Spray Foam R-Value per Inch: Open vs Closed Cell

Spray foam R-value per inch: chart and table

R-value measures how well a material resists heat moving through it by conduction. The per-inch figure tells you what each inch adds, so a foam layer's R-value is its thickness times its R per inch, as long as the per-inch number holds at that thickness.

These are published values from manufacturer technical data sheets (TDS) and one DOE guide. Brand names appear only to show the spread between makers, not as recommendations. Whatever foam your contractor sprays has its own TDS, and that's the number to use.

The HFC row comes from a 2009 DOE Building America guide, written when closed-cell foam was blown with HFCs. The same guide put open-cell at about R-3.6 per inch. Federal rules have since moved new closed-cell systems to low-GWP blowing agents; HFO vs HFC spray foam covers that change. For older closed-cell foam already in a building, about R-6 per inch is a fair planning number.

Open-cell vs closed-cell R-value per inch

Open-cell foam weighs about 0.5 lb per cubic foot. It's blown mostly with water, its cells are broken open, and they fill with air.

Closed-cell foam weighs about 2 lb per cubic foot. Its cells are sealed around a blowing agent gas that conducts heat less than air does. That trapped gas is a big part of why closed-cell has close to twice the R per inch.

Fill a 3.5-inch 2×4 cavity and you get about R-13 with open-cell, or roughly R-25 with HFO closed-cell. R per inch is only one line in that decision. Closed-cell vs open-cell in Climate Zone 5 covers vapor control, water and cost.

Initial vs aged R-value

Fresh closed-cell foam tests higher than it will stay. Over months and years, some blowing agent diffuses out of the cells and air diffuses in, and the R-value drifts down before it levels off. Open-cell foam starts out full of air, so it has little to lose.

That's why closed-cell R-values are published as aged values. The FTC's R-value rule (16 CFR 460.5) requires polyurethane insulation to be tested on samples that fully reflect the effect of aging, at a 75°F mean temperature, on the insulation alone. ASTM C518, a heat-flow-meter test on a flat specimen, is one of the test methods the rule accepts.

Labs can't wait years, so two standards speed the aging up:

• CAN/ULC-S770 cuts the foam into thin slices, which age faster, and retests them over time. The result, called long-term thermal resistance (LTTR), represents a five-year aged R-value.
• ASTM C1303 is the alternative method, built on the same thin-slice approach.

Thin slices age faster because the gas has less distance to travel. That's also why thickness matters on a data sheet. Several sheets say their tables are calculated from tests at two thicknesses, such as 1 inch and 4 inches, and products don't all move the same way. One lists R-6.3 at 1 inch and R-26 at 3.5 inches, about R-7.4 per inch. Another lists R-7.2 at 1 inch and R-25 at 3.5 inches, a slightly lower per-inch figure for the thicker layer. Read the R-value at your thickness.

Per-inch R-value vs the R-value of the whole wall

Per-inch numbers describe the foam alone. A wall or roof also has studs, plates, rafters or steel, and heat takes the easiest path through all of it.

Building Science Corporation modeled a 2×6 wall framed 24 inches on center with 5 inches of high-density (closed-cell) foam. At the R-6 per inch BSC used, the foam was R-30 installed, but the whole wall came out near R-20. The same wall with open-cell foam, filled and trimmed flush, was R-21 installed and about R-16 overall.

Steel is far worse. An article in Building Enclosure, citing ASHRAE figures, reports that an R-19 batt between 6-inch steel studs at 16 inches on center performs like about R-7.1, a 63% loss, against roughly 16% in wood studs. In steel framing and metal buildings, insulation that runs over the framing matters more than a deeper cavity fill. Pole barn insulation options covers those buildings.

Then there are diminishing returns. R-value climbs in a straight line as foam is added, but heat flow through a layer falls with 1 ÷ R, so each inch saves less than the one before. In FoamGuard's ASTM C-236 hot-box data, closed-cell foam cut heat loss through the test assembly 72% at 1 inch, 86% at 2, 90% at 3, 92% at 4, 93% at 5 and 94% at 6. How thick should spray foam insulation be turns that into thicknesses for each part of a building.

R-value doesn't measure air leakage

An ASTM C518 test runs on a solid specimen with no air moving through it. In a building, air leaking through gaps carries heat with it, around and through the insulation, and no R-value on a label accounts for that.

Spray foam is also an air barrier once it's thick enough. Data sheets list HFO closed-cell under 0.02 L/s·m² of air permeance (ASTM E2178) at 1 inch. Open-cell products are listed at 3 to 3.5 inches. Comparing insulation by R per inch alone leaves that difference out. Whatever you install, seal the leaks: see air sealing before insulation and spray foam vs fiberglass vs cellulose.

Does cold weather lower spray foam's R-value?

Published R-values are measured at a 75°F mean temperature. In January the foam in an exterior wall runs much colder. With 70°F inside and 10°F outside, its average is about 40°F.

The effect depends on the blowing agent. A gas with a higher boiling point can start to condense inside the cells near freezing, which lets more heat through. In a 2022 paper, Arkema, a blowing-agent maker, reported that penalty at 32°F in lab foams blown with HFO-1336mzz(Z), while the HFOs it tested performed alike at 50°F and 75°F. Those were hand-mixed lab foams, not sprayed insulation, and most spray foam data sheets don't list cold-temperature values. If it matters for your building, ask the manufacturer for the R-value at a lower mean temperature.

Using R-value per inch on a quote

1. Get the product name and its TDS. Look for the aged R-value and the test method.
2. Find the R-value listed at your thickness, not just the headline per-inch number.
3. Make sure the quote states the thickness and resulting R-value for each surface. How to read a spray foam quote covers what else to check.
4. Choose thickness by the job the foam has to do, not by the biggest number.

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