If you’re thinking about insulating your home, the R-value rating is the first number you’ll run into, and it shapes both your comfort and your budget. So what’s the R-value of spray foam insulation? That’s one of the most common questions property owners ask. At Platinum Spray Foam, we help homeowners pick the right foam for their project, meet local building codes and get the most out of every inch they pay for.
Quick Answer: What’s the R-Value of Spray Foam Insulation?
Open-cell spray foam is about R-3.5 to R-3.8 per inch. Closed-cell spray foam is about R-6.0 to R-7.0 per inch. In real buildings, plan on the lower end of that closed-cell range, because the number settles a bit as the foam ages (more on that below).
Open-cell foam is soft and flexible. It fills gaps, stops drafts and soaks up sound. Closed-cell foam is dense and rigid, with sealed cells filled with a gas that slows heat. Among the insulation types used in homes, it’s one of the highest in R-value per inch.
R-Value by Thickness Chart
Multiply the R-value per inch by the thickness and you get the total for your wall or roof deck. This chart uses R-3.7 per inch for open-cell and R-6.5 per inch for closed-cell, which are typical aged values. Your product’s data sheet has the exact figure.
| Thickness | Open-cell (R-3.7 per inch) | Closed-cell (R-6.5 per inch) |
| 1 inch | R-3.7 | R-6.5 |
| 2 inches | R-7.4 | R-13.0 |
| 3 inches | R-11.1 | R-19.5 |
| 4 inches | R-14.8 | R-26.0 |
| 5 inches | R-18.5 | R-32.5 |
| 6 inches | R-22.2 | R-39.0 |
A quick way to use it: a standard 2×4 wall is about 3.5 inches deep, so open-cell fills it to roughly R-13. A 2×6 wall is about 5.5 inches deep, which gets open-cell to around R-20.
R-Values for Texas Climate Zones
Building codes set minimum R-values by climate zone, and the International Energy Conservation Code (IECC) climate zone map assigns each county to a zone. On the map used with the 2015 IECC, most of North and East Texas falls in Climate Zone 3. Houston, San Antonio and the Gulf Coast are in Zone 2.
Here’s where it gets tricky. The 2021 IECC map moved a handful of Texas counties, including Dallas and Tarrant, into Zone 2, and some cities enforce a newer code than the state baseline. So check your county and which code edition your building department uses. The Department of Energy’s Building Energy Codes Program is a good place to start.
For a Zone 3 home, typical code targets look like this:
| Part of the house | Typical Zone 3 target |
| Attic or roofline | R-38 (2015 IECC) to R-49 (2021 IECC) |
| Exterior walls (wood frame) | R-20, or R-13 plus R-5 of continuous insulation |
| Floors over unconditioned space | R-19 |
Zone 2 walls and floors usually need less, around R-13 each, while attic targets are similar. Don’t let a wall target of “R-13” throw you off in Zone 3. A 2×4 wall filled with foam hits R-13, but you’ll normally need more to meet code, either a 2×6 wall or added continuous insulation.
Foam sprayed on the roof deck turns the attic into a sealed, conditioned space and keeps your ducts out of the heat, so it can perform better than its R-value alone suggests. That doesn’t automatically mean you can use less. Many codes still list R-38 to R-49 for the roof assembly, so ask your building department before planning around a lower number.
Why R-Value Alone Is Not the Whole Story
R-value measures how well a material resists heat, but how a building actually performs depends on more than that number.
- Air sealing. Batts and loose-fill slow heat but let air move through. Spray foam expands into gaps around electrical boxes and framing, so it blocks the air leaks that carry heat in and out.
- Thermal bridging. Wood studs conduct heat faster than insulation does. Closed-cell foam that runs continuously over the studs helps cut that heat path.
- Thermal aging. Closed-cell foam traps a gas that slowly trades places with air over the first few years, so its R-value drifts down a little before it levels off. Manufacturers publish an aged value, called LTTR, which predicts the R-value at about five years. That’s the number to trust, not the day-one figure.
- Moisture. Wet insulation performs worse. Closed-cell foam resists water, while open-cell can soak it up if there’s a leak.
Which Insulation Has the Highest R-Value?
Closed-cell spray foam is among the highest-rated insulation types used in homes, but it isn’t the highest available. Rigid polyiso foam board is in a similar range, at about R-5.6 to R-6 per inch. Specialty products like vacuum-insulated panels and aerogel go well beyond that, but they’re costly and rarely used in houses. For most homes, closed-cell foam is the top performer you can actually spray into a cavity.
Higher isn’t always better. Open-cell foam costs less and works great in attics and interior walls, so the “best” choice depends on the spot.
Spray Foam vs. Fiberglass and Blown-In Insulation
Here’s how spray foam stacks up against the usual alternatives:
- Fiberglass batts: about R-3.1 to R-3.4 per inch, and they lose effectiveness when air leaks around the edges. See our post on what batt insulation is and how it works.
- Blown-in cellulose: about R-3.2 to R-3.8 per inch, but it can sag and settle over time. Learn more about blown-in insulation.
- Closed-cell spray foam: about R-6.5 per inch, and it forms an air barrier and a vapor retarder at the same time.
For the full side-by-side, read platinum vs traditional insulation.
What Thickness Do You Need for Target R-Values?
The correct thickness depends on the foam type.
- R-30 (attics and roofs): about 8 inches of open-cell or about 4.5 inches of closed-cell
- R-38 (attics): about 10 inches of open-cell or about 6 inches of closed-cell
- R-49 (current code target in much of Texas): about 13 inches of open-cell or about 7.5 inches of closed-cell
There’s a practical catch. A 2×6 rafter is only about 5.5 inches deep, and a 2×10 is about 9.25. Thick open-cell layers often won’t fit in the rafters without extra framing. That’s why many Texas jobs use a hybrid of closed-cell and open-cell, or add furring. Your installer can tell you what fits.
Cost vs. R-Value Performance
Open-cell foam costs less to install and suits interior walls, sound control and sealed attic rooflines. Closed-cell costs more per square foot, but it adds rigidity, a vapor retarder and the most R-value in tight wall cavities. To estimate your project, see our post on how much spray foam insulation costs in Texas.
Conclusion
Once you know the R-value of spray foam, you can match the right foam to the right spot and stay on the right side of your local code. Open-cell is the budget-friendly air sealer with a lower R-value. Closed-cell packs more R-value per inch and blocks moisture. Either one gives lasting results when a professional installs it. To learn what R-value means in general, the Department of Energy explains it on its Energy Saver insulation page.
Frequently Asked Questions
How much R-value does 1 inch of spray foam insulation have?
One inch of open-cell foam gives about R-3.5 to R-3.8. One inch of closed-cell foam gives about R-6.0 to R-7.0, with aged values usually near R-6 to R-6.5.
Which is better for Texas attics, open-cell or closed-cell?
Both work. Open-cell is the more economical choice for deep rafter bays. Closed-cell is better where space is tight or moisture control matters.
Does spray foam lose R-value over time?
Open-cell holds steady. Closed-cell drifts down slightly during its first few years as the trapped gas and air trade places, then levels off. Ask for the manufacturer’s aged (LTTR) value.
How thick does spray foam need to be in exterior walls?
A 2×4 wall needs about 3.5 inches of open-cell or 2 inches of closed-cell to reach R-13. In Climate Zone 3, codes usually call for R-20, or R-13 plus continuous insulation, so a 2×6 wall (about 5.5 inches of open-cell) is a common route.
Does spray foam work as a vapor barrier?
Closed-cell foam acts as a vapor retarder at around 1.5 to 2 inches, depending on the product. Open-cell lets vapor pass, so some assemblies need a separate vapor retarder.
What is the highest R-value insulation?
Among common home insulation, closed-cell spray foam and rigid polyiso board are near the top. Specialty panels go higher but are rare and expensive.
What R-value do I need in my attic in Texas?
Most codes call for R-38 to R-49 in the attic, depending on your zone and the code edition your city enforces. Check with your building department.
Is a higher R-value always better?
Not always. Air sealing, moisture control and installation quality matter just as much, and a thicker layer costs more.
Ready to find the right R-value for your home? Platinum Spray Foam Insulation can help you pick the right foam type and thickness, and give you a custom quote. We serve homeowners across Texas, including Ennis, Waxahachie, Kaufman, Rockwall, Athens and Gun Barrel City. Contact us or call (214) 240-8322.