Staircases and staircase landings are often located along exterior walls or walls adjoining unconditioned spaces like garages or attics. Insulating and air blocking of the exterior wall below the stairs is sometimes overlooked. When the insulation in the exterior wall cavities is not fully aligned with (touching) the interior wall sheathing (drywall), the insulation’s effectiveness is reduced. After filling the wall cavities with insulation, the walls should be covered with a rigid air barrier material such as rigid foam insulation, OSB, or drywall that is taped at any seams and sealed to the bottom plate, top plate, and framing at the edges with caulk. The insulation should fill the cavity and be fully aligned with the rigid sheathing.

Staircases may be prefabricated or custom site-built. Prefabricated stairs are often used when the stairs will be covered with carpet, although they can be ordered with hardwood and custom finishes. Custom staircases are sometimes built when the stair treads will be hardwood.

Prefabricated stairs come with the stair treads and risers already attached to the stringer boards. The most common varieties are the notched stringer stair (Figure 1) and the routed stringer stair (Figure 2). The notched staircase uses three stringers that are cut out or notched to hold the stair step treads and risers. Routed stringer stairs use two stringer boards that are not notched. The routed stringer differs from the notched stair by utilizing two visible skirt boards as its structural support stringers. Each stringer is usually a solid finished piece of wood, typically 12 inches high and 1.25 inches thick, that is routed to hold the treads and risers. The steps are nailed and glued to the stringers. Because these stringer boards extend above and below the sides of the steps, they serve as the finish boards as well.

These materials may be installed by insulators, framers, or drywallers. This task should be included in the contract for the appropriate trade depending on the workflow at the specific job site.

How to Install a Site-Built or Prefabricated Notched Stringer Staircase on an Exterior Wall

1. Attach a spacer board to the outer side of the outside notched stringer board on the side that will attach to the wall (Figure 3). Attach the spacer board flush with the bottom edge of the stringer board. The spacer board should be thick enough to allow room to slide the drywall between the stringer and the exterior wall studs. If you want to install a finish board along the steps, use a wider spacer (e.g., a 2x4) that will accommodate the thickness of both the drywall and the finish board.

   

2. Attach the stringers at the base floor and the second floor or landing. Insulate the wall. Attach the stringer board to the exterior wall studs through the spacer board. The spacer board will give you room to slide the drywall between the riser and the exterior wall studs. 

3. Cut drywall to fit from the top of the spacer board to the ceiling. Apply caulk or joint compound along the top of the spacer (or bottom edge of the drywall). Slide the drywall behind the stringer board so it is resting on the spacer board and fasten it to the exterior wall studs (Figure 4). Install a finish board next to the drywall if desired.

   

4. Below the stairs, apply caulk to the exterior wall framing and the bottom edge of the spacer board then install rigid foam, drywall, or another sheet good from the bottom of the stringer board to the bottom plate of the wall. Caulk or tape any seams.

How to Install a Prefabricated Routed Stringer Staircase

1. If a prefabricated routed stringer staircase is used, insulate the wall, then attach the bottom end of the stringers to the floor and the top end to the second floor or landing. Fasten the staircase through the stringer boards to the exterior wall studs. Install drywall cut at a diagonal to fit from the top of the stringer/finish board to the ceiling. Apply caulk or joint compound to the seam between the top of the finish board and the drywall (Figure 5). Cover the seam with trim.

   

2. Below the stairs, apply caulk to the exterior wall framing and to the bottom edge of the stringer board, then install rigid foam, drywall, or another sheet good from the bottom of the stringer/finish board to the bottom plate of the wall. Caulk or tape any seams.