Closed-Cell Spray Foam R-Value: What Each Inch Buys You

The short answer: Closed-cell spray foam carries an aged R-value of roughly R-6 to R-7 per inch, the highest of anything we install, with some newer products published a bit above that. The number that matters is the one on the manufacturer’s data sheet for the thickness you are actually getting, because the rating is not a straight multiple of the one-inch figure. In a Minnesota house, that per-inch number decides how much foam it takes to meet code in a 2x4 wall or a basement.

People search for this number because a quote landed on the kitchen table with “2 inches closed-cell” on it and they want to know what that means. Fair question. R-value is the one spec on an insulation quote you can check yourself, as long as you know how the number is produced and where it stops telling the whole story.

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

R-value is resistance to heat flow. Higher is better, and for a given material it grows with thickness.

Closed-cell foam is dense, rigid polyurethane, around two pounds per cubic foot, and its cells are sealed shut with a blowing agent trapped inside. That trapped gas conducts heat more slowly than air, which is why closed-cell outperforms nearly everything else per inch. The common range quoted across the industry is about R-6 to R-7 per inch.

Specific products land in different places. One widely used example: Huntsman’s technical data sheet for Heatlok HFO Pro, a closed-cell foam made with the newer HFO blowing agents, lists an aged thermal resistance of R-7.4 at one inch. Other brands publish their own figures, and the data sheet for whatever foam is going into your house is the document to ask for.

Open-cell foam is a different product with a different job. Its cells are broken open and filled with ordinary air, so it runs lower. Huntsman lists its Classic Ultra Select open-cell foam at R-3.7 per inch. That is roughly half of closed-cell, which is why open-cell only makes sense where there is room for a thicker layer.

Why the data sheet says “aged”

Here is the detail most R-value charts skip. Freshly sprayed closed-cell foam tests higher than it will settle at, because over time some of the blowing agent inside the cells trades places with air, and air insulates less well.

The federal rule that governs insulation labeling, the FTC’s R-value rule at 16 CFR Part 460, deals with this directly. It requires polyurethane foam to be tested on samples that fully reflect the effect of aging. So the honest number, and the one that belongs in any comparison, is the aged number.

The same rule sets the lab conditions. R-value tests are run at a mean temperature of 75 degrees with roughly a 50-degree difference across the sample. That gives every product the same yardstick, which is the point. It also means the label describes a lab condition, not a January wall in Monticello. Treat R-value as a fair way to compare products, not as a forecast of a specific night.

Why two inches is not always twice one inch

Read that Heatlok sheet closely and something jumps out. One inch is listed at R-7.4. Three and a half inches, the depth of a 2x4 cavity, is listed at R-23.

Multiply 7.4 by 3.5 and you get almost 26. The sheet says 23. The manufacturer publishes the thicker figure separately rather than letting anyone scale up the one-inch number, and that is the right way to read it. When we talk through a thickness, we go by the line on the sheet for that thickness when one is published, and we do not multiply the best-looking figure by the number of inches.

This is also a quick check on any quote or ad you see. If someone takes a one-inch rating and multiplies it out to a wall depth, the result will usually look better than the manufacturer’s own published figure for that depth.

What R-value leaves out

R-value measures conduction through the material. It does not measure air moving through or around it, and in our climate air movement is a big part of where the heat goes.

That is the real reason closed-cell foam earns its price in the spots we recommend it. The same data sheet lists air leakage under 0.02 liters per second per square meter at one inch, which is effectively an air barrier, and a vapor permeance of 0.91 perms at one inch. The building code treats anything at or under 1.0 perm as a Class II vapor retarder, so at an inch that product is already doing vapor control on top of insulating. Fiberglass is the opposite case: air moves through it, so a batt often performs below its label. We walk through that comparison in our post on spray foam versus fiberglass batts.

The framing matters too. Softwood studs run a little over R-1 per inch, so every stud in a wall is a strip where heat takes the easy way out, no matter what is in the cavities beside it. A cavity rating is the best case for the wall, not the wall’s average.

How much R-value does Minnesota code ask for?

Minnesota amends the national energy code in its own rule, Minnesota Rules chapter 1322. Its Table R402.1.1 sets the prescriptive insulation levels, and most of the northwest metro and the I-94 corridor sits in climate zone 6, with the northern tier of the state in zone 7.

For zone 6, the table calls for:

• Ceilings: R-49.
• Wood frame walls: R-20 in the cavity, or R-13 in the cavity plus R-5 of continuous insulation or insulated siding.
• Basement walls: R-15.
• Crawl space walls: R-15.

Footnote (a) under that table is the one that matters for foam. When insulation goes into a cavity shallower than the product’s label thickness, the installed R-value still cannot be less than what the table requires. In other words, a thin wall does not get a pass on the number.

That is where per-inch performance stops being trivia. A 2x4 wall has three and a half inches to work with. To reach R-20 in that depth, the insulation has to average better than R-5.7 per inch. Standard batts cannot get there. A closed-cell foam published at R-23 for a full 2x4 cavity clears it. This comes up on additions and on older ramblers and split-levels around Maple Grove where someone wants to open a wall without reframing it deeper.

What does a basement wall need in Minnesota?

Minnesota’s rule spells out how interior spray foam on a foundation wall has to be installed. Section R402.1.1.6 says closed-cell foam on the inside of a foundation wall has to be sprayed directly onto the concrete, must leave at least a one-inch gap between the foundation wall and any framing, and cannot be punctured by utilities or fasteners used to hang a frame wall. It also sets a vapor permeance window for the foam, no more than 0.8 perms and no less than 0.3, measured by two different test procedures.

To hit R-15 on that wall, a closed-cell foam in the R-7 range needs a little over two inches, depending on the product’s published figures at that depth. Open-cell foam is allowed too, but the same section requires a separate vapor retarder and air barrier on the warm side, which closed-cell does not need.

What does each inch actually buy?

Heat flow through an assembly is inversely proportional to its R-value, which means every inch is worth a little less than the one before it. Doubling the R-value of a patch of wall halves the heat moving through that patch. Doubling it again halves what is left. The first couple of inches carry most of the benefit.

That is why we do not spray every assembly as deep as it will go. Our rule of thumb works by location:

• Rim joists. The band of framing where the house sits on the foundation leaks more air than almost anywhere else. About two inches of closed-cell handles the air sealing, the vapor control, and a solid chunk of R-value in one layer, and it is where we usually start on an existing home.
• Basement walls. Enough to reach R-15, with the one-inch gap to framing that Minnesota requires.
• 2x4 walls and cathedral ceilings. Places where the depth is fixed and the code target is high. This is the case where full-depth closed-cell makes sense.
• Open attic floors. Not a good place for deep closed-cell. The ceiling needs air sealing, and then loose fill can pile up to R-49 in space that costs nothing to use.

Thickness also has a practical limit on the day of the job. The same Heatlok sheet caps a single pass at three inches, or two inches followed by another two once the first has risen, and says to let the foam’s core cool below 100 degrees before spraying more. Closed-cell foam gives off heat as it cures, and deep lifts sprayed too fast are a fire hazard. A full 2x4 cavity is two passes, not one, and a good crew does not rush it. The same sheet also lists a winter floor of 15 degrees for the surface being sprayed, which matters on a January job in an unheated building.

Getting the number right on your quote

When you compare quotes, ask three things: which foam product, how many inches, and what the manufacturer’s data sheet lists as the aged R-value at that thickness. Those three answers tell you more than any per-inch claim in an ad. We are glad to go through them for any job, and our spray foam insulation page covers where we use closed-cell and where open-cell makes more sense.

Frequently asked questions

What is the R-value of 2 inches of closed-cell spray foam?

Roughly R-12 to R-14 for most closed-cell products, with some newer HFO foams listed a bit higher. The accurate figure is whatever the manufacturer’s data sheet publishes for two inches, since those sheets often list thicker layers separately rather than multiplying the one-inch number. Ask your installer which product they spray and look it up.

Does closed-cell spray foam lose R-value over time?

Freshly sprayed foam tests higher than it settles at, because some of the blowing agent in the cells slowly exchanges with air. Federal labeling rules require foam R-values to be tested on samples that reflect that aging, so the aged number on a current data sheet is the one to rely on for the life of the building.

Is closed-cell or open-cell spray foam better for R-value?

Closed-cell, by a wide margin. It runs around R-6 to R-7 per inch against roughly R-3.5 to R-3.8 for open-cell. Open-cell can still reach a good total where there is room for a thicker layer, but in shallow cavities, rim joists and Minnesota basement walls, closed-cell does more in less depth.

How thick should spray foam be to meet Minnesota code?

It depends on the assembly. Minnesota’s energy code calls for R-15 on basement walls and R-20 in the cavity of a zone 6 wood frame wall, or R-13 with R-5 of continuous insulation. With closed-cell foam, that usually means a little over two inches on a basement wall and close to full depth in a 2x4 wall.

If you have a quote in hand and want a second read on the numbers, or you want us to look at the assembly itself, send us the details and we will set up a walkthrough.