Open Cell vs Closed Cell Spray Foam: What Oklahoma Homeowners Need to Know

Every spray foam conversation starts with the same question: open cell or closed cell? The answer is not one-size-fits-all. These are fundamentally different materials with different densities, different R-values per inch, different vapor behaviors, and different costs. Choosing the right one depends on where it goes, what it needs to do, and what the building science demands for that specific assembly.

At Bo’s Spray Foam, we install both. We choose based on the application, not the margin.

The names describe the physical structure of the cured foam.

Open cell foam has cells that are not completely closed. The blowing agent — typically water-blown — escapes during the reaction, leaving a soft, flexible matrix. Cured density is approximately 0.5 lb/ft³. The foam is light, spongy, and compressible. It fills cavities completely, conforming to irregular framing, wiring, and plumbing penetrations.

Closed cell foam has cells that remain intact and sealed. The blowing agent stays trapped inside each cell, contributing to higher thermal resistance. Cured density is approximately 2.0 lb/ft³ — four times heavier than open cell. The foam is rigid, structural, and adds racking strength to wall assemblies. It bonds aggressively to substrates and does not compress under normal loads.

This density difference drives almost every downstream distinction between the two materials: R-value, vapor permeability, structural contribution, moisture behavior, and cost.

This is the number everyone asks about first, and it matters — but not as much as most people think.

• Open cell: R-3.7 per inch
• Closed cell: R-6.5 per inch

Closed cell delivers roughly 76% more thermal resistance per inch. That is a real, measurable advantage. But R-value per inch is only half the story. The other half is installed depth — and depth depends on the application.

In a 2x4 wall cavity (3.5 inches deep), depth is constrained by the framing. You cannot spray 5 inches of foam into a 3.5-inch cavity. Here, closed cell at 2 inches delivers R-13, meeting the 2009 IECC prescriptive requirement for walls. Open cell fills the full 3.5 inches at R-13 — same result, different path.

In an attic roof deck, depth is not constrained the same way. Rafters are typically 2x6, 2x8, or deeper. Open cell at 5.5 inches delivers R-20. Closed cell at 3 inches delivers R-19.5. Both pass Oklahoma code via the REScheck performance path.

The point: R-value per inch matters most where depth is limited. Where depth is available, open cell can match closed cell’s total R-value — just at greater thickness.

This is the concept most insulation discussions get wrong, and it is the most important one to understand.

Thermal resistance does not scale linearly with real-world performance. The relationship between R-value and heat flow reduction follows a diminishing returns curve. Here is the math:

Heat flow reduction = 1 - (1/R-value), expressed as a fraction of the uninsulated baseline.

• R-5 eliminates 80% of conductive heat transfer
• R-13 eliminates approximately 92%
• R-19 eliminates approximately 95%
• R-26 eliminates approximately 96%
• R-38 eliminates approximately 97%

Going from R-0 to R-13 captures 92 percentage points of improvement. Going from R-13 to R-26 captures only about 4 more percentage points. Doubling the insulation does not double the performance. It captures a smaller and smaller slice of the remaining heat loss.

This is why we do not push homeowners toward unnecessarily thick applications. The energy savings between R-19.5 (3 inches closed cell) and R-26 (4 inches closed cell) on a roof deck are real but modest — and the cost difference is significant. We install what the building science supports, not what maximizes the invoice.

This is where the two foams diverge most meaningfully for building science purposes.

Open cell is vapor-permeable. At standard densities, it has a permeance of roughly 10-16 perms at typical installation depths. Moisture vapor can move through it. This is neither good nor bad — it depends on the assembly. In a vented attic floor application, permeability is irrelevant because the attic is ventilated. On an unvented roof deck, vapor permeability means moisture from conditioned space can reach the roof sheathing. In Oklahoma’s Climate Zone 3, IRC R806.5 permits open cell on unvented roof decks without a Class II vapor retarder — that requirement applies only in Zones 5 through 8, where winter condensation risk is higher.

Closed cell is a vapor retarder at standard installation depths. At 2 inches, it achieves approximately 1 perm or less, qualifying as a Class II vapor retarder. This means it blocks most moisture vapor migration. On a roof deck, closed cell inherently protects the sheathing from interior moisture. In crawlspaces and below-grade walls, this vapor resistance provides an additional layer of moisture management.

For Oklahoma specifically, Climate Zone 3 is relatively forgiving on vapor control. Summer humidity is the dominant moisture load, not winter condensation. Both foam types work — but the approach to moisture management differs. The 2018 IRC R806.5 requires unvented attics in Climate Zones 1 through 3 to incorporate moisture management: supply air of at least 50 CFM per 1,000 square feet of ceiling area, a transfer fan, or mechanical dehumidification. This applies regardless of foam type.

Closed cell foam at 2.0 lb/ft³ density adds measurable racking strength to wall assemblies. FEMA has documented closed cell foam’s performance in high-wind events — it effectively glues the sheathing to the framing and stiffens the wall as a unit. In Oklahoma, where straight-line winds and tornadoes are real considerations, this is a legitimate structural benefit.

Open cell foam at 0.5 lb/ft³ adds no meaningful structural value. It is too soft and too light to resist lateral loads. It fills cavities and stops air movement, but it does not stiffen the assembly.

This is a genuine differentiator, but it should not be the primary reason for choosing closed cell. The structural benefit is a bonus — the primary decision should be driven by thermal performance, moisture management, and cost for the specific application.

Both open cell and closed cell spray foam are air barriers when installed at sufficient thickness. This is the single biggest advantage spray foam has over fiberglass batts and blown cellulose — the insulation and the air barrier are the same material in the same pass.

Air leakage is the dominant source of energy loss in most residential buildings. A house insulated to R-38 with fiberglass but leaking at 12 ACH50 will underperform a house insulated to R-19 with spray foam at 3 ACH50. The air sealing matters more than the R-value in most practical scenarios.

Both foam types, properly installed, create a continuous air barrier at the building envelope. The 2018 IRC requires new construction to achieve 5 ACH50 or less. In our retrofit work, we routinely see spray-foamed homes testing at 2 to 3 ACH50 — well below code minimum and in the range where controlled ventilation becomes important.

Closed cell costs more per board foot than open cell. The raw chemistry is more expensive, the blowing agents cost more, and the material yield per drum is lower because of the higher density. As a rough framework:

• Open cell runs lower per board foot, and a board foot is one square foot at one inch thick.
• Closed cell runs approximately 2 to 3 times more per board foot than open cell.

But cost per board foot is not the same as cost per project. The relevant comparison is cost to achieve the target R-value for a specific assembly.

In an attic roof deck: 5.5 inches of open cell (R-20) versus 3 inches of closed cell (R-19.5). The open cell application uses more board feet but at a lower cost per board foot. The closed cell application uses fewer board feet but at a higher cost per board foot. For most attic projects, open cell comes in at a lower total cost for equivalent thermal performance.

In a 2x4 wall: 3.5 inches of open cell (R-13) versus 2 inches of closed cell (R-13). Same R-value target, same cavity. Open cell uses more material but at lower cost. Closed cell uses less material but at higher cost. The total project cost gap is narrower in walls than in attics, and closed cell adds the vapor retarder and structural benefits.

We price both options for every project and let the homeowner make an informed decision.

Based on Oklahoma’s climate (Zone 3, except the panhandle in Zone 4), code requirements, and building science:

Attic roof deck (unvented attic): Both types work. Open cell at 5.5 inches (R-20) or closed cell at 3 inches (R-19.5). Both pass via REScheck performance path. Open cell is typically more cost-effective here. The 2018 IRC R806.5 moisture requirements apply to both — supply air, transfer fan, or dehumidification. Closed cell provides inherent vapor control at the sheathing; open cell relies on the conditioned attic and moisture management strategy.

Exterior walls (2x4 framing): Both types achieve R-13 prescriptive. Open cell fills the full 3.5-inch cavity. Closed cell at 2 inches hits R-13 and leaves 1.5 inches of cavity space. The remaining cavity is sometimes left empty, sometimes filled with fiberglass — a practice called “flash and batt.” For new construction and retrofits in Oklahoma, either approach meets code.

Crawlspaces: Closed cell is generally preferred on crawlspace walls and rim joists. The combination of R-value, vapor resistance, and moisture tolerance makes it the better fit for below-grade and near-grade applications. The 2009 IECC prescriptive calls for R-5/13 on crawl space walls in Climate Zone 3.

Metal buildings and pole barns: Closed cell bonds directly to metal substrates, provides vapor control to prevent condensation on the cold metal, and adds structural rigidity. Open cell on metal panels risks trapping moisture against the metal. For metal building applications, closed cell is the standard choice.

The right foam for the right assembly. That is the only honest answer to the open cell versus closed cell question.