Double 2×4 wall detail – concerns over potential for moisture in the wall

This is a homeowner forum post, not a finished project, so there's no news to report here — just one person's question about a wall design for a house addition in western New York. The homeowner is building an addition using a "double stud wall": two separate 2x4 stud walls, each filled with rockwool insulation, built side by side to create an 8-inch-thick wall. This approach can give good insulation using ordinary lumber and materials rather than specialty foam panels.

The question is where to place the vapor barrier, a layer that limits moisture from passing through the wall. The homeowner wants to put it between the two stud walls, both because it's easy to install and because it would let the inner wall serve as a channel for electrical wiring without punching holes through the barrier. The concern is whether that mid-wall location stays warm enough in winter to avoid condensation, given average winter temperatures around 25°F in that area.

For a homeowner following home performance topics, this is a useful example of the kind of moisture and insulation-placement questions that come up in double-stud wall construction, a method sometimes used in additions or new builds aiming for higher insulation levels. It does not involve a rebate, product launch, or code change, so there is nothing here that affects existing homes or current upgrade programs.