Open-Cell vs Closed-Cell Spray Foam: Which Is Right for Your Upstate Home?

We’re South Carolina Spray Foam Insulation, a family-owned company that’s spent more than 15 years insulating homes and commercial buildings across Greenville and the rest of the Upstate. The open-cell versus closed-cell question comes up on nearly every estimate we run, from attics in Greer and Taylors to crawl spaces in Anderson and Seneca. The honest answer is that neither foam is universally better — they’re different chemistries built for different jobs, and the right one depends on where it’s going and what it has to stand up to.

The Two Chemistries, in Plain Terms

Open-cell spray foam is the lighter, softer product. Its cells stay partly open after curing, so it expands aggressively on application — one pass fills a deep rafter or stud bay — and it remains vapor-permeable, meaning water vapor can move through it rather than stopping at the surface. It delivers roughly R-3.5 to R-4 per inch.

Closed-cell foam cures dense and rigid. Its cells are sealed shut, so it stops air movement, acts as a vapor retarder, and adds stiffness to whatever it bonds to. It delivers roughly R-6 to R-7 per inch — about twice the R-value of open-cell at the same thickness, which matters when you only have a few inches of depth to work with.

How Each One Handles Moisture

This is the difference that decides most jobs in our climate. The Upstate sits in IECC Climate Zone 3A — humid subtropical — and from late spring through early fall the outdoor air carries real moisture. Beneath most homes along the I-85 corridor, the Piedmont red clay releases ground dampness steadily upward into the crawl space. Put a vapor-permeable product in that environment and moisture migrates through it toward the wood framing.

That’s why closed-cell goes on every moisture-facing surface we insulate: crawl space foundation walls, rim joists, and anywhere else ground or outdoor moisture can reach the foam. Two or three inches delivers meaningful R-value and a vapor retarder in a single pass. Open-cell’s permeability isn’t a defect — in a dry attic or an interior partition there’s no moisture load for it to manage. It’s simply the wrong tool below grade, and we treat it that way.

Where Each One Belongs

Closed-cell: crawl space walls, rim joists, and foundation surfaces — anywhere moisture is part of the picture, or the cavity is shallow and you need the most R-value per inch you can get.

Open-cell: attic rooflines and floors, interior walls, and anywhere sound control matters. Open-cell dampens noise noticeably, which is why homeowners in Simpsonville and Mauldin ask for it between floors and around bonus rooms — and deep rafter bays give it the thickness it needs at a lower cost per board foot.

What Each Costs Per Board Foot

Insulation is priced per board foot — one square foot of coverage at one inch thick — so the total depends on the thickness applied and the area covered. Open-cell runs $1.00 to $1.60 per board foot installed; closed-cell runs $1.60 to $2.60 per board foot installed. Thickness moves the number proportionally: two inches of closed-cell costs roughly double the single-inch rate, three inches roughly triple. Depth required, access, and existing conditions all shift the final figure, which is why we quote only after an on-site insulation evaluation — never a price sight unseen.

Duke Energy serves most of the Upstate and offers rebates for qualifying insulation and air-sealing work. The amounts vary by measure and change annually, so confirm current amounts before you schedule. On the federal side, the 25C energy-efficiency tax credit expired December 31, 2025 — no federal insulation credit currently applies, whatever an out-of-date website tells you.

Deciding for Your Own Home

Three questions settle most of these decisions. First, is moisture involved? A crawl space, rim joist, or foundation means closed-cell, nearly every time. Second, how much depth do you have? Tight cavities favor closed-cell’s higher R-per-inch; deep, dry attic bays let open-cell reach the target R-value for less money. Third, is sound a goal? Open-cell wins there, and it isn’t close.