Spray Foam Makes the Building Stiffer

Why it stiffens a wall

A stud wall resists racking - being pushed out of square by wind - mainly through its sheathing and fasteners. Fill the cavities with a rigid foam that adheres to the studs and the sheathing on every face, and you have effectively glued the assembly together. Stud walls filled with spray-applied polyurethane show a significant increase in racking strength over empty ones.

Batt insulation contributes nothing here. It is a loose material resting in a space.

It also cannot shrink, settle or fall down

Because it is attached rather than resting, there is nothing for gravity to act on. Forty-year-old fiberglass in a Maine attic has usually compressed to half its thickness and slid down the rafter bays. Cured foam is in the same position it was sprayed in, and as an inert plastic it stays there as long as the building does.

The quiet

People notice this more than they expect to. Sealing every air path also seals the paths sound was traveling along - traffic, wind, rain on a metal roof, aircraft. Open-cell is the better of the two for sound because it is softer and absorbs rather than reflects, which is why it is worth considering in interior walls where heat is not even the point - a bedroom wall, or between a garage and living space.

And design flexibility

Because closed-cell delivers roughly R-6 to R-7 per inch, a 2x4 wall can hit a number that would otherwise need a deeper assembly. On a renovation where you cannot make the wall thicker, that is sometimes the whole reason to use it. More on R-value.