Spray Foam Types Explained: Open Cell vs Closed Cell for Maximum Efficiency

Closed-cell spray foam delivers roughly double the R-value per inch (R-6.0 to R-7.0) compared to open-cell spray foam (R-3.5 to R-3.8), making it the stronger performer for thermal resistance, structural reinforcement, and moisture control. Open-cell foam costs significantly less upfront and excels at sound dampening, filling large cavities quickly, and providing a high-performance air seal in interior applications where moisture exposure is minimal. For maximum energy efficiency in moisture-prone or space-constrained areas like basements, crawl spaces, and exterior walls, closed-cell is the clear choice, especially when combined with strategies outlined in spray foam insulation energy efficiency guide. For large open areas like attics and interior partitions where budget and sound control matter more, open cell delivers strong value.

TLDR / Key Takeaways

• Closed cell spray foam delivers R-6.0 to R-7.0 per inch, while open cell delivers R-3.5 to R-3.8 per inch, meaning closed cell achieves roughly double the thermal resistance at the same thickness
• Open cell foam typically costs $1.50 to $3.50 per square foot installed, while closed cell foam ranges from $2.50 to $7.50 per square foot, depending on thickness and project conditions
• Closed-cell foam acts as a Class II vapor retarder at 1.5 inches and a near-complete vapor barrier above 2 inches, while open-cell foam remains vapor permeable and may require a separate vapor control layer in cold climates
• Open cell foam absorbs approximately 70% of sound at 3 inches thickness, making it far superior for acoustic applications
• Closed-cell foam adds structural rigidity to wall assemblies at roughly 2 pounds per cubic foot density, while open-cell foam is soft and flexible at roughly 0.5 pounds per cubic foot
• The U.S. Department of Energy recommends matching insulation R-value to your climate zone, with colder zones requiring R-49 to R-60 for attics and R-20 to R-38 for walls

Head-to-Head Performance Comparison

Understanding how these two foam types differ at a technical level helps determine which product delivers the best return on investment for each specific application. The differences go well beyond R-value and affect moisture management, structural performance, and long-term durability.

According to the U.S. Department of Energy’s Energy Saver program, R-value measures thermal resistance, and the higher the R-value, the greater the insulating effectiveness. The required R-value depends on your climate zone, the type of heating and cooling system, and the part of the building being insulated.

How Each Foam Type Works

Open-cell spray foam is a low-density, semi-rigid material that expands dramatically during application, often reaching 100 times its original volume. The cells within the foam are left open, which gives the material its soft, spongy texture and allows moisture vapor to pass through. This high expansion rate means it fills wall cavities and attic spaces thoroughly with relatively little material, which contributes to its lower cost. The open cell structure also gives it strong sound-absorbing properties, with a Noise Reduction Coefficient (NRC) of approximately 0.70 at 3 inches of thickness U.S. DOE Building America. 

Closed-cell spray foam is a medium-density, rigid material with cells that are completely sealed off from one another. This closed-cell structure traps a gas within the foam, which provides its significantly higher R-value per inch. The rigid nature of the foam also adds measurable structural strength to the assembly. Research from the Building Science Corporation confirms that closed-cell foam applied at thicknesses over 2 inches provides sufficient vapor diffusion control for even very cold climates, eliminating the need for a separate polyethylene vapor barrier in most wall assemblies.

Cost Breakdown

Cost is often the deciding factor for property owners and contractors, especially on larger projects. While closed-cell foam delivers superior performance in several categories, it comes at a premium that needs to be weighed against the specific requirements of each project.

According to HomeGuide’s spray foam insulation cost guide, open cell spray foam costs $1.50 to $3.50 per square foot with installation, while closed cell ranges higher depending on thickness and project complexity. This Old House reports that material and labor for closed-cell can run $1.25 to $1.50 per square foot for the foam itself, with total project costs climbing based on preparation, cavity depth, and access challenges.

Where Each Foam Type Performs Best

The right choice depends on the specific building assembly, climate conditions, and project priorities. Here are the scenarios where each foam type delivers the strongest results.

Open-cell spray foam is the better choice when:

• Insulating large, open attic spaces where cost per square foot matters
• Sound control is a priority, such as between bedrooms, home offices, or shared walls in multi-family construction
• The cavity depth allows full fill without sacrificing R-value requirements
• Moisture exposure from the exterior is not a concern, such as interior partition walls
• Budget constraints require maximizing the coverage area over peak thermal performance

Closed-cell spray foam is the better choice when:

• Insulating basements, crawl spaces, or below-grade walls where moisture intrusion is a real concern
• Cavity depth is limited, and the highest possible R-value per inch is needed, such as in 2×4 wall framing
• Structural reinforcement of the building assembly is desirable
• The application requires a vapor retarder or vapor barrier as part of the insulation layer
• Exterior continuous insulation is being applied under cladding

Real-World Scenarios

The following examples illustrate how foam type selection plays out across different project types and conditions.

Factors That Influence the Decision

Several variables beyond R-value and cost affect which spray foam type delivers the best long-term performance and value. Our team evaluates each of these on every project before recommending a product.

• Climate zone: Colder climates demand higher R-values, which favors closed-cell in space-constrained assemblies. The DOE recommends R-49 to R-60 for uninsulated attics in climate zones 5 through 8
• Cavity depth: Shallow cavities like 2×4 walls (3.5 inches) benefit more from closed cell’s higher R-value per inch, while 2×6 walls (5.5 inches) give open cell room to achieve competitive total R-values
• Moisture exposure: Below-grade applications, coastal properties, and high-humidity environments favor closed-cell’s low vapor permeability
• Budget: Open cell delivers roughly twice the coverage per dollar, making it practical for large area applications where peak R-value is not critical
• Sound requirements: Multi-family construction, home theaters, and office partitions benefit significantly from open cell’s acoustic absorption
• Structural needs: Closed cell adds measurable rigidity to wall and roof assemblies, which matters in some engineering scenarios
• Fire code requirements: Both foam types require a thermal barrier (typically 1/2 inch drywall) to meet building code, but the specific code path may vary by jurisdiction

Who This Is For (and Who It Is NOT For)

Spray foam insulation is the right choice for:

• New construction projects where air sealing and insulation can be installed simultaneously for maximum efficiency
• Retrofit projects targeting hard-to-reach areas like rim joists, crawl spaces, and irregular cavities
• Properties where moisture control is a concern, especially when using closed-cell in below-grade or high-humidity applications
• Building owners targeting the highest possible energy efficiency ratings and long-term utility savings

Spray foam may NOT be the best fit for:

• Projects with extremely tight budgets where fiberglass batts or blown-in cellulose can meet code requirements at a lower cost
• Walls that may need future access for wiring or plumbing modifications, since spray foam is permanent
• Properties where occupants have chemical sensitivities, as spray foam off-gassing during and immediately after installation, require proper ventilation and curing time

Get a Professional Insulation Assessment

Choosing between open-cell and closed-cell spray foam is not a one-size-fits-all decision. The right product depends on your building’s specific assembly details, climate zone, moisture exposure risk, and performance priorities. Our team at Lamothe Insulation & Contracting evaluates every project individually to recommend the foam type and application method that delivers the best efficiency, durability, and value for your situation.

Contact us today at (508) 847-0119 or [email protected] to discuss your project and get a detailed, no-surprise quote tailored to your property.

Sources

• U.S. Department of Energy – Energy Saver: Insulation – Comprehensive guide covering R-value requirements, insulation types, and climate zone recommendations from the federal agency responsible for energy policy.
• Building Science Corporation – RR-0912: Spray Polyurethane Foam Vapor Retarder Research – Peer-reviewed research report on vapor control performance of open cell and closed cell spray foam in residential wall assemblies across multiple climate zones.
• U.S. DOE Building America – Spray Foam Insulation Guide – Technical guide from the Department of Energy’s Building America program covering spray foam properties, installation practices, and performance specifications.

Frequently Asked Questions

Can open-cell spray foam be used in a basement?

Open-cell foam is generally not recommended for basement walls because it absorbs and retains moisture. Closed-cell foam’s low vapor permeability and rigid structure make it the better choice for below-grade and basement rim joist applications where moisture is a persistent concern.

How long does spray foam insulation last?

Both open-cell and closed-cell spray foam are designed to last the lifetime of the building when properly installed. Neither type settles, sags, or degrades over time like fiberglass or cellulose, making them a long-term investment in energy efficiency.

Does spray foam require a vapor barrier?

Closed-cell spray foam applied at 2 inches or thicker functions as its own vapor retarder, eliminating the need for a separate polyethylene barrier in most wall assemblies. Open-cell foam does not control vapor diffusion and may require an additional vapor control layer in colder climates.

Is spray foam insulation worth the higher cost compared to fiberglass?

Spray foam delivers a significantly higher R-value per inch, creates a complete air seal, and does not settle or degrade. For homeowners targeting maximum energy efficiency, lower utility bills, and improved indoor comfort, spray foam typically pays for itself through energy savings over time.

Can spray foam be installed in existing walls?

Spray foam can be installed in existing walls through small access holes, but the process is more invasive and costly than in new construction. Rim joists, attics, and crawl spaces are the most common and cost-effective retrofit locations for spray foam insulation.