The interior wall had water stains at the basebord, some distortion in the laminate floor near the baseboard and the distinct odor of damp, decaying wood.
I pulled the baseboard off and cut away the drywall 12 inches up for the entire length of the wall. The sill plate is damp and one stud at the bottom of the stairway shows signs of rot or termite damage. The insulation was damp at the ends but not at the 12 inch mark.
Currently, there is nothing between the poured concrete for the stairway and the studs. No plywood sheathing, no plastic, nothing. It looks like at some point in the past there was something there. Two of the studs have scraps of plastic sheet between them and the sill plate. I cannot tell if the plastic sheet was on the interior or exterior side of the studs. On the exterior side of the studs near the top end of the stairs there is plywood sheathing about 12 inches wide, but broken off in a ragged edge. It is waterlogged.
My best guess is that there was/is a leak between the stairs and wall near the upper end of the stairs. The water is flowing down the concrete and then running along the sill plate toward the corner of the wall. At some point in the past the interior wall was removed and the plastic sheeting and plywood sheathing were ripped out and then the interior wall was restored. Obviously, this is not the right solution.
My question is two-fold:
If this were new construction, how would one construct a stairway parallel to an exterior wall.
How can the wall as currently constructed be repaired such that water infiltration cannot occur in the future.