of our air barrier strategy, using the extra-thick poly is worth it, and we’ve seen the results firsthand on previous projects.

The Stego Wrap rolls we purchase are just over 1,900 sq. ft., so one is usually enough for a house. All the overlap joints and pipe penetrations are sealed with their proprietary tape to create as solid an air barrier as possible. The continuous barrier will help prevent vapor diffusion upward from the ground to the concrete.

The plastic is held in place by the pressure-treated 2×6 plate along the perimeter. (As I mentioned in a previous blog post, the 2×6 also prevents the slab edge insulation from rising up during the concrete pour.) Once the exterior walls are framed, we tape the plastic to the Zip System sheathing to continue the air barrier up the outside of the wall.

Foundation Air and Vapor Strategy 2

The 15-mil-thick vapor barrier is run long; overhanging each edge.

Foundation Air and Vapor Strategy 3

Each penetration and seam is taped.

Foundation Air and Vapor Strategy 4

Once the walls are raised the edges of the vapor barrier will be taped to the exterior of the Zip Sheathing.