Open-cell spray foam
Also called: open cell foam, half-pound foam, low-density spray foam, ocSPF
Light, soft spray polyurethane foam (about 0.5 lb per cubic foot) with interconnected cells: R-3.5 to R-3.8 per inch, an excellent air seal but vapor-open and not water-resistant. $0.60–$1.20 per board foot installed; the usual choice for attic roof decks and interior walls.
Open-cell foam expands 100 times or more as it’s sprayed, filling a cavity in one pass and air-sealing it completely. It’s cheap per board foot, quiet (it absorbs sound), and flexible enough to move with framing without cracking. Because its cells are open it lets water vapor pass and will soak up liquid water, so it isn’t used where it can get wet — rim joists, crawl spaces, below grade or on metal buildings — and in cold climates it may need a vapor retarder on the warm side, or a layer of closed-cell foam against the sheathing first.
Its low R-value per inch means depth: R-30 is about 8.5 inches, and a 2×6 wall filled full gets about R-20. That’s why it’s the standard foam for an unvented attic roof deck, where rafters are deep and the foam is often left exposed under an ignition barrier, and for interior walls where sound control matters more than R-value.
Open-cell isn’t widely sold in DIY kits; the two-part kits at hardware stores are closed-cell. A contractor quoting “spray foam” for an attic without saying which type usually means open-cell — ask, and get the depth and R-value in writing.