Spray polyurethane foam has become one of the most powerful tools in modern building envelopes, prized for its ability to insulate and air-seal in a single application. But spray foam is not a single product. It comes in two fundamentally different forms, closed-cell and open-cell, and treating them as interchangeable leads to expensive mistakes. Understanding how each behaves helps you place the right foam in the right assembly.
The Basic Chemistry of Two Foams
Both foams are created on site by mixing two liquid components that react and expand within seconds. The difference lies in the structure of the bubbles, or cells, that form as the foam rises. In open-cell foam, the cells are not fully enclosed, so they remain interconnected and filled with air. This produces a soft, spongy, low-density material. In closed-cell foam, the cells are completely sealed and filled with a gas, creating a rigid, dense material that is far more resistant to the passage of air, water, and vapor.
This structural distinction drives every practical difference between them. Open-cell foam is light and flexible, typically weighing about half a pound per cubic foot. Closed-cell foam is heavy and hard, around two pounds per cubic foot, and it adds noticeable structural rigidity to the surfaces it bonds to.
Thermal Performance and Space
Closed-cell foam delivers higher thermal resistance per inch, generally in the range of R-6 to R-7, while open-cell foam offers roughly R-3.5 to R-3.8 per inch. When cavity depth is limited and you need to reach a high R-value in a thin space, closed-cell foam is the clear winner. In a shallow roof rafter or a thin retrofit cavity, those extra R-values per inch can be the difference between meeting code and falling short.
Open-cell foam, by contrast, makes the most sense where depth is abundant and cost matters. Because it expands much more aggressively, a small amount of material fills a large cavity, which lowers the per-cavity cost. In a deep attic or a wide wall cavity where you have room to spare, open-cell foam can achieve the target R-value economically while still providing excellent air sealing.
Air and Moisture Behavior
Both foams stop air leakage when applied at adequate thickness, and that air-sealing ability is the main reason builders reach for spray foam at all. The crucial difference is how they handle water vapor. Closed-cell foam is a vapor retarder once it reaches a couple of inches of thickness, meaning it slows the diffusion of moisture through the assembly. This makes it valuable in situations where you want to keep moisture out, such as below-grade walls, rim joists, and the underside of roof decks in certain climates.
Open-cell foam is vapor-permeable. It allows moisture to pass through, which can be an asset or a liability depending on the design. In some assemblies, this permeability lets a roof or wall dry toward the interior, which is desirable. In others, particularly humid climates with air-conditioned interiors, an unprotected open-cell roof can allow moisture to reach a cold sheathing and condense. The same property that helps one assembly dry can harm another, which is why climate and assembly design must guide the choice.
Water Resistance and Flood Resilience
Closed-cell foam does not absorb water readily. It can be exposed to bulk water and remain largely unaffected, which is why it is recommended in flood-prone areas and below grade. Open-cell foam, with its interconnected cells, will absorb and hold water if it gets wet, and it must dry out afterward. In a basement or a coastal home where flooding is a genuine risk, this distinction alone can justify the higher cost of closed-cell foam.
Sound, Structure, and Strength
The two foams diverge in how they handle other building demands as well.
- Open-cell foam is a better acoustic absorber. Its soft, porous structure dampens airborne sound, making it a favorite for interior partitions, media rooms, and walls where noise control matters.
- Closed-cell foam adds racking strength and rigidity to a structure because of its density and adhesion, a meaningful benefit in high-wind regions.
- Open-cell foam is more forgiving of expansion and contraction, flexing with the building rather than cracking.
- Closed-cell foam resists physical damage and provides a harder, more durable surface.
Cost and Practical Trade-offs
On a per-cavity basis for a given R-value, the comparison is nuanced. Closed-cell foam costs more per board foot, but because open-cell needs more thickness to reach the same R-value, the gap narrows in deep cavities. The general pattern is that open-cell foam is more economical where space is generous and high vapor resistance is unnecessary, while closed-cell foam earns its premium where space is tight, water resistance is critical, or added rigidity is welcome.
Matching Foam to Application
The practical guidance that emerges is straightforward once you know the properties. Choose closed-cell foam for below-grade walls, rim joists, flood-prone areas, thin cavities that must hit a high R-value, and unvented roof assemblies in climates where you want to control vapor. Choose open-cell foam for interior sound-dampening walls, deep attic and wall cavities where space is abundant, and assemblies designed to dry inward.
Whichever you select, both foams demand professional installation, proper ratios, controlled temperatures during curing, and attention to thickness for fire-code-required ignition barriers. Spray foam rewards expertise and punishes shortcuts. Used thoughtfully, with the right type in the right place, it remains one of the most effective ways to build a tight, well-insulated, durable envelope.