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 at right.)
The trickiest seal is under the stringers. Here are two options:
- Attach a building gasket (closed cell foam or bulb) to the underside of the stringer or spacer before hanging drywall.
- 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.
As an option, you could insulate the wall and cover it with a sheet of polyethylene as described on the next page.
Stairs without Spacer
Stairs with Spacer
 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.
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