I’m installing a radiant barrier to the walls in an old house that never even had insulation. I’m using Kraft paper back rolls of fiberglass insulation between the 2×4’s. Where exactly should the radiant barrier be installed? Against the outside vapor barrier before installing the insulation, or on the “sheet-rock” side of the insulation?
As a general rule you want the vapor barrier on the inside of the wall system rather than the outside. That’s because moisture usually comes from the inside of the wall rather than outside, so you want to stop its migration from getting into the wall system. You can install a radiant barrier on the inside of the stud wall and that will function as a vapor barrier as well. In order to get the air space the radiant barrier needs, you will have to install furring strips on the studs after the radiant barrier is in place and then attach the drywall.