Cantilevers are sometimes overlooked during air sealing. The open cavity between joists offers an easy path for air leakage, even if it's fully insulated. The insulation seldom stops all air movement. The principle of sealing cantilevered floors applies to any location where a heated space occurs above an unheated space, including a tuck-under garage and a room above a porch. Here are two methods of sealing these potential leakage sites.

Blocking Between Floor Joists

With this method the subfloor forms a portion of the air barrier, so all joints between the sheets must be sealed with caulk or construction adhesive. When joists meet the outside wall at a right angle, install blocking (wood or rigid insulation) between the floor joists to block air movement. Run a bead of caulk around all four sides of the blocking. If you use foam blocking, be sure the sealant material is compatible. When floor joists run parallel to the outside wall, position one joist to sit directly on top of the wall below, even if it means adding a joist. Set the joist on a gasket or a bead of construction adhesive and then add another on top before laying the subfloor.

Sealed Exterior Rigid Insulation

If you install rigid insulation on the exterior surface of walls, you have another option for air sealing. Since the foam sheets already cover the entire wall surface, you can take it one step further for air sealing. First, use a compatible sealant or adhesive to seal the foam boards to the framing at the locations indicated in the drawing. Then use a high-quality construction tape to cover the joints between sheets.