Last updated: June 13, 2026 | Written by Diana Mills | Reviewed by Robert Kowalski
One of the most common questions we hear from homeowners and new contractors is: “What’s the difference between open-cell and closed-cell spray foam, and which one should I use?” It’s a fundamental question with a nuanced answer — and getting it wrong can cost you thousands of dollars in performance, materials, or remediation.
This guide covers everything you need to know to choose the right foam type for your application.
The Chemistry Difference in Plain English
Both open-cell and closed-cell polyurethane foam are created by mixing two liquid components (called “A” and “B” sides) that react on contact to create a foam. The key difference is what happens to the bubbles (cells) in the foam as it cures:
- Open-Cell Foam (0.5 lb density): The cell walls break as the foam expands, creating an interconnected, sponge-like structure. This makes the foam soft and squishy, with a density of about 0.5 lb per cubic foot, and gives it excellent sound absorption properties. Water and air vapor can move through it.
- Closed-Cell Foam (2 lb density): The cell walls remain intact as the foam cures, trapping the blowing agent gas inside each cell. This creates a rigid, dense structure (about 2 lb per cubic foot) that resists both air and liquid water transmission. It’s hard, strong, and is a vapor retarder.
Side-by-Side Comparison
| Property | Open Cell (0.5 lb) | Closed Cell (2 lb) |
|---|---|---|
| Density | 0.5 lb/cu ft | 2.0 lb/cu ft |
| R-Value per inch | R-3.7 to R-3.9 | R-6.5 to R-7.0 |
| Air Permeance | Air barrier at 3.5″+ | Air barrier at 2″+ |
| Vapor Permeance | Permeable (Class III vapor retarder) | Low perm (Class II at 2″+) |
| Water Resistance | Absorbs water; dries out | Highly water resistant |
| Structural Contribution | None | Yes – adds racking strength |
| Sound Attenuation | Excellent (STC improvement) | Moderate |
| Cost per Board Foot (installed) | $0.44–$0.65/bf | $1.00–$1.50/bf |
| Typical Applications | Interior walls (sound), attic deck, interior rafter bays | Below grade, rim joists, roof assemblies, exterior walls in cold climates |
When to Use Open-Cell Foam
Attic Rafter Bays (Hot Roof/Unvented Attic)
Open-cell foam in attic rafter bays creates an unvented (hot roof) assembly that brings the attic inside the thermal envelope. At 5.5″ to 7″ thickness, it achieves R-21 to R-27 and creates an excellent air barrier. Because it’s vapor-permeable, it allows drying to the interior — which is the correct design for certain climate zones. Open-cell at 5.5″ costs roughly 60% less than equivalent closed-cell, making it the cost-winner for large attic applications in warmer climates.
Important caveat: In Climate Zones 5, 6, and 7, open-cell foam in unvented attic assemblies requires either a vapor retarder on the warm side or a vapor-permeable roofing assembly to prevent condensation. Check with a building scientist or review IRC Section R806.5 for your specific situation.
Interior Partition Walls (Sound Control)
Open-cell foam’s sponge-like structure makes it exceptional for sound absorption. A 3.5″ application of open-cell foam in a 2×4 wall cavity can improve the wall’s STC (Sound Transmission Class) rating by 8-12 points compared to an empty cavity, and 4-6 points compared to fiberglass batts. For home theaters, bedrooms adjacent to mechanical rooms, or party walls in multifamily construction, open-cell is the foam of choice.
When to Use Closed-Cell Foam
Rim Joists and Crawlspaces
Closed-cell is almost universally the correct choice for rim joists and crawlspace walls. Its water resistance, vapor retarder properties, and superior R-value per inch make it ideal for these moisture-prone, space-constrained applications. See our full rim joist insulation guide for details.
Below-Grade Walls and Foundations
Any application where the foam may come in contact with moisture should use closed-cell. Below-grade walls, foundation walls, and concrete block walls are all closed-cell applications. Open-cell foam in these locations will absorb groundwater and serve as a moisture reservoir.
Exterior Walls in Cold Climates (Zone 5+)
In very cold climates, the dewpoint within the wall assembly can fall within the insulation layer itself. If this happens with open-cell foam, condensation forms within the foam — a serious moisture problem. Closed-cell foam, as a vapor retarder, prevents this by controlling where vapor can diffuse. This is not a concern in warmer climates.
The Hybrid Approach
As discussed in our spray foam vs fiberglass guide, the hybrid closed-cell + fiberglass approach is increasingly popular. A thin layer (1-2″) of closed-cell foam on the cold sheathing side, followed by open-cell or fiberglass to fill the remaining cavity, captures the vapor control and air sealing benefits of closed-cell while dramatically reducing material costs.
Quick Decision Guide
| Application | Recommended Foam Type |
|---|---|
| Rim joists | Closed-Cell (2 lb.) |
| Crawlspace walls | Closed-Cell (2 lb.) |
| Attic rafter bays (Climate Zones 1-4) | Open-Cell |
| Attic rafter bays (Climate Zones 5-8) | Closed-Cell or engineered hybrid |
| Interior sound walls | Open-Cell |
| Exterior walls, new construction (Zone 5+) | Closed-Cell or hybrid |
| Below-grade walls/foundation | Closed-Cell (2 lb.) |
| Roof deck (flat/low-slope) | Closed-Cell (roofing grade) |
Still not sure? Post your specific application details in our Community Forum, and our experts will give you a recommendation for free. Or visit our Resources page to download the complete application selection guide.
Diana Mills is the founder and editor of Foam Insulation Review. With over 15 years of experience in the building materials and spray foam industry, she has tested hundreds of products, interviewed leading contractors, and published in-depth guides for homeowners and professionals alike. Diana is passionate about helping people make smarter insulation decisions.
