My house has an addition that is connected to the old part of the house by a long staircase. The backside of the stair is exposed to an uninsulated garage (covered by drywall but no insulation). We can feel cold air rolling off these stairs. The stair is maybe 20 linear feet and the drywall panel (easily accessible portion) in the garage is maybe 6'-8' long. I want to cut this section out, insulate up into each cavity, add vapor barrier if missing, and patch the drywall.
1) I'm assuming I'll want vapor barrier (6 mil plastic) against the stairs, followed by insulation. Since this is a dry area, an external sheet of Tyvek is not needed. Sound reasonable?
2) Should I consider a spray insulation kit or just fiberglass batts? Would foil-backed foam boards be ideal here?
3) I have a suspicion that one entire side of the cavity will not have a vapor barrier (it's an internal wall). In that case I may need to extend the vapor barrier along that side and tie into the barrier between the garage and house (let's hope that part exists!). Would that change my decision on #2?
I'm going to punch a hole in the drywall tonight to see what I'm dealing with.