Open Cell vs Closed Cell Spray Foam

If you've started researching spray foam insulation, you've likely run into two options: open cell and closed cell. Both are spray foam — but they perform very differently, and choosing the wrong one for your application can cost you thousands in energy bills or moisture damage.

As a Mass Save® certified insulation contractor serving East Boston and Greater Boston, we install both types every week. Here's exactly how to choose.

The Core Difference

The names describe the cell structure of the foam after it expands:

• Open cell foam — cells are broken open. Air fills the gaps. Soft, flexible, excellent sound dampening, lower cost.
• Closed cell foam — cells are sealed shut. Gas fills the gaps. Rigid, vapor-impermeable, higher R-value per inch, higher cost.

R-Value Comparison

When to Use Open Cell Foam

Open cell is our recommendation for most interior applications in Greater Boston homes:

• Interior wall cavities (sound between rooms)
• Attic rafters in conditioned attic spaces (when you want the attic to breathe slightly)
• Interior floor joists between living spaces
• Budget-conscious projects where maximum coverage matters more than maximum R-value

When to Use Closed Cell Foam

Closed cell is the right choice for moisture-critical and exterior applications:

• Rim joists (always — this is where most moisture infiltration occurs)
• Basement and crawl space walls
• Rooflines in cold climates (IECC Zone 5A requires minimum R-49 total)
• Any exterior-facing application in Massachusetts
• When you need vapor control without a separate vapor barrier

The New England Specific Rule

Massachusetts sits in IECC Climate Zone 5A — a mixed-humid climate with cold winters and humid summers. This creates a specific challenge: moisture can drive inward in summer and outward in winter, meaning your insulation needs to handle both directions.

Our recommendation for most Greater Boston homes: use 2 inches of closed cell on the exterior-facing surface (rim joists, basement walls) and open cell for interior cavities. This hybrid approach gives you vapor control where you need it and cost savings where you don't.

Does Mass Save Cover Spray Foam?

Yes. Mass Save® rebates cover qualifying spray foam insulation in Massachusetts homes. Most homeowners receive 75–100% of project costs covered. As a certified Mass Save contractor, Smooth Construction handles all paperwork and applies your rebate at the time of invoice — you pay nothing upfront for the covered portion.

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Learn more: Spray Foam Insulation · Open Cell Foam · Closed Cell Foam · Attic Insulation · Air Sealing

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