Closed cell spray foam is a rigid, 2-lb density polyurethane insulation that seals air, blocks moisture vapor, and delivers about R-6.1 per inch.
Wondering what is closed cell spray foam and whether it fits your project? It is a spray-applied plastic insulation that expands into a hard, dense layer. Unlike its softer cousin, closed cell foam traps gas inside sealed bubbles, which is exactly why it insulates so well per inch. For a gardening or outbuilding project where space is tight, that thin-layer performance is the main draw.
How It Works And What It Feels Like
Two liquid chemicals meet at the spray nozzle and react, expanding rapidly to fill the cavity. The result is a rigid board-like material that is roughly four times denser than open-cell foam and about ten times stronger in compression. It cures with a hard, sandable surface rather than the soft, spongy texture of open-cell foam.
Because the cells stay sealed, the blowing agent stays trapped inside each one. That trapped gas is what delivers the thermal performance.
R-Value, Water, And Air: The Specs That Matter
That high per-inch value makes it the right pick for a 2×4 wall or a shallow roof bay where open-cell foam would not meet code.
| Property | Closed Cell Spray Foam | Why It Matters |
|---|---|---|
| Density | About 2.0 lb/ft³ | Rigid, structural feel |
| R-value | R-6.1 per inch | High insulation in thin layers |
| Closed-cell content | Over 90% | Seals gas inside each cell |
| Water absorption | Under 2% by volume | Hydrophobic, resists moisture |
| Vapor permeance | 0.8 perms at 1-inch | Acts as a vapor retarder |
| Air impermeability | Very low at 1-inch | Functions as an air barrier |
| Compressive strength | 47 psi | Much stronger than open-cell |
The closed cells give it a dual personality. It blocks air movement, so it is an effective air barrier when installed continuously. It also resists water vapor, with the Johns Manville comparison guide noting it is far more moisture-resistant than open-cell foam.
Common Mistakes To Avoid
The biggest error is assuming closed cell is always better. It costs more than open cell and absorbs sound poorly, so a workshop ceiling might perform better with the cheaper, softer foam. Another misstep is treating it as universally waterproof — it resists water, but bulk-water management and drying design still matter.
Fire protection is also easy to overlook. Ignition barriers may be required in occupied spaces under code, so check local requirements before you commit.
Where It Earns Its Keep
Closed cell foam shines in crawl spaces, rim joists, metal buildings, and any spot with limited depth. It adds rigidity to wall assemblies and handles the air-sealing job that fiberglass cannot. Ready to compare products for your project? See our tested roundup of the best closed cell spray foam options for the ones worth buying.
FAQs
Is closed cell spray foam worth the higher cost?
It depends on the job. For thin-wall retrofits, crawl spaces, or any assembly where maximum R-value per inch matters, the premium price is justified. In large open areas where sound absorption or budget is the priority, open-cell foam often delivers better overall value.
Can closed cell foam get wet and still work?
The material itself absorbs almost no water, so a single wetting event does not ruin its insulating value. But it is not a substitute for proper drainage and waterproofing. Bulk water trapped against any structure will find a way to cause trouble, so design for drying regardless.
Does closed cell spray foam stop air leaks completely?
When applied at the correct thickness with no gaps, it performs as an air barrier. The spray method fills cracks, seams, and irregular shapes that batts and boards miss. Complete sealing depends on proper coverage, so application quality matters as much as the material itself.
References & Sources
- Johns Manville. “Spray Foam: Open Cell vs. Closed Cell.” Compares density, cost, and moisture resistance of both foam types.