When you're looking to insulate your home or building, spray foam is often presented as a superior option to traditional materials like fiberglass or cellulose. But then you're faced with another choice: open cell or closed cell? Understanding the fundamental differences between these two types of spray foam insulation is crucial for making the right decision for your project.
The primary difference lies in the structure of the cured foam, which dictates its physical properties, density, and performance.
Structure: This foam consists of tiny bubbles, or cells, that are deliberately left open or broken. The cells are not completely encapsulated.
Density: It is significantly lighter and less dense, typically weighing around $0.5$ pounds per cubic foot ($pcf$).
Texture: It is soft, spongy, and flexible once cured.
Air/Moisture Barrier: Open cell foam is an excellent air barrier but is permeable to water vapor (it can absorb and hold water). This means it is generally not recommended for exterior applications or areas prone to flooding.
Structure: The cells in this foam are completely closed, packed tightly together, and filled with an insulating gas (usually an HFO or HFC blowing agent).
Density: It is much denser and more rigid, usually weighing between $1.75$ to $2.0$ $pcf$ or higher.
Texture: It is hard and rigid once cured.
Air/Moisture Barrier: Closed cell foam acts as both an air barrier and a vapor barrier. Because of its rigidity and ability to repel water, it can actually add structural strength to the wall assembly.
The R-value is the measure of thermal resistance, or how well the insulation resists heat flow.
| Feature | Open Cell Spray Foam | Closed Cell Spray Foam |
| R-Value (per inch) | $R-3.5$ to $R-3.7$ | $R-6.0$ to $R-7.0$ |
| Air Barrier | Excellent | Excellent |
| Vapor Barrier | No (Vapor permeable) | Yes |
| Sound Dampening | Superior (Due to soft, open structure) | Good |
| Water Resistance | Low (Absorbs water) | High (Repels water) |
| Structural Benefit | No | Yes (Adds rigidity) |
| Expansion Rate | High (Expands 100x its liquid volume) | Low (Expands 40x-60x its liquid volume) |
Key Takeaway: Closed cell foam offers nearly double the R-value per inch compared to open cell. This is critical in applications where space is limited, such as in existing exterior walls or cathedral ceilings, where maximizing R-value in a thin profile is necessary.
| Consideration | Open Cell Spray Foam | Closed Cell Spray Foam |
| Material Cost | Generally less expensive per square foot | Generally more expensive per square foot |
| Common Applications | Interior walls, ceilings, attics, floor joists. | Exterior walls, metal buildings, roofs, crawl spaces, below-grade applications. |
Because open cell foam has a much lower density and higher expansion rate, you need less raw material to fill a given space, making it the more budget-friendly option for large, easy-to-fill areas like attic floors or between roof rafters.
Closed cell foam’s ability to repel water and its higher density make it the necessary choice for any area where moisture control is a top priority or where structural integrity is desired, such as metal framing or in flood zones.
Choosing between open cell and closed cell foam is a matter of balancing your needs for R-value, cost, and location.
If your budget is a concern and you are insulating a dry, large interior space (like an attic or interior wall), open cell is a fantastic choice for its cost-effectiveness, excellent air sealing, and superior noise reduction.
If space is limited, you need the absolute highest R-value, or you are insulating a damp/exterior area (like a basement, crawl space, or metal building), closed cell is the superior and necessary choice due to its high R-value, vapor-sealing capabilities, and structural rigidity.
Before proceeding, always consult with a certified spray foam installer to determine the best product for your specific building codes and performance goals.
The Truth About Spray Foam Roofing Warranties: What Every Contractor Needs to Know
