Insulation R-Value Comparison: The Chart, and What Backs Each Number

The short version. The per-inch comparison chart is a convenience, not a measurement. No US agency publishes one. The figures come from trade groups and manufacturers, and they are reached by dividing a tested R-value by a thickness. Do that division yourself on three published data sheets and you get three different behaviors: on a stone wool batt the per-inch figure falls as the product gets thicker, on a fiberglass blanket it rises, and on an XPS board one product returns 5.0, 5.4 or 5.5 per inch depending on the test temperature and the aging method. Use the chart to sort materials into rough tiers. Use the product’s own sheet, at the thickness you plan to install, to size anything.

Where the numbers on a comparison chart come from

Search for an insulation R-value chart and you will find the same table on dozens of sites: fiberglass around 3, cellulose around 3.5, mineral wool around 4, closed-cell spray foam around 6 or 7. It is rarely sourced, and the reason is that there is no single authority to source it to.

What exists instead is this. The DOE Insulation Fact Sheet gives two rules of thumb, and gives them for a specific job: working out what is already in your attic. Multiply fiberglass batt thickness by 3.2, multiply foam board or sprayed-on foam by 5. ENERGY STAR publishes recommended total R-values by climate zone, which is the number a project gets sized against. Between those two sits a gap, and the trade associations fill it: the Cellulose Insulation Manufacturers Association for cellulose, the Spray Foam Coalition for professional foam, NAIMA for fiberglass and stone wool, each publishing a figure for its own members’ products.

Those figures are not invented. They are just further from the test than they look, because each one has been through a division.

What a published sheet actually states

Under 16 CFR 460.5, an insulation R-value has to come from one of four named ASTM tests, run on the insulation alone with no airspace, at a mean temperature of 75 F. Nothing in that rule produces a per-inch number. It produces a value for a specimen at a thickness.

Manufacturers publish exactly that, as pairs. Here is what four of them state, and what the division gives.

Product	Published by the maker	Per inch, by division	Test named
ROCKWOOL Comfortbatt stone wool	R15 at 3.5 in, R23 at 5.5 in, R30 at 7.25 in, R38 at 9.5 in	4.29, 4.18, 4.14, 4.00	ASTM C518
Owens Corning EcoTouch fiberglass blanket	R-10 at 3.4 in, R-13 at 4.3 in, R-19 at 6.3 in, R-30 at 9.25 in	2.94, 3.02, 3.02, 3.24	ASTM C177 / C518
Owens Corning FOAMULAR 250 XPS board, aged 180 days	R-5.0 at 1 in, R-10 at 2 in, R-20 at 4 in	5.00, 5.00, 5.00	ASTM C518 at 75 F mean
The same FOAMULAR 250 board, long-term method	R-5.0 at 1 in, R-10.6 at 2 in, R-22.0 at 4 in	5.00, 5.30, 5.50	CAN/ULC S770-03

Read down the first column and the per-inch figure drops by seven percent across the range. Read down the second and it climbs by ten percent. Read the third and fourth together and one board gives a flat 5.0 under one method and a rising 5.0 to 5.5 under another. The same sheet also states R-5.4 at one inch at a 40 F mean temperature, which is the identical board tested colder.

Owens Corning does not bury this. The sheet says it in a footnote, in its own words: R-values vary depending on many factors including the mean temperature at which the test is conducted, and the age of the sample at the time of testing. It then names which of its several figures is the one printed on the product, the 180 day value at 75 F, because that is the one used to compare products.

That footnote is the whole subject of this page. A comparison chart picks one of those numbers and prints it as the property of the material.

Why the rules restrict the per-inch form

16 CFR 460.20 tells sellers not to give an R-value for one inch or per inch in labels, fact sheets, ads or promotional material. There are two exceptions, and the important one requires actual test results proving the R-value per inch does not drop as the material gets thicker. Even then the seller has to state how much it drops and add a warning sentence.

The Comfortbatt line above is what that rule is written about. Nothing there is wrong or misleading: the maker published four honest pairs. It is the division that introduces the error, and the size of the error depends on which pair you divide.

This site does the division anyway, in the comparison tables, so that eleven canned kits sold in different can sizes can be lined up in one column. Every one of those figures carries the thickness and the document it came from, and none of them is a claim that the foam delivers that value at any depth you like. Where a maker reported at two inches, that is what the record says.

Cellulose does not have a per-inch number you can measure with a tape

Cellulose is the row where the chart breaks most cleanly. CIMA’s guidance says it is reasonable to assume an R-factor of 3.8, and then says the thing the chart cannot carry: R-values for cellulose are based on settled thickness and bag count, not installed thickness.

Loose fill settles. A contractor who blows in enough depth on the day and not enough material by weight leaves an attic that measures the right number of inches and holds less insulation than the label promises. That is why the coverage charts on a cellulose bag give a minimum bag count per thousand square feet alongside the depth, and why both have to be met. Settled thickness is determined under the FTC rule’s methodology; installed thickness is checked under ASTM C1374.

A per-inch figure for cellulose is therefore a description of correctly installed material, not a measurement you can take with a ruler in someone else’s attic.

Where canned DIY kits land

Professional foam has trade-body figures: open-cell starting at 3.6 per inch, closed-cell starting at 5.7 per inch, both from the Spray Foam Coalition, neither with a test method attached. The coalition’s own page tells the reader to ask the seller for the fact sheet on R-values, which is the correct advice and the reason this site exists.

Canned kits are a narrower market and the pattern there is sharper. Of the eleven kits in our yield table, seven print exactly 5.66 per inch. Not a range, not a similar figure: the identical number, across brands with different can sizes, different sellers and different countries of origin. Four of those seven can point at a data sheet, a coverage chart or an application guide that prints it. Three cannot, and on those the number lives in store copy or a marketplace table with nothing behind it. The remaining four kits cluster between 4.0 and 4.12, and three of those four got there by halving a two-inch figure.

So 5.66 is not a finding about any one of those foams. It is the number the category prints. Treat it the way you would treat a price that is identical at seven shops: as evidence about the supply chain, not about the product.

The number your project is actually sized against

Material R-value is measured on the insulation alone, excluding any airspace. A wall is not that. The DOE fact sheet is blunt about the gap: heat flows through the framing as well as the cavity, so the overall R-value of a wall or ceiling is somewhat different from the R-value of the insulation itself, and in metal-framed walls the assembly can fall to as low as half the insulation value.

For a homeowner that reorders the questions. Recommended total R-values by climate zone come first, from ENERGY STAR. Then the cavity depth you have. Then which material reaches that total in that depth, which is where the per-inch tiers are genuinely useful: if a 2x4 cavity has to hit a high number, a material at 3 per inch cannot get there and one at 6 can. Then, and only then, the product’s own sheet at the thickness you will install.

How to use a comparison chart without being misled by it

• Use it for tiers, not for arithmetic. Fibrous materials around 3 to 4.3 per inch, rigid boards and closed-cell foams around 5 to 6, is a fair summary of the sheets above.
• Before sizing anything, find the pair. A tested R-value and the thickness it was tested at, on a document with a date on it.
• Check the mean temperature if the figure is for a rigid or closed-cell foam. The FTC test is at 75 F. A colder test gives a better number for the same board.
• Check the aging basis on any closed-cell product. Aged and long-term figures are both legitimate and they are not the same figure.
• For loose fill, check the bag count as well as the depth.
• Add layers if you like. Do not treat the sum as the wall.