Open-cell foam is the lighter, softer cousin of closed-cell - roughly half a pound per cubic foot of mostly-air structure. It won't do everything closed-cell does, and it isn't supposed to. Where the job is an interior wall, a quiet room, or an attic on a budget, open-cell delivers an air-seal at a lower cost per board foot, and its one standout trait - sound dampening - is something closed-cell can't match.
What it does well
- Interior walls and demising walls. Air-seals and insulates at R-3.5 to R-3.8 per inch without the premium of closed-cell.
- Sound control. Its open, spongy cell structure absorbs airborne noise far better than dense foam - the reason it shows up in media rooms, shared walls and bedrooms.
- Budget attic work. Where a vapour barrier isn't the job and depth is available, open-cell reaches attic R-values for less.
Where it does not belong
Open-cell is vapour-permeable and absorbs water, so it stays out of the places closed-cell owns: below grade, crawlspaces, exterior walls, metal buildings, and anything exposed to moisture. Put it where it can breathe and stay dry, and it's excellent; put it below grade and it's the wrong tool.
The honest trade
Choosing between the two isn't about which foam is "better" - it's about matching density to the assembly. A good installer will often spec both in one house: closed-cell where moisture and R-per-inch matter, open-cell where sound and budget lead. Read the other side of the decision on the closed-cell page, then get a quote and let a vetted installer spec the mix.