Understanding the difference between open-cell and closed-cell spray foam—and how their R-values compare—is essential when choosing insulation for walls, roofs, or crawlspaces. R-value measures thermal resistance: the higher the R-value per inch, the better the material resists conductive heat flow. But R-value alone doesn’t tell the whole story for spray foam because the foam’s cellular structure, density, air-sealing ability, moisture behavior, and installation thickness all affect real-world performance.
Open-cell spray foam is lighter, less dense, and has a spongy structure with cells that are intentionally left open. Its R-value is typically in the range of about 3.5 to 4.0 per inch. Open-cell excels at filling irregular cavities and provides excellent air-sealing because it expands into gaps, but it is vapor-permeable and will absorb some moisture. Closed-cell spray foam, by contrast, is denser with fully sealed cells filled with gas; its R-value is higher—commonly around 5.5 to 7 per inch—and it provides both a strong air barrier and a moisture/vapor retarder. Closed-cell also adds structural rigidity and can perform better in thin applications where high R-value per inch is needed.
Because of these differences, the choice between open- and closed-cell foam often comes down to application goals, budget, and local climate. Closed-cell’s higher R-value per inch makes it preferable where space is limited or where a vapor/air barrier is desired, but it is more expensive and less forgiving if moisture gets trapped. Open-cell is more cost-effective for larger volumes and can be advantageous in assemblies that need to dry to one side. Installation quality, building details, and considerations like thermal bridging, ventilation, and building codes will ultimately determine how well either foam performs in a specific project.
This article will dig deeper into the numbers and mechanics behind R-values for both foams, explain how density, temperature, and aging affect thermal performance, compare moisture and air control properties, and offer guidance for selecting the right spray foam type for different scenarios. By the end you’ll understand not just the nominal R-values, but the practical implications for energy efficiency, durability, and cost.
R-value per inch comparison between open-cell and closed-cell spray foam
Open-cell spray foam typically has an R-value around 3.2–3.8 per inch (commonly quoted ~3.5), while closed-cell spray foam is considerably higher, commonly in the 6.0–7.0 per inch range. R-value per inch measures the thermal resistance provided by a given thickness of insulation; a higher R-value per inch means more resistance to heat flow for the same thickness. These are typical laboratory or manufacturer ranges — actual installed performance will vary with formulation, application quality, and the assembly in which the foam is installed.
The difference in R-value per inch stems from fundamental differences in cellular structure, density, and the gas trapped inside the foam cells. Closed-cell foam is dense (often around 1.5–2.0+ pounds per cubic foot) and its cells are sealed, trapping low-conductivity blowing agent gases that greatly reduce conductive heat transfer. Open-cell foam is much less dense (roughly 0.4–0.7 pcf), has interconnected cells filled with air, and therefore conducts heat more readily. The higher density and sealed cells of closed-cell foam also reduce small-scale convective currents and offer better resistance to moisture diffusion, both of which help the material achieve a higher R-value per inch.
For practical decisions, R-value per inch is only one factor. Closed-cell’s higher R/inch makes it preferable where space is limited (rim joists, thin wall assemblies) or where a vapor/air barrier and structural rigidity are desired; it’s often the choice for exterior-facing or flood-prone applications. Open-cell is less expensive by volume, provides excellent air sealing in many assemblies, and offers good sound dampening, but requires greater thickness to reach the same R-value and is vapor-permeable. Also keep in mind that whole-assembly performance (thermal bridging through framing, air leaks, and installation quality) often has equal or greater effect on comfort and energy use than the nominal per-inch R-value of the foam itself.
Foam density and cellular structure effects on R-value
Foam density and cellular structure are the primary physical reasons R-value differs between spray-foam types. Open-cell foam is low density (typically ~0.4–0.7 lb/ft3) with large, interconnected cells that are mostly filled with air; that cellular geometry yields a lower thermal resistance per inch (commonly around 3.5–4.0 per inch). Closed-cell foam is much denser (commonly ~1.8–3.0 lb/ft3 for building products) with tightly packed, discrete cells that trap a low-conductivity blowing gas inside each cell; the small cell size, thicker cell walls and trapped gas produce a substantially higher R-value per inch (commonly in the 6–7 per inch range). In short, higher density and closed, gas-filled cells reduce conductive heat transfer and increase R-value per unit thickness.
Density and cell structure also determine how R-value changes with formulation, aging, and environmental exposure. Within closed-cell products, higher-density formulations and more effective low-conductivity blowing agents give higher initial R-values; however some blowing agents slowly diffuse out over years and air diffuses in, which can reduce long-term R-value somewhat. Open-cell foam uses ambient air as the cell gas, so its per-inch R-value is lower but more stable over time (no gas diffusion losses), while its open-cell network makes it more sensitive to moisture uptake and compression—both of which can reduce effective thermal performance in the field. Manufacturing quality, cell uniformity, and installation (coverage, voids, overspray or compression) also alter real-world R-values regardless of nominal per-inch numbers.
Practically, the R-value difference driven by density and cellular structure dictates application choices. When thickness is limited or a high thermal resistance is required in a thin assembly, closed-cell foam is often chosen because its higher density and closed-cell structure deliver more R-value per inch and better moisture resistance and rigidity. When cost, cavity-filling ability, or sound dampening are priorities and more thickness is available, open-cell foam can be the better option despite its lower R-value per inch. Remember that R-value per inch is only one factor: air-sealing behavior, moisture/vapor characteristics, long-term gas diffusion effects, and proper installation quality all influence the actual thermal performance of a sprayed-foam assembly.
Air sealing, moisture resistance, and vapor permeability implications
Both open-cell and closed-cell spray foams can form effective air barriers when applied as a continuous layer, which often produces greater real-world energy savings than the nominal R-value alone. Open-cell foam expands a lot and conforms well to irregular cavities, sealing small gaps and reducing convective air movement; however, because it is lower density, it can be more susceptible to air movement through poorly detailed transitions or if the application is thin or discontinuous. Closed-cell foam, being much denser and stiffer, tends to form a tighter continuous air barrier at thinner thicknesses and resists bulk air flow more readily; this makes closed-cell especially useful where a compact, high-performance air seal is needed (e.g., rim joists, shallow cavities, or portions of an enclosure that are hard to access).
Moisture resistance and vapor-permeability differ sharply between the two and drive important assembly decisions. Closed-cell foam is relatively impermeable to water vapor and can act as a vapor retarder (often a Class II or better, depending on thickness), and it resists bulk water intrusion and holds very little liquid water if wet. That characteristic both protects the insulation R-value from moisture-related loss and can add some structural rigidity, but it also means that closed‑cell can trap moisture inside a wall if the overall assembly has no other drying path. Open-cell foam is vapor‑open (high permeance) and will allow an assembly to dry toward the interior or exterior; it can also absorb and retain some liquid water, which can temporarily reduce insulation effectiveness and raise mold risk if persistent wetting occurs. Because of these contrasts, the choice between open and closed cell should consider climate (which side of the assembly needs to dry), potential for bulk water exposure, and whether a continuous vapor retarder is desired.
R-value per inch is a major technical difference that affects thickness, cost, and design. Open-cell spray foam typically has an R-value of roughly 3.5 to 4.0 per inch, while closed-cell typically ranges from about 6 to 7 per inch—so closed-cell delivers substantially higher thermal resistance in the same thickness. That higher R-value plus lower permeability means you can meet thermal code requirements in thinner cavities with closed-cell, but you also must be cautious about vapor control and potential condensation risks in certain climates or wall assemblies. Conversely, open-cell’s lower R-value requires more thickness to reach the same insulating level but permits drying and can be a better choice where moisture must escape the assembly. In practice, you should balance R-value needs, air‑sealing performance, vapor control strategy, cavity depth, and cost to choose the appropriate foam type for the specific building and climate.
Long-term performance, aging, settling, and R-value retention
Long-term R-value retention for spray foam depends on cell structure, trapped gases, and how the foam is installed and exposed to moisture or mechanical stress. Closed-cell foam achieves a higher initial R-value per inch because its cells are sealed and filled with low-conductivity blowing agents; historically some closed-cell products lost some of that initial advantage over years as blowing gases diffused out and air diffused in, but modern formulations reduce that effect. Open-cell foam is lower density and filled with air, so its per‑inch R-value starts lower and generally shows less change from gas diffusion effects; however both types can lose effective insulating performance if the installed foam is compressed, damaged, or contaminated by moisture.
Settling and mechanical changes are more likely with low-density open-cell foam: because it is softer and more flexible, it can sag or compact in tall wall cavities or between studs if not applied at recommended thickness or if installed in multiple thin lifts improperly. Closed-cell foam is rigid and resists sagging and settling, so its installed thickness and thus long-term insulating contribution are more stable. Moisture interactions also matter: closed-cell foam is much less vapor-permeable and is better at resisting bulk water intrusion and long-term moisture-related R-value loss; open-cell foam can absorb and hold water (reducing its insulating effectiveness while wet) but also dries faster and allows drying of assemblies. In short, environmental exposure and installation quality strongly influence long-term retention of R-value for either product.
When comparing R-values directly, typical industry ranges are about 3.5 per inch for open-cell and roughly 6–7 per inch for closed-cell (exact numbers depend on product and manufacturer). Because closed-cell provides higher R/in and tends to maintain installed thickness and resist moisture intrusion, its long-term effective insulation value per inch of applied foam is usually greater. That said, a realistic assessment should use manufacturer-rated aged R-values, account for potential settling or compression in the chosen application, and ensure proper application technique and thickness to preserve long-term performance for either open- or closed-cell foam.
Application requirements, thickness/coverage, cost, and code considerations
Open-cell and closed-cell spray foams have different R-values per inch—open-cell roughly 3.5–3.8 R/in and closed-cell roughly 6–7 R/in—because closed-cell foam is denser and its cells are filled with low-conductivity gas, while open-cell foam’s broken cells are filled with air. That fundamental R-value difference directly drives application requirements and thickness/coverage calculations: to meet a given target R-value you will need substantially more inches of open-cell than closed-cell. For example, to reach about R‑13 you’d need roughly 3.5–4 inches of open-cell but only about 1.8–2.0 inches of closed-cell (using typical R/inch ranges); for high attic R-values (R‑38+) the disparity becomes even larger. These per‑inch differences also affect board‑foot coverage and how you estimate material quantities for a job.
Cost and installation implications flow from the R-value and physical differences. Closed‑cell foam is more expensive per board‑foot than open‑cell, but because it provides higher R per inch you may need less material and less space to meet the same insulation requirement; this sometimes narrows the effective cost gap. Closed‑cell also tends to add structural stiffness and resists bulk water intrusion, which can reduce secondary costs in some assemblies. Open‑cell is cheaper by volume, better at sound attenuation, and can be preferable where vapor permeability or filling odd-shaped cavities is important. Installation practice differs: closed‑cell often requires higher-pressure equipment, tighter control of substrate and temperature, and strict lift/thickness limits to avoid excessive exotherm; open‑cell has different lift limits and curing behavior. These technical requirements affect labor skill, speed of application, and jobsite handling, so installer selection and adherence to manufacturer specifications are crucial.
Code and regulatory considerations further influence which foam and what thickness are appropriate. Building codes and local climate zones set minimum cavity and continuous‑insulation R-values, and they also define moisture and vapor control requirements; closed‑cell foam, at certain thicknesses, can act as a vapor retarder and in some cases count toward structural or continuous insulation requirements, while open‑cell is vapor‑permeable and may require a separate vapor retarder in cold climates. Both foam types typically require an approved thermal/ignition barrier (such as 1/2″ gypsum or an approved coating) when exposed in occupied spaces unless using listed alternatives, and acceptance criteria (permittable thickness, use in soffits/attics/crawlspaces) vary locally. Because of variable R/inch performance, installation constraints, and differing moisture and fire properties, get local code guidance and manufacturer documentation and have a qualified installer calculate required thickness and coverage for your specific R‑value goals and climate.