Like the rim joist of a floor between conditioned spaces, a stair built against an outside wall poses an air sealing challenge. The sequence below offers one solution to the stair dilemma. It has the advantage of delaying the sealing work until the last possible moment, so sealing doesn't interfere with framing or other earlier work. This option works when you attach the stair support directly to the studs. Although cutting the drywall is tedious, applying sealant is easy.

If you prefer to attach a spacer to the studs, the drywall will be easier to attach, but getting the sealant where you need it will be more difficult. (See section drawing below.)

The trickiest seal is under the stringers. Here are two options:

1. Attach a building gasket (closed cell foam or bulb) to the underside of the stringer or spacer before hanging drywall.
2. Wait until after drywall to install the finish treads. The joint can be caulked as you remove each temporary tread. This option is shown below.

	Step 1 Attach the stringer directly to the studs. Hold the temporary treads away from the wall enough to allow the drywall to slip behind them.
	Step 2 Hang drywall. Fit drywall close to the stringer. A gap between 1Ú8 and 1Ú2 in. is ideal for filling with caulk.
	Step 3 As you remove the temporary treads to install the finish ones, run a bead of caulk between the drywall and the stringer. Seal the top and bottom.

This article appeared in Energy Source Builder #30 December 1993 ©Copyright 1993 Iris Communications, Inc.