Dropped soffits are common above the cabinets in kitchens and bathrooms. If attached to exterior walls and ceilings, they can be a major air leakage problem. Because soffits are typically framed before the drywall is installed, the backside is open to the wall cavities and the top is open to the floor or ceiling above. This offers an easy way for air and water vapor to move into the outside walls and roof. When the soffit opens into a cavity between floors, air can travel freely through much of the building until it finds a route to the outside. Recessed lights often installed in the soffit can make matters worse.

Sealing this air leak is difficult, because the two possible solutions conflict with the typical construction sequence. You could delay building the soffit or insulate and air seal earlier.

Option1: First Drywall, Then Soffit

The most effective air seal would be formed by framing the soffit after the wall is insulated and covered with drywall. In this case, the drywall forms an effective barrier to the top of the wall and across the ceiling. Because joints between sheets of drywall will not yet be taped, seal them with a high quality caulk (polyurethane or silicone). Next frame the soffit. Drywall can be attached and drywall finishing can proceed.

Option 2: First Plastic, Then Soffit

This option requires that you have a roll or two of insulation and a sheet of 6-mil polyethylene handy during framing. First, place the insulation in the exterior wall (and ceiling, if necessary). Run a bead of sealant down the face of the top plate and across the blocking. Staple the plastic over the insulated wall, pushing it firmly into the sealant. Run the plastic about 12 inches past where the soffit will be. The plastic and drywall will make a continuous air barrier over the top of the soffit.

You may want to devise your own variation of this idea. Here's one option. You could frame the entire soffit as you normally would. Let the insulators install the insulation. Cover the wall behind the soffit with the plastic sheet. Caulk the plastic to the soffit framing. This option has the advantage of fitting better into the construction sequence. On the down side, the work is tedious and offers the least reliable air sealing.