Double wall design and condensation

A homeowner in Nova Scotia is planning a new single-story house on screw piles and asked for advice on a "double wall" design: drywall, a 2x4 stud wall with fiberglass insulation, a 5-inch layer of rigid foam in the middle, a second 2x4 stud wall with fiberglass, plywood sheathing with taped seams, a moisture barrier, and metal siding outside. The question was about whether this would work and what kind of moisture barrier to use.

A building science expert responded that the "moisture barrier" the homeowner meant is really a WRB, or weather-resistant barrier, something like Tyvek housewrap installed over the exterior sheathing, behind the siding. He warned against using poly sheeting in this spot, since the thick foam layer in the middle already blocks moisture flow well, and adding poly would create a "double vapor barrier" problem that can trap moisture inside the wall.

The expert also questioned the overall design. He said these foam-in-the-middle double stud walls tend to cost more and take more labor to build, without necessarily performing better. Instead, he suggested skipping the rigid foam layer entirely and simply filling the same wall depth with thicker fiberglass batts or dense-packed cellulose insulation. By his rough math, that approach could reach roughly R41, a strong insulation value, without the added expense and complexity of the foam layer.

For anyone considering a similar double-wall build, the exchange is a reminder that adding foam to a wall assembly isn't always the most efficient path to a well-insulated, moisture-safe house, and that WRB placement and vapor-barrier choices matter for avoiding trapped moisture.