A standard feature in many kitchens is a 6- or 12-inch-tall soffit that runs at least partly along an exterior wall. Unless this soffit is properly air sealed, it creates a big air leak in the ceiling that wastes energy and draws unconditioned air into the house. It also provides a path for moist air from the surrounding conditioned space to leak into the attic, where it deposits condensation and reduces the R-value of the insulation.
Best practice is to hang drywall on the walls and ceiling before building the soffit, effectively isolating it from the exterior walls and ceiling joist bays. But the typical construction sequence is to build the soffit during the framing stage. No one gives it a second thought after the walls and ceiling are insulated and the drywall is hung, and by then it’s too late to fix air leaks.
Here’s a simple way to solve the air-leak problem while still framing the soffit before the drywall is in place.
Instead of framing the soffit against the studs [1], first fasten a thin piece of plywood or similar material against the studs in the section of exterior wall that falls within the soffit [2]. Cut the plywood to match the length and depth of the soffit and butt any seams over studs. Caulk the plywood at the perimeter as well as at any mid-span butt joints to make sure no air can be exchanged between the soffit and the exterior wall. Then frame the soffit against the plywood.
Next, close off the top of the soffit to prevent air leaks into the attic. Tack a piece of plywood or thin rigid foam in place over the opening and seal the perimeter with spray foam [3]. If a seam can’t be avoided, scab a narrow strip over the seam and foam the edges in place.
With the soffit sealed along the exterior wall and along the ceiling above [4], batts or loose-fill insulation can be added in the attic floor. Once sealed, the soffit no longer allows air movement from outside to inside, or from the conditioned space into the unconditioned attic.