Open-Cell vs. Closed-Cell Spray Foam: A National Comparison Guide

If you only read one section, read this one. The table below lines up the core specs side by side — density, R-value per inch, vapor behavior, typical national cost, and the assemblies each foam tends to fit best.

Density is simply how much material is packed into a cured cubic foot. Open-cell foam's tiny bubbles rupture and interconnect as it cures, leaving a soft, spongy material at roughly half a pound per cubic foot. Closed-cell foam's bubbles stay sealed and intact, packing in roughly four times the material at around two pounds per cubic foot. That difference in raw material is the single biggest reason closed-cell costs more — you're paying for meaningfully more foam chemistry per square foot sprayed.

R-value per inch is a measure of resistance to heat flow, and it's where the density difference pays off thermally. Open-cell lands around R-3.5 to R-3.7 per inch; closed-cell lands around R-5 to R-7 per inch. In practice that means hitting a code-required R-value target takes noticeably less material thickness with closed-cell foam — useful in a shallow assembly like a metal-building roof purlin space, where there isn't much cavity depth to work with in the first place.

Vapor permeability is the property that trips people up most, because it's the one that isn't just about R-value math. Open-cell foam is vapor-permeable at any practical thickness — moisture vapor passes through it, which lets an assembly dry in whichever direction it needs to, a genuine advantage in some wall designs. Closed- cell foam, once applied at sufficient thickness, functions as a vapor retarder, meaning it blocks that vapor drive almost entirely. Neither behavior is inherently better; it depends entirely on whether the assembly underneath needs to dry outward, needs a vapor block, or needs something in between. This is the property that climate region and assembly type both bend around, which is why it gets its own two sections below.

The textbook answer to “which foam should I use” changes noticeably once you factor in climate, because vapor control matters far more in some regions than others. Spray foam insulation contractors think in terms of climate zones rather than state lines, and this site organizes that same logic into five broad regions for buyer education (see find contractors by region for the full breakdown).

In Northeast and Cold-climate regions, high heating-degree-days push attic and wall R-value targets to the top of the national range, and vapor drive direction (moisture moving from warm interior air toward cold exterior surfaces in winter) makes vapor-retarder detailing a real consideration on roof decks and in certain wall assemblies — closed-cell gets specified there more often than the national average.

In Southeast and Humid climates, the concern flips direction — warm, moisture-laden outdoor air pushing vapor inward — and closed-cell's vapor-retarder behavior is frequently the right tool on roof decks, in coastal construction, and in hurricane-zone assemblies where wind-driven moisture is a real risk. This is the region where the open-cell-versus-closed-cell decision leans closed-cell more consistently than anywhere else in the country.

Southwest and Hot-Dry regions are the opposite case: with low ambient humidity, vapor drive is a minor concern next to cooling load and air-sealing, so open-cell foam's lower cost and strong air-sealing performance make it the default across most assemblies, roof decks included.

Midwest homes see a genuine four-season swing — real summer humidity and real winter cold in the same envelope — so the decision tends to hinge on the specific assembly (roof deck versus attic floor, for instance) more than the region as a whole. Pacific and Marine climates are mild but persistently damp year-round, which is often enough moisture exposure to tip roof decks and exterior assemblies toward closed-cell even without the temperature extremes seen elsewhere.

None of this is a precise zone-by-county rule — it's a directional pattern, and the exact code-required vapor-retarder class for your specific assembly and jurisdiction is something your matched contractor will confirm, not something to assume from a region name alone.

Climate region sets the backdrop, but the specific assembly you're insulating usually settles the decision. Here's how the two foams typically get assigned across the four assemblies that come up most often:

Roof decks. This is where the two foams diverge most sharply. A sealed (unvented) roof-deck assembly puts the insulation directly against roof sheathing that experiences real moisture exposure from both directions, and closed-cell's higher R-value per inch and vapor-retarder behavior are frequently the default in humid and cold-climate regions. Open-cell can still work on roof decks in drier climates, but it's more often paired with a separate vapor-retarder membrane when it's used there.

Interior walls. Nearly the opposite pattern — interior wall cavities aren't exposed to bulk moisture or extreme temperature swings on either side, so open-cell's lower cost, strong air-sealing, and sound-dampening properties make it the default choice in the large majority of climates and assemblies. Closed-cell in an interior wall is usually reserved for cases where the added racking strength matters structurally, not for moisture control.

Exterior and below-grade assemblies. Foundation walls, rim joists, and any surface with direct or likely exposure to ground moisture or bulk water lean closed-cell almost everywhere in the country, regardless of region — its closed cell structure resists water absorption in a way open-cell's spongier structure simply doesn't, which matters most where a leak or moisture intrusion is a realistic possibility rather than a remote one.

Metal buildings. Metal building roof and wall panels condense moisture readily against a cold metal skin, and the cavity depth in a purlin or girt space is often shallow. Closed-cell's higher R-value per inch fits more thermal performance into a shallow space, and its adhesion and vapor-retarder behavior are well suited to a metal substrate — which is why it's the more common recommendation for commercial and agricultural metal-building envelopes. See commercial spray foam for more on how that bidding process typically works.

Nationally, closed-cell foam runs roughly twice what open-cell costs per square foot — directionally $3.00-$5.00 per square foot for closed-cell versus $1.50-$3.50 for open-cell, translating to roughly $1.00-$1.80 per board foot versus $0.40-$1.00 per board foot in the trade pricing most contractors actually quote from. That gap is real and it adds up fast across a whole roof deck or building envelope, which is exactly why it's worth being deliberate about where the upcharge is buying something and where it isn't.

The upcharge is generally worth paying on roof decks in humid or cold climates, on exterior and below-grade surfaces with real moisture exposure, on metal buildings with shallow cavity depth, and anywhere added racking strength has genuine structural value. In each of those cases you're paying for a property — moisture resistance, higher R-value per inch in limited depth, or rigidity — that open-cell foam structurally can't deliver, not just paying more for the same outcome.

The upcharge is usually not worth paying on interior walls, standard attic floors in dry or mixed climates, and sound-dampening applications, where open-cell delivers the practical result — a well air-sealed, well-insulated cavity — at a meaningfully lower material cost. A contractor who defaults to closed-cell everywhere because it's the higher-margin sale, rather than because the assembly calls for it, is worth questioning on that point directly.

A reputable contractor should be able to answer these questions specifically, assembly by assembly, rather than giving one blanket answer for the whole house:

A contractor who answers these clearly, assembly by assembly, is demonstrating that they've actually scoped your project rather than applying a one-size-fits-all quote. That's the same instinct worth applying when you're comparing multiple quotes side by side — foam type and thickness are exactly the two details that make quotes comparable or incomparable in the first place.

Related reading: Open-Cell Spray Foam: A Buyer's Guide · Closed-Cell Spray Foam: A Buyer's Guide · How to Compare Spray Foam Insulation Quotes