Spray Foam R-Value: What the Number Needs Before You Can Use It

The short version. An R-value means something only with three things attached: the exact product, the thickness it was reported at, and the test it came from. Under US rules an insulation R-value must come from one of four named ASTM tests, run at a 75 F mean temperature on the material alone. A per-inch figure is not allowed on labels or in ads except in two narrow cases. Both canned kits we track publish an R-value per inch based on a superseded German conductivity method. That does not make the number wrong. It means the basis a US buyer would need has not been shown.

What an R-value is, and the test it has to come from

R-value measures resistance to heat flow under stated laboratory conditions. Higher means more resistance. It is not coverage, not air sealing, not durability, and not a property of your wall.

Section 460.5 of the FTC’s R-value Rule is unusually specific about where the number comes from. For insulation other than reflective products, the test must be ASTM C177 (guarded hot plate), ASTM C518 (heat flow meter), ASTM C1363 (hot box) or ASTM C1114 (thin heater). It must be run at a mean temperature of 75 F with a temperature difference of 50 F plus or minus 10 F. It must be run on the insulation alone, excluding any airspace. Results from the two heat-flux methods must be reported only under ASTM C1045.

One clause matters for spray polyurethane in particular. For polyurethane, polyisocyanurate and extruded polystyrene, the rule says the tests must be done on samples that fully reflect the effect of aging on the product’s R-value. Closed-cell foam is gas-filled, and its thermal resistance changes as that gas exchanges with air, which is why ASTM C1303 exists to predict long-term thermal resistance by accelerating that aging in the laboratory.

So a complete R-value claim is not one number. It is a number plus a thickness, a method, a mean temperature, an aging basis and a document you can ask for.

The per-inch rule most listings ignore

Section 460.20 is short and blunt: in labels, fact sheets, ads or other promotional materials, do not give the R-value for one inch or the R-value per inch of your product. There are exactly two exceptions. One covers sellers under an existing FTC cease and desist order, who may petition to amend it. The other is the substantive one.

Under that second exception a seller may give a per-inch figure only where actual test results prove that the R-value per inch of the product does not drop as it gets thicker. Even then the seller may list a range, must say exactly how much the R-value drops with greater thickness, and must add this statement:

“The R-value per inch of this insulation varies with thickness. The thicker the insulation, the lower the R-value per inch.”

That is the reason this site never divides a reported R-value by its thickness, and never multiplies one to reach another depth. Doing so would manufacture a claim that no test supports, and it would assume the linearity the rule was written to question. The Commission discussed exactly that assumption in its 2018 final-rule notice.

Two boundary questions come up often. First, does the rule reach a canned foam that calls itself a sealant rather than insulation? Section 460.22 covers a product other than a fenestration product that is marketed in whole or in part to reduce residential energy use by slowing heat flow. It must be tested under section 460.5, and advertised R-values must fairly reflect those results. Second, how precise may the number be? Section 460.11 requires rounding to the nearest tenth, or to the nearest whole number at 10 and above.

What the tracked kits actually publish

Both one-component kits in our research set publish the same thermal statement, and both state it the way the rule reserves for a narrow exception.

Document	Thermal statement	Basis given
Kraken Bond FastCoat TDS rev. 1	Thermal conductivity 0.025 W/m.K; R-value 5.66 per inch	DIN 52612 for the conductivity; no ASTM method, laboratory or report number for the R-value
FastCoat product page	R-5.66 per inch, and R-11.32 at 2 inches	No test basis stated on the page
Sprayman SprayCoat TDS	Thermal conductivity 0.025 W/m.K; R-value 5.66 per inch	DIN 52612 for the conductivity; no ASTM method, laboratory or report number for the R-value
SprayCoat product page	R-value 5.66 per inch	No test basis stated on the page

Three observations follow, and none of them is a finding against a product.

DIN 52612 is not one of the methods the US rule names. It is a German guarded-hot-plate method for thermal conductivity, and its publisher’s record shows it superseded in 2001 by DIN EN 12664, DIN EN 12667 and DIN EN 12939. A conductivity measured under a European method is a real measurement. It is simply not the basis section 460.5 sets for an R-value used in a US insulation claim. The data sheets also name no ASTM test, laboratory or report number for the R-value they print.

A per-inch figure is the form the rule restricts. Section 460.20 permits it only on proof that per-inch resistance does not fall with thickness, together with the disclosure quoted above. Neither data sheet we have shows that proof or that statement. The obligation sits with the seller, not with the buyer, and a seller may hold documentation we have not seen. So we record the claim, record what is missing, and do not rank on it.

The doubled figure is the maker’s arithmetic, not a second test. R-11.32 at 2 inches is 5.66 taken twice. We do not repeat that calculation as if it were an independent result, and we would not do it for any other brand either.

These questions are asked here in public and left open until a document answers them, for every brand in the order. Our claim-status methodology defines how a documented claim differs from an accepted one, and the ranked kit list shows where each product’s documents stand today. The reviews guide sets out what our own disclosure still owes a reader.

One seller that states the number the way the rule asks

The clearest example we have found of a complete claim comes from a wide-spray can rather than a kit. DAP describes its Wall and Cavity Foam as meeting an ASTM C518 aged R-value of 4.1 at 1 inch. That sentence carries a named method the rule accepts, a thickness, and an aging basis.

Set it beside the four kit data sheets we hold. Kraken Bond, Sprayman, Akfix and Stanley each print R-5.66 per inch, and each derives it from a thermal conductivity of 0.025 W/m.K measured under DIN 52612. That figure works out at about R-5.77 an inch, so the printed 5.66 is the same measurement rounded down rather than a second test. DIN 52612 is a German conductivity method and is not one of the four the FTC rule names, and none of the four sheets states whether the specimen was aged.

Product	Stated R-value	Method behind it	Aging basis
DAP Wall and Cavity Foam	4.1 at 1 inch	ASTM C518, named	stated as aged
Kraken Bond FastCoat	5.66 per inch	DIN 52612 conductivity	none stated
Sprayman SprayCoat	5.66 per inch	DIN 52612 conductivity	none stated
Akfix ThermCoat	5.66 per inch	DIN 52612 conductivity	none stated
Stanley SuperCoat	5.66 per inch	DIN 52612 conductivity	none stated

The gap is 28 percent, and working backwards from 4.1 gives a conductivity near 0.035 W/m.K against the 0.025 the sheets print. That is the size of gap the aging rule exists to catch, but do not read it as a measurement of aging here. These are two different products from two different makers, and one figure is aged while the other names no basis at all, so the difference carries product, formulation and method all at once. What it shows is that a per-inch number with no basis printed next to it cannot be compared with one that has a basis.

Two things this does and does not show. It shows that the one product stating an aged, method-named figure states a much lower one, which is what a reader should expect once aging is counted. It does not show that the other four foams are really 4.1. They are different formulations, we have measured nothing, and 4.1 belongs to DAP’s foam alone. What a reader can do with it is ask any seller the question DAP has already answered: is your figure initial or aged, and under which method?

When two R-values can be compared, and when they cannot

The same logic in table form, for a page you might be reading with two product tabs open:

Available evidence	Reported basis	How we treat it
Exact-product R-value with supporting documentation	Same thickness, compatible test basis	Direct comparison may be shown, with its conditions
Exact-product R-values at different thicknesses	Unlike thickness	Not directly comparable; do not divide either value to create a per-inch figure
An R-value from a listing with no thickness or evidence source	Incomplete basis	Claim incomplete; kept out of performance ranking until verified
Different formulation, test method, temperature or reporting basis	Unlike evidence	Not directly comparable until documentation establishes a common basis

Material R-value is not whole-wall performance

Even a fully documented material R-value describes the material, not your building. Framing, fasteners, voids, uneven thickness and other thermal bridges change heat flow through the finished assembly, and air leakage and moisture behavior are separate control questions. Section 460.5 makes the boundary explicit at the test bench: the value is measured on the insulation alone, excluding any airspace.

Our sister publication SprayFoamGuides covers the building-level question in R-value, thickness and board feet. For the volume of material a job needs, which is a different calculation entirely, use the coverage and yield guide and the spray foam calculator.

What this means for a buyer

• Ask for the value at your thickness. Not per inch, and not a doubled figure. The number you can act on is one that was tested and reported at a depth you will actually install.
• Ask which ASTM test produced it. Section 460.5 names four. A conductivity figure from a European method is a measurement, but it is not that basis, and neither tracked data sheet names an ASTM test or report for its R-value.
• Treat a per-inch number as a flag, not a spec. The rule allows one only with proof that per-inch resistance does not fall with thickness, plus a specific disclosure sentence. If neither appears, the figure has not been placed on the footing the rule expects.
• Keep R-value away from coverage and fire. A can’s rated yield and its Class A surface-burning result are separate claims on separate evidence.
• Ask about aging for closed-cell foam. The rule requires samples that fully reflect aging, and ASTM C1303 exists for long-term thermal resistance. A value with no aging basis is incomplete rather than wrong.
• Do not rank on the biggest number. Documentation quality, the thickness you can actually install, coverage, application control, fire requirements and the assembly all sit alongside thermal resistance.

A short record beats a remembered number. Save the product page and the data sheet with their dates, the value with its thickness, the method if one is named, and a line for what is still missing. That last line is usually the most useful thing on the page.