R-value drift: what the label number doesn't tell you

Last reviewed: August 4, 2026

Every insulation quote leans on one number: R-value. Here’s what the sales sheet doesn’t say — that number was measured in a laboratory at one moment in the product’s life, at one temperature. Real buildings get neither. Two different things quietly move the real number, and the trade calls both “R-value drift”: what age does to gas-blown foams, and what temperature does to everything. Neither is a scandal. Both are physics you should know before you compare quotes by the label.

(For years the best-known page on this subject lived in the technical library of Foam-Tech, a Vermont foam contractor. The company wound down, and its domain now belongs to an unrelated software firm — only an archived copy survives. This page is our maintained treatment of the same question, with the sources brought current.)

Drift with age: the blowing agent story

Closed-cell foams — closed-cell spray foam, polyiso board, XPS board — insulate as well as they do partly because their cells are filled with a captive blowing agent that conducts heat more poorly than air. That gas doesn’t stay put forever. Cell walls are slightly permeable, so over months and years air works in and some of the blowing agent works out, and the foam’s R-value settles down toward its long-term aged value. Fresh foam is the best it will ever test; aged foam is what you own.

Products with nothing but air in them — open-cell foam, fiberglass, cellulose, mineral wool — have no captive gas to lose, so age doesn’t move their R-value the same way. (Open-cell spray foam’s cells are open by design; it reaches its air-filled state almost immediately, which is why its lower per-inch number is at least an honest one.)

This isn’t fine print — it’s federal labeling law. The FTC’s R-value Rule says it plainly:

“For polyurethane, polyisocyanurate, and extruded polystyrene, the tests must be done on samples that fully reflect the effect of aging on the product’s R-value.” — 16 CFR § 460.5(a)(1), accessed August 2026

So any R-value in an ad, label, or fact sheet for these foams is already required to be an aged basis, not a day-one brag. The industry’s standardized way of producing that number is LTTR — Long-Term Thermal Resistance — from slice-and-accelerate aging tests under ASTM C1303, “Standard Test Method for Predicting Long-Term Thermal Resistance of Closed-Cell Foam Insulation,” or Canada’s CAN/ULC S770. The LTTR convention: an estimate of the R-value after five years, used as a 15-year design average — with the honest caveat, raised even by insulation manufacturers, that drift doesn’t stop at year five. LTTR is the right number to compare; it is still an estimate, not a floor.

What this means for your quote: when an installer says closed-cell runs about R-7 per inch — the rule-of-thumb a 30-year installer uses too — the number to put in writing is the aged R-value from the specific product’s data sheet. Same thickness, same price, two products can age to different numbers. Ask for the data sheet; the honest ones print it.

Drift with temperature: the label is a 75-degree number

The second kind of drift resets daily. The R-value Rule defines the label test at

“a mean temperature of 75 degrees Fahrenheit and with a temperature difference of 50 degrees Fahrenheit” — 16 CFR § 460.5(a)

A pole barn in January is not a 75°F-mean lab. Every product’s R-value is temperature-dependent; the direction and size of the change vary by material, and two cases are well documented:

• Polyiso board in the cold. Building Science Corporation’s testing found “a decrease in R-value as ‘outside’ temperatures go below freezing,” concluding that polyiso’s thermal performance falls off as mean temperature deviates from the 75°F label condition — Info-502, Temperature Dependence of R-values in Polyisocyanurate Roof Insulation. Their fix is telling: thicker polyiso, or a hybrid stack with cold-tolerant insulation on the cold side.
• Loose-fill fiberglass in very cold attics. The archived Foam-Tech library reported U.S. Department of Energy testing in which loose-fill fiberglass fell from R-17.8 at mild temperatures to roughly R-9 with the cold side at -18°F — about half its rated value — while loose-fill cellulose held or slightly improved, and a 1986 comparative test it cited had polyurethane foam holding far more of its value in the same deep-cold, windy conditions. Those specific figures come from that archived page, not from our own testing — and they describe loose-fill attic products under extreme temperature swings, not batts in a wall.

Closed-cell spray foam’s advantage here is structural: no loft to slump, no air to convect inside it, and an air seal that doesn’t depend on staying fluffed. That’s the honest version of the “foam performs when it’s cold” pitch — true, for reasons you can inspect.

What a building owner actually does with this

• Compare quotes on aged numbers. Label R for gas-blown foams must already reflect aging (that’s the FTC rule) — so ask each bidder for the product data sheet and put its aged/LTTR value, not a verbal per-inch figure, next to the price. The cost page’s board-foot math works the same either way.
• Design for your January, not the lab’s 75°F. In cold country, material choice matters more than catalog R suggests — and if part of your stack is polyiso, BSC’s cold-climate caveat above is the one to raise with your builder.
• Remember why the real number matters: a wall performing below its label R runs colder interior surfaces, and colder surfaces are how a building crosses the dew point and starts dripping. That story — the one that actually wrecks buildings — is the condensation guide’s whole subject.
• Don’t over-buy thickness to chase drift. Aging drift is real and already priced into the label number; it is not a reason to double the inches. The 87% rule still governs where your money stops working.

If a quote in front of you claims a number that smells like a day-one value, or a salesman waves away the aged question — that’s exactly the kind of thing the form below exists for. We sell no foam and no boards; the physics doesn’t care whose logo is on the drum.