Closed-Cell Spray Foam R-Value: Per Inch, Aged, and What the Receipt Must Show

Illustrative

Closed cell spray foam R value, the short answer

A closed cell spray foam R value is a number printed by the maker, for one product, at one thickness, measured one way. Two things move it: how old the foam is when it is measured, and how thick the layer is. Read it from the product's own data sheet, never from a general figure.

• Per inch: the sheets we opened print values near 6 per inch, and one prints 6.5 at one inch. Different products, different test years, different numbers.
• Aged vs fresh: a fresh reading and a long-term reading are not the same number. The aged one is the one to buy by.
• Thickness: a thicker layer can show a higher value per inch than a thin one, so R per inch times inches is the wrong way to state a job.
• On paper: a receipt for installed insulation should show area, thickness and R-value, taken from the maker's data.

What the foam is and what a data sheet contains is on what closed-cell foam is. This page is only about the R-value line: what it means, why it drifts, and how it should be written down. What R your building needs is a code question, covered in code R-values by edition, and how many inches of foam it takes to stop condensation is on how thick closed-cell foam should be.

R per inch, honestly

R-value is resistance to heat flow, so a higher number slows heat more. For foam sold by the layer, the number you meet first is R per inch, and it varies more than most people expect.

Here is what the documents we opened actually print.

Two cautions about reading that table. First, the first two rows are old and general. They are good for sizing up closed-cell against open-cell, and poor for stating the number on a job. Second, the last two rows are different products from different makers, so the 6.5 and the 6 are not a contest. They are two products' own claims, each valid only for its own foam.

None of these is our product. The sheet for the foam PolyJacks will actually spray is a separate item, under the table section further down.

Why the R drops after spraying

Closed-cell foam is a mass of tiny sealed cells. Each cell holds a gas, and that gas is part of why the foam resists heat flow. Neal Ganser likened the blowing agent to the argon between window panes. Over time the gas in a cell slowly trades places with air, and the foam's resistance settles lower than the number measured when it was fresh.

Test laboratories handle this in a standard way. Intertek describes CAN/ULC S770 as a method for closed-cell foams that tests the full thickness first, then thin slices to speed up the aging, then retests the slices at intervals. The result is a long-term thermal resistance, or LTTR, which the page says represents a five-year aged value. In short: the maker slices the foam thin so the gas can leave in months rather than years, and reports where the number lands.

Three practical points follow.

1. LTTR is a laboratory prediction, not a reading on your building. It tells you the value the maker expects the product to settle at. It does not tell you what any single job measured.
2. The sheet should say which kind of number it prints. An "initial" or "design" value and an "aged" or "LTTR" value are different claims. If the sheet prints one and not the other, ask the maker for the other.
3. Aging is not the same as failing. The drop is a settling of the measured value, which the aged number already includes. A building insulated by the aged number has what it paid for. What else can wear on foam over the years is on what degrades spray foam.

Where the out-of-state sources fit. The DOE guide is national, Intertek is a testing company with no stated location, and the Huntsman sheet is Canadian. Gas moving out of foam cells and heat moving through foam are physics and material behavior, so they apply in Kansas and Missouri as they do elsewhere. What does not carry over is Canadian code language on the Huntsman sheet, which we do not use.

Thickness bands: why R per inch is not a constant

The Huntsman sheet is useful because it prints long-term values at three thicknesses. This is the sheet's own table, with one arithmetic column added by us.

How we converted: one RSI is 5.678 R, and we divided by the inches in each band. The sheet itself rounds to R-12, R-18 and R-26. The per-inch column is our arithmetic, not the maker's claim.

What the table shows is that doubling the layer from 50 mm to 100 mm took the value from 2.06 to 4.50 RSI, which is more than double. So the average value per inch rose with depth for this product. We make no claim that every foam behaves this way, and the sheet gives no reason. The point is narrower: when a maker lists values by thickness band, that list is the answer, and a single per-inch figure multiplied by inches is a shortcut that can miss it in either direction.

The same sheet also lists a design value of R-6 per inch. A design number and a measured band can both appear on one sheet without contradiction, because they answer different questions.

How many inches for R-30? Use the sheet, not the multiplication

A buyer asks for a target R and wants inches. Dividing 30 by 6 gives 5, and it feels like an answer. It is not one the rules support, and for the product above it is not even answerable.

Take the Huntsman table. Its tallest band is R-26 at 100 mm. R-30 lies above everything the sheet lists, so the sheet cannot say how thick R-30 is. To state R-30 for that product, the maker would have to publish a tested value at the thicker band, or give a rule for stacking passes. Anything in between is a guess, and a guess does not belong on a signed document.

So the method for any product is this.

1. Find the sheet's long-term table and the test standard it names.
2. Find the band that contains your target R.
3. If the target falls between two printed bands, ask the maker how the in-between depth is stated. Do not interpolate on a receipt.
4. If the target is above the tallest band, ask what the maker publishes for a thicker layer.
5. Check whether the building needs that R at all. In a shop that only needs the steel kept above the dew point, the answer may be a thinner layer than the code R for a heated room; see how thick closed-cell foam should be.

What your receipt must show

The Federal Trade Commission has a rule on home insulation sales. Its section on what installers must tell customers says the receipt for standard installed insulation must show the coverage area, the thickness and the R-value, and must be dated and signed by the installer (16 CFR 460.17, via Cornell LII). Two other lines in the same section matter for foam.

• Use the maker's data. To figure out the R-value, the section says to use the data the manufacturer gives you.
• No multiplying. The section tells the installer not to multiply the R-value for one inch by the number of inches installed.

It also says not to add up coverage areas or R-values for different parts of the house. So the section implies a receipt written section by section, each area with its own thickness and its own R, with no total R for the building.

That rule is aimed at homes. Most of our work will be shops, barns and steel buildings, so the rule is not a legal description of the job. We use its fields anyway, because they are the right ones to ask for on any foam job. What to ask a contractor about the sheet and the receipt is covered in how to choose a spray foam contractor.

A receipt for a foam job should read like this, section by section. These are blank fields, not figures.

How we measure and write these at the free visit is in the free estimate visit.

FAQ

What is the R-value of closed cell spray foam per inch?

It depends on the product, so read it off the data sheet. The sources we opened print values close to 6 per inch, with one product at 6.5 for its first inch. Aged values are the ones to rely on, and they run lower than a fresh reading.

Does R-value per inch change with thickness?

On some sheets it does. The Huntsman sheet above lists three bands, and the value per inch rises from the thinnest to the thickest. The federal receipt rule says not to multiply the one-inch value by the inches, so the band that matches your thickness is the one to state.

Does spray foam lose R-value?

Closed-cell foam's measured value settles lower than the fresh reading as the gas in its cells exchanges with air. Test labs report an aged figure, the LTTR, to account for it. Sheets that print only a fresh number leave the question open.

How many inches of spray foam for R-30?

Use the product's thickness table, not R per inch times inches. If R-30 is above the sheet's tallest listed band, the sheet cannot answer it, and the maker has to.

Book a free on-site measurement

At the visit we measure the surfaces, ask what the building is used for and tell you which R targets apply to it. A written estimate follows, built from the product's sheet. If your building needs a code R you will see the thickness the sheet supports for it, and where the sheet leaves a gap we say so.

Book a free on-site measurement

Pick "Spray foam insulation" on the booking form. We work within about 35 miles of Spring Hill, KS.

Spray foam insulation is a new PolyJacks service line run by its own crew and rig.

Who wrote this: PolyJacks spray foam team. How it was made: drafted with AI assistance from the sources below, then checked against each source.

Sources

• Intertek, "CAN/ULC S770" (no date or author shown): intertek.com
• Neal Ganser, "Closed-Cell Foam Beats Open-Cell Foam," GreenBuildingAdvisor, October 28, 2009: greenbuildingadvisor.com
• U.S. Department of Energy, Building America, "Which Spray Foam Is Right For You?" (Steven Winter Associates, 2009): www1.eere.energy.gov
• Huntsman Building Solutions, Heatlok Soya HP commercial sell sheet (Canada; the file's web address carries October 2024): huntsmanbuildingsolutions.com
• UPC (Universal Polymers), "UPC 2.0 HL HFO Technical Data Sheet and Spray Guidelines," code UPC-TDS-2.0-HFO 091720EA, hosted by Sprayworks Equipment: sprayworksequipment.com
• Federal Trade Commission, 16 CFR 460.17, What installers must tell their customers, via Cornell LII: law.cornell.edu