Air Sealing Sill Plates

Air seal above-grade sill plates adjacent to conditioned space to minimize air leakage.

• Air seal between the sill plate and the sub-floor with caulk, foam, or an equivalent material. 

• Install a foam gasket beneath sill plates that are sitting on concrete or masonry and adjacent to conditioned space to both air seal and serve as a capillary break between the concrete and the sill plate.

Air leakage can occur at any seam in the building envelope, but buildings are especially susceptible at the sill plate. This is because they’re often attached to hard and rough concrete, which prevents a smooth seam; and because of the stack effect which pulls in air at the bottom of the building. The best way to air seal the sill plate is to place a sill sealer (also called a sill gasket) on the concrete before laying down the sill plate. Sill sealer is a pliable foam product that is available in varying widths up to 10 inches wide. It comes in rolls and is rolled out over the concrete along the foundation perimeter. The flexible sill sealer product conforms to any irregularities in the surface of the concrete. A waterproof closed-cell foam product should be selected that will both air seal and provide a capillary break preventing any moisture that migrates up through the concrete from reaching the wood of the sill plate. The rot-resistant product also prevents insect and rodent intrusion. Some builders seal the sill plate to the foundation wall with two large beads of caulk but a sill seal product that covers the whole sill plate area is preferable because of its waterproofing capability and inherent uniformity.