Spray Foam Chemistries: Closed-Cell vs. Open-Cell

Understanding The Chemical Architectures

While both open-cell and closed-cell spray foam combine two liquid resins at the spray gun, their chemical architectures dictate entirely different applications.

If you are trying to answer the broader search query best spray foam insulation, read this chemistry page alongside our product-standard guide on best spray foam insulation. That page explains why we use Accufoam AF1 for open-cell work and Accufoam CC when the assembly needs closed cell.

If you specifically need thickness math, read our closed-cell spray foam R-value page next.

Open-Cell Foam (Half-Pound Foam): The cells within the foam are intentionally left open, allowing the chemistry to expand massively, up to 100 times its liquid size. It cures into a soft, sponge-like material. It is a phenomenal air barrier and sound dampener, but it allows water vapor to pass through it. It is the dominant choice for above-grade exterior walls and unvented roof decks because it fills complex cavities rapidly at a highly competitive cost.

Closed-Cell Foam (Two-Pound Foam): The cells remain completely closed and packed tightly together, trapping dense insulating gas inside. It cures into a rigid, rock-hard plastic. Because of this density, its expansion rate is much lower, around 30x. However, it boasts a massive R-value per inch and acts as a Class II vapor retarder, actively resisting bulk water and moisture vapor. It is the mandatory choice for high-moisture environments.

Builder and Developer Notes

Specifying the wrong foam chemistry in the wrong assembly leads to catastrophic moisture failures. Relying on a one-product-only subcontractor usually signals a lack of technical expertise.

Where to specify open-cell:

• Unvented roof decks: Excellent for grabbing complex rafter trusses and keeping HVAC in conditioned space.
• Above-grade framed walls: Provides full cavity fill and aggressive air sealing.
• Interior partitions: Great for sound attenuation between bedrooms and media rooms.

Where to specify closed-cell:

• Crawl space foundations: Mandatory for perimeter block walls and band/rim joists. Read our deep dive on encapsulating crawl spaces vs. traditional sub-floor batts.
• Shallow framing: When you must hit a high R-value but only have a 2x4 wall or shallow rafter depth.
• Metal buildings: Applies directly to steel decking without trapping condensation risk against the metal.

Scope language to include in your bid request: Ensure your contractors clarify their exact product intent. Bidding “spray foam” is not enough. You must specify the depth and the cell type, for example, “3 inches of closed-cell in the crawl space, 6 inches of open-cell at the roofline.”

Risk flags to avoid:

• Water wicking: Never use open-cell foam below grade, on a foundation wall, or near exterior masonry that can absorb moisture.
• Thermal barriers: Both open-cell and closed-cell foams are cellular plastics and require code-approved thermal or ignition barriers, usually half-inch drywall or intumescent paint, depending on the assembly’s accessibility.

Request a Takeoff for Both Chemistries

Comparison Table: Open-Cell vs. Closed-Cell

Metric	Open-Cell (0.5 lb)	Closed-Cell (2.0 lb)
Typical R-Value per Inch	R-3.7 to R-3.9	R-6.5 to R-7.0
Air Sealing capability	Excellent	Excellent
Moisture / Vapor Resistance	Poor (Vapor Permeable)	Excellent (Class II Vapor Retarder)
Structural Rigidity	None (Soft, flexible)	High (Adds racking strength)
Acoustic Dampening	Superior (Absorbs frequencies)	Moderate (Deflects sound)
Cost per Board Foot	Lower (High expansion yield)	Higher (Dense, lower yield)

Local Relevance: Designing in Middle Tennessee

Nashville’s volatile climate forces builders to respect both thermodynamics and fluid dynamics. In Climate Zone 4A, summer humidity demands strict vapor management.

Due to the heavy use of crawl spaces in Davidson and Williamson counties, closed-cell foam is the undisputed king of sub-grade foundation encapsulation. However, pushing closed-cell foam throughout the entire above-grade envelope of a massive custom home is often cost-prohibitive overkill. The smartest local builders specify a hybrid approach: closed-cell below grade for water defense, open-cell at the roof deck and 2x6 exterior walls to handle the massive latent heat load at a manageable price point.

Homeowner Notes

If you request a spray foam quote and a contractor insists that open-cell foam is useless, or conversely that closed-cell foam is the only real foam, you are speaking to a salesperson, not a building-science-minded contractor. Both products are required to build a high-performing house. Expect your estimate to combine the products precisely where their respective chemistries perform best.

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References

We define our application rules based on strictly vetted science:

• Building Science Corporation - Deep research on vapor permeability and cell structures in mixed-humid climates.
• Spray Foam Alliance (SPFA) - Yield dynamics and application safety of SPF chemistry.
• International Code Council (ICC) - Code definitions for vapor retarders matching closed-cell performance.