An architect recently told a friend of mine that he was designing a flash-and-batt wall with the spray foam insulation installed on the interior, and the batt insulation toward the exterior, “because that way the vapor barrier will be on the right side of the assembly.”
Anyone who wants to address this architect’s suggestion will need to unpack the assumptions behind his statement. For example, it’s worth discussing whether walls need vapor barriers. It’s also worth discussing why flash-and-batt walls almost always have the spray foam insulation near the exterior rather than the interior of the wall assembly.
To help people like this architect, my friend suggested that I write an entry-level article on wall design, starting from first principles.
When designing a wall, builders can choose from a wide range of materials. Available insulation materials include rigid foam, spray foam, fiberglass batts, mineral wool, and cellulose. Sheathing options include OSB, plywood, gypsum sheathing, and Zip sheathing (with or without adhered polyisocyanurate insulation).
To put these materials together wisely, a designer needs to understand a few basic principles involving water entry, vapor transmission, and air leakage. Moreover, a designer needs to understand these principles well enough to sense which types of assemblies are risky and which are safe.
To point designers in the right direction, I’ve boiled my advice down to five principles.
Regardless of what type of wall you are building, strive for airtightness. The tighter the wall, the better it will perform.
If you can’t control air flow through your walls, it’s hard to maintain the interior conditions than most homeowners expect. Air leaks cause many problems: they increase heating and cooling costs, of course, but they can also lead to moisture problems. During the winter, exfiltrating air can carry a lot of moisture,…