Open-Cell Spray Foam Insulation

Open-cell spray foam gets its name from the tiny, broken air cells inside the cured material — cells that stay open rather than sealed shut, which is what makes the foam soft, flexible, and lighter than its closed-cell counterpart. It typically delivers an R-value in the R-3.5 to R-3.7 per inch range, and because it's applied as a liquid that expands on contact, it fills gaps around wiring, plumbing, and odd framing angles that pre-cut batts routinely miss.

That same open-cell structure makes the foam vapor-permeable rather than a vapor barrier, meaning it allows some moisture vapor to pass through rather than trapping it inside a wall cavity. In most above-grade, interior applications that's exactly what you want — it lets a wall assembly dry out in whichever direction it needs to rather than sealing moisture in on one side. It's also why open-cell isn't the right call for crawl spaces, rim joists, or other areas with regular moisture exposure; that's a job for closed-cell foam instead.

Interior wall cavities are the most common application — open-cell foam fills the stud bay completely, stopping the air movement that a compressed or gapped batt lets through, and adding real sound-dampening between rooms in the process. It's a popular upgrade for additions and full remodels, where the wall is already open and a single application can replace what would otherwise be a batt-and-vapor-barrier job.

Attic floors and the underside of roof decks in a conditioned, above-grade attic are another strong fit, especially in homes where ductwork or HVAC equipment sit in the attic space and benefit from being brought inside the building's thermal envelope. Open-cell is also a favorite for home theaters, media rooms, and any shared wall where reducing sound transfer matters as much as temperature control.

The most common question we get on a walkthrough is which foam a given space actually needs, and the honest answer is that it depends on moisture exposure and available cavity depth more than personal preference. Open-cell costs less per square foot and performs beautifully in interior, above-grade applications, but it isn't moisture-resistant and it delivers less R-value per inch than closed-cell foam, which matters in a shallow cavity where every fraction of an inch counts.

For crawl spaces, rim joists, exterior walls in tight cavities, and roof decks exposed to real moisture risk, closed-cell foam is the better tool for the job — see our closed-cell spray foam service for that comparison in detail. Most whole-home projects actually use both: open-cell where it makes sense on cost and interior walls, closed-cell where moisture resistance and R-value per inch matter most. We'll walk the job with you and recommend the mix that fits your home and your budget rather than defaulting to one product everywhere.