Closed-cell spray foam
Also called: closed cell foam, 2-lb foam, medium-density spray foam, ccSPF
Dense, rigid spray polyurethane foam (about 2 lb per cubic foot) with sealed gas-filled cells: R-6 to R-7 per inch, a vapor retarder at 1.5–2 inches, water-resistant and structurally stiffening. $1.30–$2.50 per board foot installed; nearly all DIY kits are closed-cell.
Closed-cell foam is the higher-performance, higher-cost option. Its cells are sealed and filled with a blowing agent, which gives it about twice the R-value per inch of open-cell foam, makes it a Class II vapor retarder at about 1.5–2 inches, keeps it from absorbing water, and adds enough rigidity that it measurably stiffens a wall or roof. It’s specified where those properties matter: rim joists, crawl space and basement walls, garages and pole barns, metal buildings, under floors, and cold-climate roof decks where the foam has to keep warm indoor air from reaching cold sheathing.
It costs more per board foot but needs less depth for the same R-value, so on a finished job the gap with open-cell is smaller than the per-foot prices suggest. It cures with heat, which is why it’s sprayed in 1–2 inch lifts — a single thick pass can overheat, shrink from the framing or scorch.
DIY two-part kits are almost all closed-cell, at $1.30–$2.00 per board foot — about what a contractor charges installed, since contractors buy foam by the drum. Like all spray foam, it needs an ignition barrier or thermal barrier when left exposed.