17/09/2022
Double-stud wall vs. exterior insulation—which makes the most sense?
Here are some considerations when choosing between these two high-performance wall assemblies:
Double-stud walls, which are two framed stud walls separated by a gap with the entire assembly filled with insulation, are often thought to be susceptible to moisture accumulating in the cavity. But is that true? calls it the “yeti of building science”—a beast of myth not reality. The argument: if the assembly is well air-sealed and includes a rainscreen to promote drying to the exterior, the threat is alleviated.
A GBA Expert Member adds: Double-stud walls and those with exterior foam can be both be detailed so they are simple or complicated. That makes more difference to their longevity than which one you choose.
Another reader recommends using either CDX or RTD plywood for the sheathing on double-stud walls, saying, when combined with dense-pack cellulose, air-sealing, and a rainscreen, the sheathing wetting risk is near zero.
Many green builders try to stay away from foam because of its environmental impact. If it is attractive for a higher R-value over the double-stud wall with cellulose, one option is to use mineral wool instead.
Those in favor of exterior foam say it keeps the exterior side structural sheathing warmer, which usually means drier. Building scientist agrees, saying, a wall with sufficient continuous exterior insulation has greater wintertime moisture safety than a double-stud wall. He cites studies proving continuous exterior insulation keeps the exterior sheathing warmer, reducing risks of condensation, compared to a double-stud wall that has sheathing at cold outdoor temperatures.
That said, he’s not against double-stud walls. For details from Kohta on how to ensure the longevity of both types of walls, click the Linkin.bio.
