Open Cell vs Closed Cell Spray Foam Insulation: Which Is Right for Your Project?

Spray foam insulation comes in two distinct forms (open cell and closed cell) and choosing the wrong type for an application is one of the most common and costly mistakes in building science. This guide explains the real differences, where each excels, and how to match the product to the project.

The Fundamental Difference

Spray polyurethane foam (SPF) is created when two liquid components (an isocyanate and a polyol resin) are mixed in the spray gun's impingement chamber and expand on contact with the substrate. What happens during that expansion determines whether you get open cell or closed cell foam.

In open cell foam, the bubbles that form during expansion are intentionally left open and interconnected. The result is a soft, spongy material with a low density (around 0.5 lb/ft³). In closed cell foam, those bubbles are sealed shut with a low-conductivity gas trapped inside, producing a rigid, dense material (1.7–2.0 lb/ft³) with dramatically higher thermal resistance.

Side-by-Side Comparison

When to Use Open Cell Spray Foam

Open cell foam is the right choice when you need cost-effective air sealing and thermal performance in a conditioned space where moisture management is handled separately by the HVAC system or a vapor barrier.

Best applications for open cell foam:

• Interior walls: superior sound attenuation and full cavity fill at lower cost
• Interior side of roofdecks in warm and mixed climates (Zones 1–4) where drying to the exterior is possible
• Cathedral ceilings where thermal bridging at rafters is a concern
• Basement walls in conditioned basements above grade where vapor drive is not a concern
• Large commercial cavities where fill volume is high and budget is a constraint

ThermoSeal's open cell products (ThermoSeal 500 and ThermoSeal 800) deliver R-4.1 to R-4.5/inch and excel at eliminating air infiltration, which accounts for up to 40% of a building's energy loss.

When to Use Closed Cell Spray Foam

Closed cell foam earns its higher price point in applications where you need maximum R-value in minimal thickness, moisture resistance, or structural contribution, often all three at once.

Best applications for closed cell foam:

• Roofdecks in all climate zones: the impermeable cell structure prevents condensation regardless of climate
• Crawlspaces and below-grade walls: resists bulk water and vapor drive from soil
• Rim joists: tight space requires high R-value in thin application; closed cell at 2–3 inches achieves R-13 to R-20
• Metal buildings: prevents condensation on steel framing and adds structural rigidity
• Extreme climate zones (Zones 6–8) where vapor drive is intense
• Projects with limited cavity depth: achieves required R-values in less space than any other insulation type

ThermoSeal HFO is our most advanced closed cell product, achieving R-7.4/inch (the highest available in the industry) while using a next-generation HFO blowing agent with a Global Warming Potential (GWP) of just 1, compared to roughly 1,030 for older HFC-blown foams.

Can You Use Both in the Same Building?

Yes, and in many high-performance buildings, combining the two types is the optimal strategy. A common hybrid approach:

• Closed cell foam on the roofdeck (moisture control, high R-value)
• Open cell foam in the walls (cost-effective air sealing and sound control)
• Closed cell foam in the rim joist (maximum R-value in minimal space)

This approach captures the performance advantages of each product type where they matter most, without paying closed cell prices throughout the entire building envelope.

R-Value Requirements by Climate Zone

The 2021 International Energy Conservation Code (IECC) sets minimum R-value requirements that vary by climate zone. In colder zones, the higher R-value per inch of closed cell foam means you can meet code requirements with a thinner application, important in retrofits where cavity depth is limited.

• Climate Zones 1–2: Attic R-38, walls R-13 (open cell typically sufficient)
• Climate Zones 3–4: Attic R-49, walls R-20 (open cell with adequate depth or closed cell)
• Climate Zones 5–8: Attic R-49 to R-60, walls R-20+ci (closed cell strongly preferred) for roofdeck and rim joist

The Bottom Line

Open cell and closed cell spray foam are not interchangeable; they solve different problems. Open cell delivers cost-effective air sealing and sound control in interior and above-grade applications. Closed cell delivers maximum thermal performance, moisture resistance, and structural contribution in demanding environments.

The best insulation strategy uses each product where it performs best, guided by local climate zone, building type, and code requirements. A ThermoSeal certified installer can assess your specific project and recommend the right product and thickness for every zone in the building.