Open-Cell vs Closed-Cell Spray Foam in Chattanooga, TN

The basic differences

Open-cell foam has a density of about half a pound per cubic foot. The tiny cells in the cured foam are broken open, which makes it soft, flexible and breathable to water vapor. It expands aggressively, roughly a hundred times its liquid volume, so it fills deep cavities in a single pass. Its insulating value runs about R-3.5 to R-3.8 per inch.

Closed-cell foam runs about two pounds per cubic foot. The cells stay intact and hold the blowing agent, which is why it insulates at about R-6 to R-7 per inch. It is rigid, it adds measurable strength to framing it bonds to, it does not absorb water, and at roughly two inches it qualifies as a Class II vapor retarder. It expands less, about thirty to forty times, so it goes on in thinner lifts and costs more per inch of coverage.

Neither is better in the abstract. Each is better somewhere specific.

Where open-cell wins in a Chattanooga house

Roof decks and attics are the classic open-cell application when the assembly is designed for it. A roof deck with five to seven inches of open-cell brings the attic inside the envelope at the lowest cost per R. It also dampens sound noticeably, which people appreciate in a house under a metal roof or near a busy road like Highway 153 or I-24.

Interior walls, floors between stories and the walls of a bonus room over a garage are all good open-cell territory. There is no moisture drive across those assemblies, so the vapor permeability is irrelevant, and the fill quality is what matters.

The caveat in our climate: open-cell on a roof deck can allow indoor humidity to reach the cold sheathing in winter and condense. The solution is to manage indoor humidity, apply a vapor retarder coating, or use a closed-cell flash layer against the deck. We work through that on the assessment based on your roof, your HVAC and how the house is used.

Where closed-cell is the only right answer

Anywhere moisture is part of the picture. Crawl space walls, rim joists, basement walls, the underside of a floor over an open pier foundation, and metal building walls where condensation is the whole problem all call for closed-cell. Open-cell against damp masonry or cold metal will absorb moisture and hold it against the substrate.

Thin cavities are the other case. If you have a 2x4 wall and need to hit a meaningful R-value, closed-cell gets you about R-13 in two inches and leaves room in the cavity; open-cell would need the full depth and still land lower. Same logic applies to cathedral ceilings with shallow rafters.

Closed-cell also adds structural rigidity. On a metal building or an older house with undersized framing that is a real benefit, and it is one of the reasons it is the standard for spraying the underside of metal roof panels.

Cost, thickness and mixing the two

Per inch, open-cell is the less expensive product. Per unit of R-value the gap narrows, and once you account for the fact that closed-cell is doing moisture control that open-cell cannot, the comparison depends entirely on the assembly. The written scope we give you prices the recommended product for each surface, and where two approaches are both defensible we show both.

Hybrid assemblies are common and often the smartest option. A flash-and-batt roof, with one to two inches of closed-cell on the deck followed by open-cell or batts to fill the rafter bay, gets vapor control from the closed-cell and cheap R-value from the fill. A crawl space might get closed-cell on the walls and rim with nothing on the floor above. Matching the product to each surface rather than picking one for the whole house is how the budget goes furthest.