Which Spray Foam Has Better R-Value: Open-Cell or Closed-Cell Spray Foam?

Closed-cell spray foam delivers a significantly higher R-value per inch than open-cell spray foam, approximately R-6.0 to R-7.0 per inch compared to roughly R-3.5 to R-3.8 per inch for open-cell. In a straight thermal resistance comparison, closed-cell wins by nearly double. But higher R-value per inch does not automatically mean better real-world performance for every application. Whole-wall R-value, moisture control, cavity depth, climate zone, and the specific area being insulated all factor into which product delivers better results. In standard 2×4 or 2×6 framed walls, the whole-wall R-value difference between the two foam types narrows considerably due to thermal bridging through wood framing. Understanding where each foam excels and where it falls short is the difference between a high-performing building envelope and one that wastes material and money.

TLDR / Key Takeaways

• Closed-cell spray foam delivers roughly R-6.0 to R-7.0 per inch; open-cell delivers roughly R-3.5 to R-3.8 per inch
• Both types are considered air-impermeable when installed at standard application thicknesses
• Closed-cell qualifies as a Class II vapor retarder at 1.5 inches thick, while open-cell remains vapor-permeable even at full cavity depth
• Whole-wall R-value differences between the two are smaller than per-inch R-value differences due to thermal bridging through wood framing
• Closed-cell is required in IECC Climate Zones 5 and higher for unvented attic and cathedral ceiling assemblies
• Open-cell is rated “not acceptable” for below-grade and flood-prone applications by building science authorities
• FEMA classifies only closed-cell spray foam as an acceptable flood-damage-resistant insulation material
• The right choice depends on climate zone, cavity depth, moisture exposure, and the specific assembly being insulated

R-Value Per Inch: The Raw Numbers

R-value measures thermal resistance, or how well a material resists heat flow. The higher the number, the better the material insulates. When comparing spray foam types on a per-inch basis, the difference is clear.

Source data adapted from the DOE Building America “Which Spray Foam Is Right For You?” guideline.

The nearly 2:1 R-value advantage of closed-cell foam makes it the logical choice when cavity depth is limited. A 2-inch application of closed-cell foam reaches approximately R-12, while 2 inches of open-cell delivers only about R-7.2.

Why Per-Inch R-Value Can Be Misleading

The R-value printed on a product data sheet measures the insulation material in isolation. It does not account for the entire wall assembly. As building science expert Martin Holladay explained in Fine Homebuilding, whole-wall R-value is what actually matters for energy performance.

In a typical 2×4 framed wall, roughly 25% of the wall area is wood framing. Heat flows through those studs regardless of how high the R-value of the insulation in the cavities is. When installers apply closed-cell foam in a 2×4 cavity, they often stop at about 3 inches to avoid the difficulty of trimming the dense cured foam. This leaves studs partially exposed, increasing thermal bridging and reducing the whole-wall assembly performance. Open-cell foam, being softer and easier to trim, fills the full 3.5-inch cavity completely, which keeps studs fully covered and reduces thermal bridging.

The result: the whole-wall R-value difference between a wall insulated with open-cell versus closed-cell spray foam is much smaller than the per-inch R-value difference would suggest. Paying significantly more for closed-cell foam in a standard stud wall cavity may not deliver proportional energy savings.

Performance by Application and Climate Zone

Different parts of the building envelope have different requirements. The Building Science Corporation’s Residential Spray Foam Guide provides detailed guidance on when each foam type is preferred, acceptable, or not acceptable.

Frame Wall Cavities

• Cold Climates (IECC Zones 5-8): Both types work. High-density closed-cell foam qualifies as a Class II vapor retarder at 1.5 inches and is preferred for condensation control. Open-cell can work but requires an interior vapor retarder such as vapor-retardant paint.
• Hot-Humid Climates: Closed-cell is preferred because it provides a redundant moisture control layer. Open-cell is acceptable but requires careful exterior drainage plane detailing and is not recommended behind brick veneer due to solar-driven moisture risk.
• Hot-Dry Climates: Both are preferred. Moisture concerns are minimal in these regions.

Sloped Roof Rafters and Unvented Attics

• IECC Climate Zones 5 and Higher: Only high-density closed-cell spray foam should be used. The foam must meet minimum thickness requirements specified in the IRC based on climate zone to prevent condensation on the interior surface.
• IECC Climate Zones 4 and Below: Both types are acceptable, provided appropriate moisture management measures are in place.

Foundations and Below-Grade Spaces

• Open-Cell: Rated as “not acceptable” for below-grade applications due to its moisture-permeable nature and ability to absorb and hold water.
• Closed-Cell: Preferred for basement foundation walls, especially in Climate Zones 5 and higher. The material acts as both an air barrier and vapor retarder while resisting water absorption.

Band Joists and Rim Joists

• Both types work well in this small but critical area. Closed-cell is preferred due to its moisture resistance and vapor control, but open-cell is acceptable and can be a cost-effective option where budgets are tight.

Whole-Wall R-Value Comparison

To illustrate how the two foam types compare in an actual wall assembly, consider a standard 2×6 wall with a 25% framing factor:

These calculations use the U-factor method described in the Fine Homebuilding analysis, where whole-wall U-factor equals the insulation U-factor multiplied by the insulated area percentage, plus the framing U-factor multiplied by the framing area percentage. The example shows that 3 inches of closed-cell foam in a 2×6 cavity can actually underperform a fully filled open-cell application because the exposed studs create more thermal bridging.

Flood Resistance: A Critical Distinction

In flood-prone areas, the choice between open-cell and closed-cell spray foam is not close. According to FEMA Technical Bulletin 2, closed-cell spray polyurethane foam is rated as acceptable for use below the base flood elevation in Special Flood Hazard Areas, while open-cell and all other inorganic insulation types such as fiberglass and cellulose are rated unacceptable.

This matters for any building located in a designated flood zone, but it also matters for basements, crawlspaces, and coastal construction where moisture exposure is a routine concern rather than an exception. Closed-cell foam’s hydrophobic nature means it does not absorb or retain water, while open-cell foam can hold up to one-third of its volume in water, leading to mold, rot, and structural damage.

Real-World Application Scenarios

The best way to understand the R-value decision is through specific project scenarios. Here are realistic examples our team encounters regularly:

Factors That Influence the Decision

Several variables determine which spray foam type will deliver the best performance on any given project:

• Climate zone: IECC Climate Zones 5 and higher require closed-cell for unvented roofs and prefer it for condensation control in walls and foundations
• Cavity depth: Shallow cavities benefit more from closed-cell’s higher per-inch R-value; deep cavities can achieve high total R-value with lower-cost open-cell
• Moisture exposure: Below-grade, flood-prone, and hot-humid environments favor closed-cell due to its vapor-retardant and hydrophobic properties
• Budget constraints: Closed-cell foam carries a higher installed cost per board foot than open-cell, though the per-R-value cost difference is smaller
• Structural requirements: Closed-cell adds racking strength to frame walls and can serve as a structural adhesive in modular construction
• Building code requirements: Local adoption of the IRC and IECC may mandate specific minimum foam thicknesses and types for certain applications

Who Closed-Cell Spray Foam Is Best For

• Builders and property owners in IECC Climate Zones 5 through 8 where condensation control is essential
• Projects involving below-grade insulation, basements, and crawlspace encapsulation
• Buildings located in FEMA-designated flood hazard areas
• Unvented attic and cathedral ceiling assemblies in cold climates
• Properties requiring structural reinforcement or wind uplift resistance
• Commercial buildings where moisture control redundancy is a priority

Where Closed-Cell Spray Foam Is NOT the Best Choice For

• Large-volume interior wall cavities in hot-dry or mild climates where open-cell achieves code-required R-values at lower cost
• Budget-constrained projects in warm climates where whole-wall performance differences are minimal
• Projects where the building assembly already includes a continuous exterior rigid foam insulation layer that handles vapor control
• Applications where sound attenuation is a primary goal, since open-cell provides superior acoustic performance with higher noise reduction coefficients

Get Expert Spray Foam Recommendations for Your Project

Choosing between open-cell and closed-cell spray foam requires more than comparing R-value numbers on a data sheet. The right decision depends on your climate zone, the specific building assembly, moisture conditions, and budget. Our team at All Foam & Insulation, LLC evaluates every project individually to recommend the foam type and application thickness that delivers the best long-term performance and value. Whether you are insulating a new build, retrofitting an existing structure, or solving a specific moisture problem, we have the experience to get it right the first time.

Contact us today by calling us at (541) 826-9600 or emailing [email protected] to discuss your insulation project with our team.

FAQs

Does closed-cell spray foam always provide better insulation performance than open-cell?

No. While closed-cell has a higher R-value per inch, the whole-wall performance difference narrows significantly in standard framed walls due to thermal bridging through wood studs. In deep cavities with mild climates, open-cell can deliver comparable whole-wall R-values at a lower installed cost.

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

Open-cell spray foam should not be used in below-grade applications. It is moisture-permeable and can absorb and retain water, which leads to mold growth and material degradation. Closed-cell is the correct choice for foundations, basements, and crawlspaces.

At what thickness does closed-cell spray foam become a vapor retarder?

Closed-cell spray foam qualifies as a Class II vapor retarder at approximately 1.5 inches of thickness, achieving a perm rating below 1.0. This means it controls both air and vapor movement at that depth, which is why building codes specify minimum thickness requirements based on climate zone.

Is closed-cell spray foam worth the extra cost in a warm climate?

In warm, dry climates, the higher cost of closed-cell may not deliver proportional energy savings for standard wall cavities. However, it can still be worthwhile for unvented attics, flat roofs, band joists, and any assembly where moisture control or structural reinforcement is needed.

Why does FEMA only approve closed-cell spray foam for flood zones?

FEMA’s Technical Bulletin 2 rates insulation materials based on their ability to withstand prolonged contact with floodwater without significant damage. Closed-cell foam is hydrophobic and does not absorb water, while open-cell and other insulation types absorb and retain water, leading to mold, rot, and the need for full replacement after flooding.

Sources

• DOE Building America – Which Spray Foam Is Right For You? – Comprehensive guideline on physical properties, climate-specific applications, and performance comparisons of open-cell and closed-cell spray polyurethane foam for residential construction, published by the Department of Energy’s Building America program through the Consortium for Advanced Residential Buildings.

• Building Science Corporation – Residential Spray Foam Guide (GM-2102) – Authoritative guide by Joseph Lstiburek covering spray foam use in walls, roofs, and foundations, including IRC code requirements for insulation thickness by climate zone and condensation control tables adapted from the 2018 International Residential Code.

• Fine Homebuilding – Closed-Cell Foam Between Studs Is a Waste – Analysis by Martin Holladay demonstrating that whole-wall R-value differences between open-cell and closed-cell spray foam in standard stud walls are much smaller than per-inch R-value differences suggest, using U-factor calculations that account for thermal bridging through wood framing.

• FEMA Technical Bulletin 2 – Flood Damage-Resistant Materials Requirements (2025) – Official FEMA guidance rating closed-cell spray polyurethane foam as “acceptable” and open-cell foam as “unacceptable” for use below the base flood elevation in Special Flood Hazard Areas, with detailed material classification tables for structural and finish building materials.

• Fine Homebuilding – Understanding R-Value and Spray Foam: Closed-Cell vs Open-Cell – Building science coverage of open-cell and closed-cell spray foam R-values per inch, including practical installation considerations such as trimmability, cavity fill depth, and the impact on whole-wall thermal performance.