Open cell and closed cell spray foam are both polyurethane, but they behave like different materials on the job site. The choice changes the R-value per inch, the vapor behavior, the cost per square foot, and the structural strength of the assembly. The technical differences between open cell and closed cell polyurethane foam systems show up first in density: open cell weighs about 0.5 pound per cubic foot, closed cell about 2 pounds. That single number drives most of the decisions that follow.
How Open Cell and Closed Cell Foam Differ
The names describe the physical structure of the cured foam. Open cell foam forms cells that burst during expansion, leaving a sponge-like matrix of connected air spaces. Closed cell foam forms sealed bubbles that trap a blowing agent inside, which gives the material its higher density and higher R-value.
The direct open cell vs closed cell comparison for spray polyurethane foam covers the specifications side by side. The short version: open cell is softer, lighter, cheaper, and more vapor-open; closed cell is rigid, denser, pricier, and a vapor barrier at typical thicknesses.
Two densities from the same chemistry
Both foams start as two liquid components, an isocyanate and a polyol resin, that react and expand when mixed at the spray gun. The formulation controls how much the foam rises and whether the cells stay closed. The same chemistry therefore produces both the soft open cell product and the rigid closed cell product.
What the density difference means in practice
Density drives the properties. At 0.5 pound per cubic foot, open cell is light enough to cut with a knife and compresses under thumb pressure. At 2 pounds, closed cell is firm enough to stand on and stiffens the wall assembly it is sprayed into.
| Property | Open cell | Closed cell |
|---|---|---|
| Density | 0.4 to 0.5 lb per cu ft | 1.5 to 2.0 lb per cu ft |
| R-value per inch | R-3.5 to R-3.6 | R-6.0 to R-6.5 |
| Vapor permeability | Permeable, about 10 perms at 1 inch | Vapor barrier at 2 inches or more |
| Water absorption | Absorbs moisture | Resists moisture |
| Structural strength | Minimal | Adds racking strength |
| Typical cost per board foot | $0.30 to $0.50 | $0.75 to $1.10 |
Thermal Performance and Air Barrier Properties
R-value is the headline number, but the air barrier properties matter just as much. Both foams seal the cavity completely, which stops the convection and air leakage that degrade batt insulation. A wall filled with either foam does not need the separate air sealing that fiberglass requires.
The Green Building Advisor analysis argues that open cell foam beats closed cell foam for most wall applications, mainly because the lower cost buys more coverage and the vapor-open structure dries better. The article is worth reading before a contractor quotes the job.
R-value per inch vs installed R-value
Closed cell delivers R-6 per inch, so a 2×6 wall at 5.5 inches reaches about R-33, while open cell reaches about R-19 to R-20 at the same depth. In a 2×4 wall, closed cell hits about R-24 while open cell lands near R-13 to R-14. The gap shrinks when the whole wall is considered, because both foams eliminate the convection losses that cut into batt performance.
Air barrier continuity
Both foams act as the air barrier when they fill the cavity edge to edge, which means the drywall and sheathing do not have to carry the air barrier duty alone. Gaps at the edges are the main failure mode, so application quality matters more than the foam type.
Cost, Coverage, and Installation
Open cell costs about half as much per board foot as closed cell. A board foot is 1 square foot of surface at 1 inch of thickness, so a 2×6 wall, 5.5 inches deep, uses 5.5 board feet per square foot of wall area.
Typical installed pricing runs $0.30 to $0.50 per board foot for open cell and $0.75 to $1.10 for closed cell. On a 2,000-square-foot ranch with 4,000 square feet of wall and roof area at an average 5 inches of foam, the spread between the two systems can reach $6,000 to $10,000.
The buyer’s guide to spray foam for residential construction walks through quotes, coverage charts, and contractor questions before anyone signs. Always ask for the installed board foot price, not the per-square-foot price, because the depth changes the math.
What a board foot means
One board foot is 1 inch of foam on 1 square foot of surface. A contractor quoting $0.90 per board foot on a 5.5-inch-deep wall is charging about $4.95 per square foot of wall. Comparing quotes requires converting every bid to the same units.
DIY kits vs professional installation
Two-component DIY kits ship 200 to 600 board feet of closed cell foam for $300 to $900, which pencils out near professional pricing before the learning curve is factored in. Kits work for small jobs like rim joists and crawl spaces. Whole-house work belongs with a certified installer, because thickness, temperature, and coverage errors are expensive to fix.
Moisture, Vapor, and Condensation
Vapor permeability separates the two foams more than any other property. Open cell lets water vapor pass through, so the wall can dry to the inside or outside. Closed cell blocks vapor at about 2 inches of thickness, which turns the foam itself into the vapor retarder.
The buyer’s guide to open cell and closed cell spray foam compares the vapor and moisture behavior in the context of real assemblies. The rule of thumb: open cell in walls that need to dry, closed cell in assemblies where moisture control is the priority.
Where vapor permeability matters
In a cold climate, an open cell wall needs a vapor retarder on the interior side to keep winter humidity out of the foam. Closed cell provides that protection from within the cavity. In a hot, humid climate, closed cell keeps exterior moisture from migrating inward, which can be the deciding factor.
Condensation on cold surfaces
Closed cell foam on the interior of a roof deck keeps warm, humid air away from the cold sheathing, preventing condensation. Open cell on the same roof deck needs a vapor retarder and careful detailing to do the same job.
Structural Strength and Noise Reduction
Closed cell foam is stiff enough to add real strength to a framed wall. Manufacturer tests and ICC evaluation reports show that a 2×4 wall with 2 inches of closed cell foam on one side resists racking forces significantly better than an unfoamed wall, which matters in high-wind regions and for shear walls.
Open cell foam provides almost no structural contribution, but it outperforms closed cell on sound. The open, porous structure absorbs noise, while the dense closed cell reflects it. For interior walls between bedrooms, open cell or cellulose is the better acoustic choice.
The renovation industry breakdown of spray foam insulation: open cell vs closed cell covers the noise and strength trade-offs in homeowner terms, including which foam suits attics, walls, and basements.
Racking strength and shear
When closed cell foam is used as part of a shear wall, the foam’s adhesion to both the framing and the sheathing distributes lateral loads across the panel. Some designs use the foam to replace a layer of structural sheathing, but the engineering must come from the foam manufacturer’s evaluation report, not guesswork.
Sound transmission through foam-filled walls
A wall filled with open cell foam and two layers of drywall typically outperforms the same wall with fiberglass batts on sound tests. Closed cell foam performs closer to a rigid panel, which helps block low-frequency noise but reflects mid-range sound instead of absorbing it.
Matching the Foam to the Assembly
No single foam wins every job. The right choice depends on the assembly, the climate, and the budget. These patterns hold across most projects:
- Roof decks: closed cell when the roof cannot dry to the outside and there is no room for a vent channel; open cell with a vapor retarder when drying is possible.
- Exterior walls: open cell for most wood-frame walls, closed cell in hot-humid climates and for thin walls.
- Basements and crawl spaces: closed cell for the below-grade walls, because it resists moisture and adds strength.
- Interior noise walls: open cell, or cellulose, for sound absorption.
- Rim joists and band boards: closed cell for the small cavities where R-value per inch and vapor blocking both help.
The performance, cost, and installation comparison for open cell and closed cell polyurethane foam pulls the numbers together for a final pass. The pattern from real projects: closed cell for thickness-constrained assemblies, open cell everywhere else.
A decision checklist
- Is the cavity thick enough that R-3.5 per inch reaches your target? If yes, open cell is usually the value pick.
- Does the assembly need a vapor retarder or a moisture barrier? Closed cell provides both at 2 inches.
- Will the assembly dry if it gets wet? Open cell dries faster, closed cell keeps water out.
- Does the wall carry structural loads? Closed cell adds strength, open cell does not.
- What does the quote say per board foot? Convert everything to board feet before comparing.
Retrofit and addition work
Open cell foam is the standard choice for most retrofits because it expands into irregular cavities and seals around obstructions without over-pressurizing the wall. Closed cell in a retrofit can push drywall off the framing as it expands, so the crew must control thickness carefully.
The open cell versus closed cell decision comes down to three questions: how thick the cavity is, whether the assembly needs vapor blocking, and what the board foot price is. Most homes settle on open cell for walls and closed cell for basements and roofs. The dedicated comparison of insulating basement walls with spray foam shows how the choice plays out below grade, where moisture does the most damage.