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Sill Seal with Flashing Tape Air Barrier

November 10, 2015

The biggest crack in the house, revisited

We published an airtight sill detail animation a little while ago based on various good ideas assembled in the past: hanging the wall sheathing below the mud sill, caulking between the plywood and the foundation, sealing the sill seal to the foundation, etc.

Because after all, the joint between the foundation and the framing is often the biggest crack in the house.

When I showed the animation to David Joyce, one of our technical committee members, he told me about another way that his company does it using peel and stick flashing tape instead of all the caulks, adhesives, and sealants. Basically, the flashing tape acts as a Z-flashing that bridges the gap from inside on foundation to outside of wall sheathing.

Dave lays the tape down sticky side up (stuck to the underside of the sill plate), so that he can wrap it up around the wall sheathing.

The inside of the flashing tape is folded down and covered with spray foam when they insulate the basement walls and the outside wrapped up and stuck to the wall sheathing.

Dave still uses the standard sill seal to act as a cushion between the concrete and the wood. If the concrete punctures the tape, it will be a steady stream of capillarity into the mudsill, so he installs the tape over the foam sill seal.

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