Open-cell foam
It's soft and spongy, with about R-3.5 to R-4 per inch (typical published figures). It's vapor-permeable, good at blocking sound and generally lower cost. It's common in attics and interior walls.
Closed-cell foam
It's dense and rigid, with about R-6 to R-7 per inch (typical published figures). It's a vapor retarder at typical thicknesses and adds rigidity. It costs more per inch and is common in crawl spaces, rim joists and metal buildings.
Quick guide
- Attic roof deck in a typical home: often open-cell
- Crawl space and rim joists: closed-cell
- Metal shop or barn: closed-cell
- Tight 2x4 walls needing more R-value: closed-cell
- Sound control between rooms: open-cell
Bottom line
Many projects use both. A local installer can recommend the right foam for each area after looking at your home.