New Title

Open-cell and closed-cell spray polyurethane foam both provide insulation and air sealing, but they do not perform the same way. The difference in cell structure changes density, R-value per inch, vapor permeability, rigidity, moisture behavior, expansion, and where the foam is appropriate to use. Choosing the right foam is not simply a matter of choosing the higher R-value. The entire building assembly has to be considered.

Open-cell foam has a lower-density structure in which many of the cells are interconnected. It expands significantly and creates a soft, flexible foam. Closed-cell foam has a much denser structure with cells that are predominantly closed and filled with blowing agent. The result is a more rigid foam with substantially greater resistance to vapor movement and a higher R-value per inch.

Open-Cell Spray Foam

Open-cell foam is the lower-density option and is generally used where a high level of air sealing is desired without the cost and density of closed-cell foam. It expands significantly after application, allowing it to fill cavities and irregular areas.

• Lower density and softer, more flexible finished foam
• Lower R-value per inch than closed-cell foam
• Excellent air-sealing capability when installed correctly
• More vapor permeable than closed-cell foam
• Can be a cost-effective way to create a large, continuous air-sealing and insulation layer
• Appropriate applications depend heavily on climate, assembly design, moisture exposure, and manufacturer requirements

Closed-Cell Spray Foam

Closed-cell foam is a higher-density, rigid foam that provides more R-value in less thickness. Its low vapor permeability also gives it an important role in assemblies where vapor control is part of the design.

• Higher R-value per inch than open-cell foam
• Much lower vapor permeance than open-cell foam
• Rigid structure that adds strength and dimensional stability to the treated assembly
• Excellent air-sealing capability when installed correctly
• Useful where limited space makes higher R-value per inch important
• Often the preferred foam where the assembly requires stronger vapor-control characteristics

R-Value Is Only Part of the Decision

It is easy to compare spray foam by R-value alone, but that can lead to the wrong decision. An insulation system has to control heat flow, air movement, and moisture appropriately for the assembly. Open-cell foam can provide excellent air sealing while remaining vapor open. Closed-cell foam provides both air sealing and substantially greater resistance to vapor diffusion. The correct choice depends on what the building assembly needs.

One of the most important differences between the two foams is how readily water vapor can move

through them. DOE Building America guidance identifies open-cell foam as vapor permeable and closed-

cell foam as a vapor retarder at typical thicknesses. That distinction becomes especially important when

the foam is installed directly against a roof deck or another surface where condensation potential has to be

considered.

Moisture control is not the same thing as air sealing. Both open-cell and closed-cell foam can provide an effective air barrier when properly installed, but open-cell foam does not provide the same vapor-control characteristics as closed-cell foam.

Why ISO Does Not Recommend Open-Cell Foam on Metal Buildings

Metal buildings require special consideration because the roof and wall surfaces are often metal panels rather than a conventional wood roof deck. Metal can change temperature rapidly, and when warm, moisture-laden air contacts a sufficiently cold metal surface, condensation can occur.

Open-cell foam is not vapor impermeable. Because it allows substantially more vapor movement, we do not recommend spraying open-cell foam directly against metal roof panels in our metal-building applications. The concern is not that open-cell foam cannot insulate metal. The concern is the moisture behavior of the complete assembly and the potential for moisture to reach the metal surface.

Closed-cell foam is the preferred spray foam for these applications because its much lower vapor permeance provides an additional layer of moisture control directly at the metal assembly. Its higher R- value per inch also allows substantial thermal resistance without requiring the same thickness as open-cell foam.

This is why ISO treats metal buildings differently from many residential attic applications. The foam has to be selected based on the surface being sprayed, the building's use, humidity conditions, roof configuration, and the complete assembly—not simply the price per square foot.

Open Cell in Residential Roof Assemblies

Open-cell foam can be an appropriate choice in residential roof assemblies when the climate, humidity levels, roof construction, and vapor-control strategy support its use. DOE Building America guidance notes that open-cell foam can be used in Climate Zones 1 through 3 when interior humidity is not high, while more vapor control is needed as climate conditions become more demanding.

For Oklahoma applications, ISO evaluates the specific roof assembly rather than treating every attic as identical. The existing roof deck, ventilation, HVAC configuration, interior humidity, foam thickness, and the intended finished assembly all matter.

Closed Cell Where Thickness Is Limited

Closed-cell foam is particularly useful when there is not enough available depth to install a large amount of insulation. Because it provides more R-value per inch, it can achieve substantial thermal resistance in a smaller space.

Both open-cell and closed-cell spray foam are capable of creating excellent air seals when the foam is installed continuously and detailed correctly. This is one of the major advantages of spray foam compared with insulation systems that leave gaps around penetrations and irregular surfaces.

Air sealing helps reduce uncontrolled movement of outdoor and attic air through the building envelope. In an attic application, this can help reduce the amount of hot attic air moving into the living space and can improve the overall performance of the insulation system.

Density and Physical Properties

Open-cell and closed-cell foam are fundamentally different materials in terms of density and physical behavior. Open-cell foam is softer and more flexible. Closed-cell foam is denser and more rigid. Those differences affect where each product makes sense and how the finished assembly responds to moisture, temperature, and physical demands.

How ISO Chooses Between Them

ISO does not choose open cell or closed cell based solely on which product has the higher R-value or which option costs less. We look at the application and the assembly first.

• What surface is being sprayed?
• Is the assembly wood, metal, masonry, or another material?
• How much insulation thickness is available?
• What level of R-value is required or desired?
• Does the assembly require additional vapor control?
• What are the interior humidity conditions?
• Is the space residential, commercial, industrial, or specialty use?
• What will the completed assembly 100k like after the foam is installed?
• What additional ignition, thermal, or fire-protection requirements apply to the finished assembly?

The Bottom Line

Open-cell and closed-cell spray foam are both high-performance insulation products, but they are not interchangeable in every application. Open cell is an effective air-sealing insulation system where its vapor-open characteristics are appropriate. Closed cell provides higher R-value per inch and significantly greater resistance to vapor movement, making it the better choice for many assemblies where moisture control is a primary concern.

For metal buildings in particular, ISO recommends closed-cell spray foam rather than open-cell foam because of the way metal surfaces interact with temperature and moisture. The goal is not just to insulate the building—it is to build a spray foam system that performs correctly as part Of the entire building assembly.

ISO can evaluate your building and determine which spray foam system is appropriate for the application.

DOE Building America guidance distinguishes open-cell and closed-cell spray foam by their vapor-control characteristics and provides climate-specific guidance for roof assemblies. Building Science Corporation research likewise identifies closed-cell foam as substantially more vapor-resistant than low-density open-cell foam.