An insulation R-value chart shows how much thermal resistance each material provides per inch and how many inches you need to hit a target R-value. R-value measures resistance to heat flow: higher numbers slow heat loss and heat gain more. Total R-value equals the R-per-inch of the material multiplied by its installed thickness, so a 10-inch layer of fiberglass batt at R-3.2 per inch delivers about R-32.
The chart below is the fast reference. It leads with R-value per inch by material, then the thickness needed to reach the common attic targets, then the 2021 IECC climate-zone recommendations that decide which target applies to your roof and attic.
Insulation R-value chart per inch by material
R-value per inch is the single number that lets you compare materials fairly. Rigid foams and closed-cell spray foam pack the most resistance into the least thickness, which matters where roof or cavity depth is limited. Fiberglass and cellulose cost less per R but need more depth. The ranges below reflect published manufacturer and Department of Energy figures at standard 75 degree F test conditions.
| Material | R-value per inch | Common use |
|---|---|---|
| Closed-cell spray foam | R-6.0 to R-7.0 | Roof decks, cathedral ceilings, tight cavities |
| Polyisocyanurate (polyiso) board | R-5.6 to R-6.5 | Low-slope commercial roofs, continuous exterior |
| XPS rigid foam board | R-4.5 to R-5.0 | Below grade, continuous exterior sheathing |
| EPS rigid foam board | R-3.6 to R-4.4 | Continuous exterior, roof cover board |
| Mineral wool (rockwool) batt | R-3.8 to R-4.3 | Walls, fire-rated assemblies, sound control |
| High-density fiberglass batt | R-3.7 to R-4.3 | 2×4 and 2×6 walls at higher R |
| Open-cell spray foam | R-3.5 to R-3.8 | Roof decks, interior walls, sound control |
| Dense-pack cellulose | R-3.5 to R-3.8 | Enclosed walls, dense-pack cavities |
| Loose-fill cellulose | R-3.2 to R-3.8 | Attic floors, blown application |
| Standard fiberglass batt | R-3.0 to R-3.7 | Walls, floors, standard batts |
| Blown-in (loose-fill) fiberglass | R-2.2 to R-2.7 | Attic floors over drywall |
Two numbers stand out for roofers and homeowners. Closed-cell spray foam roughly doubles the R-per-inch of loose-fill fiberglass, so it hits code in a fraction of the depth. Blown-in fiberglass sits at the bottom of the chart, which is why an attic that looks full at 10 inches may still fall short of R-30.
Fiberglass insulation R-value chart
Fiberglass R-value depends on form and density. Standard batts run R-3.0 to R-3.7 per inch, high-density batts reach R-3.7 to R-4.3, and blown-in (loose-fill) fiberglass settles at R-2.2 to R-2.7 per inch. That spread is why the same brand can label one product R-13 and another R-15 at the same nominal thickness. The batt table below gives the standard nominal R-values sold at retail.
| Fiberglass batt R-value | Nominal thickness | Typical framing |
|---|---|---|
| R-11 | 3.5 in | 2×4 wall (low density) |
| R-13 | 3.5 in | 2×4 wall (standard) |
| R-15 | 3.5 in | 2×4 wall (high density) |
| R-19 | 6.25 in | 2×6 wall, floors |
| R-21 | 5.5 in | 2×6 wall (high density) |
| R-30 | 9.5 in | Attic, cathedral ceiling |
| R-38 | 12 in | Attic floor |
| R-49 | 15.5 in | Cold-climate attic |
Note the R-19 to R-21 detail: an R-19 batt is designed for a 6.25-inch cavity, so compressing it into a 5.5-inch 2×6 wall drops its real performance. A purpose-made R-21 high-density batt fills that same 5.5-inch cavity without compression, which is why builders spec R-21 for 2×6 walls.
How thick does insulation need to be for each R-value?
Insulation thickness by R-value is found by dividing the target R-value by the material’s R-per-inch. To reach R-49 in an attic you need about 15 inches of fiberglass batt, 13.5 inches of cellulose, or 8 inches of closed-cell spray foam. The table maps the four common attic targets to each material so you can size a job or sanity-check a quote.
| Target R-value | Blown fiberglass | Fiberglass batt | Loose-fill cellulose | Open-cell spray foam | Closed-cell spray foam |
|---|---|---|---|---|---|
| R-30 | 12 to 13 in | 9.5 in | 8 to 9 in | 8 to 8.5 in | 4.5 to 5 in |
| R-38 | 15 to 16 in | 12 in | 10.5 to 11 in | 10 to 11 in | 5.5 to 6.5 in |
| R-49 | 19 to 20 in | 15.5 in | 13.5 to 14 in | 13 to 14 in | 7 to 8 in |
| R-60 | 23 to 25 in | 19 in | 16.5 to 17 in | 16 to 17 in | 8.5 to 10 in |
Depth matters on a roof where headroom or rafter depth is fixed. A 2×10 rafter gives about 9.25 inches of cavity. Filling it with open-cell foam reaches roughly R-33, while closed-cell foam in the same depth can hit R-55 or more, which is often the only way to meet a cold-climate ceiling target without furring down the rafters.
Ceiling and attic insulation R-value by climate zone
The 2021 International Energy Conservation Code (IECC) sets minimum ceiling and attic R-values by climate zone, and most jurisdictions adopt it or a close version. Attic requirements run from R-30 in the warmest zone to R-60 in the coldest. The Department of Energy recommendations track closely. Find your zone on the IECC climate map, then read across.
| Climate zone | Example regions | Attic / ceiling R-value | Wood-frame wall | Floor |
|---|---|---|---|---|
| Zone 1 | South Florida, Hawaii | R-30 | R-13 to R-15 | R-13 |
| Zone 2 | Gulf Coast, Phoenix | R-49 | R-13 to R-15 | R-13 |
| Zone 3 | Atlanta, Los Angeles | R-49 | R-20 or R-13+5 | R-19 |
| Zone 4 | Virginia, Kansas | R-60 | R-30 or R-20+5 | R-19 |
| Zone 5 | Chicago, Denver | R-60 | R-30 or R-20+5 | R-30 |
| Zone 6 | Minneapolis, Vermont | R-60 | R-30 or R-20+5 | R-30 |
| Zone 7 | Northern Minnesota | R-60 | R-30 or R-20+5 | R-38 |
| Zone 8 | Interior Alaska | R-60 | R-30 or R-20+5 | R-38 |
The jump from Zone 1 to Zone 2 is the one people miss: it goes straight from R-30 to R-49, skipping R-38. So a builder in the Gulf Coast who tops out at R-38 is a full step below current code. Zones 4 through 8 all share the R-60 ceiling target under the 2021 IECC, which is a notable increase from the older R-49 many older homes carry.
How to read the chart for a real roof or attic job
To use the chart, find your climate zone first, read the attic target, then pick a material and the thickness that hits it. Zone decides the number; material and available depth decide how you get there. Existing insulation counts toward the total, so measure what is already in the attic before you add.
- Locate your climate zone on the IECC map and read the attic/ceiling R-value from the zone table above.
- Measure existing insulation depth and multiply by that material’s R-per-inch to get the R-value already in place.
- Subtract the existing R-value from your target to find the R-value still needed.
- Pick a material and use the thickness table to convert the remaining R-value into inches to install.
- Check clearances for soffit vents, recessed lights (IC-rated only for direct contact), and rafter depth before you commit to a depth.
One field caution: R-value assumes the insulation is dry, uncompressed, and installed without gaps. Compressed batts, wind-washed loose-fill near eaves, and missing air sealing all cut real-world performance below the labeled number. For roof and attic assemblies, pairing the right R-value with proper attic ventilation keeps moisture from degrading the insulation and the deck above it.
Rigid foam and continuous insulation R-value
Rigid foam boards carry the highest R-per-inch among common products and are used as continuous insulation over sheathing or roof decks, where they also cut thermal bridging through framing. Polyiso leads at R-5.6 to R-6.5 per inch, XPS follows at R-4.5 to R-5.0, and EPS runs R-3.6 to R-4.4. A “+5” in a wall code line, such as R-20+5, means R-20 in the cavity plus R-5 of continuous foam on the exterior.
| Rigid foam | R-value per inch | R-value at 2 in | Notes |
|---|---|---|---|
| Polyiso | R-5.6 to R-6.5 | R-11 to R-13 | R drops in cold; verify aged R-value |
| XPS | R-4.5 to R-5.0 | R-9 to R-10 | Moisture resistant, below grade |
| EPS | R-3.6 to R-4.4 | R-8 to R-9 | Lowest cost per R, drainable |
Polyiso carries one caveat the label rarely shows: its R-value can fall in cold temperatures, so some designers derate it in Zone 6 and above. On low-slope commercial roofs, polyiso is still the dominant board because of its high R-per-inch and code acceptance, a pattern covered in our roofing material carbon report.
FAQ
What R-value do I need for my attic?
Attic R-value depends on your climate zone under the 2021 IECC. Zone 1 needs R-30, Zones 2 and 3 need R-49, and Zones 4 through 8 need R-60. The Department of Energy recommends R-49 to R-60 for most homes in cold climates and R-30 to R-49 in warm ones. Check your local code, since some jurisdictions still enforce older R-49 minimums.
What is the R-value of fiberglass insulation per inch?
Fiberglass R-value ranges from R-2.2 to R-2.7 per inch for blown-in loose-fill, R-3.0 to R-3.7 for standard batts, and R-3.7 to R-4.3 for high-density batts. Loose-fill sits lowest because it is fluffier and less dense. To estimate total R-value, multiply the R-per-inch by the installed depth measured in the attic.
How many inches of insulation do I need for R-49?
Reaching R-49 takes about 19 to 20 inches of blown-in fiberglass, 15.5 inches of fiberglass batt, 13.5 to 14 inches of loose-fill cellulose, or 7 to 8 inches of closed-cell spray foam. Denser and higher R-per-inch materials need less depth, which matters when rafter or joist depth is limited.
Which insulation has the highest R-value per inch?
Closed-cell spray foam has the highest R-value per inch of common insulations at R-6.0 to R-7.0, followed by polyisocyanurate rigid board at R-5.6 to R-6.5. These materials hit code targets in the least thickness, which makes them the usual choice for shallow roof decks, cathedral ceilings, and continuous exterior insulation.
Does more R-value always mean better performance?
Not always. R-value only measures conductive heat resistance in a dry, uncompressed, gap-free layer. Air leaks, missing air sealing, compressed batts, and moisture can each cut real performance well below the labeled R-value. In practice, air sealing plus a code-level R-value often outperforms a higher R-value installed poorly, and roof assemblies also need adequate ventilation to stay dry.
How is total R-value calculated?
Total R-value equals the material’s R-per-inch multiplied by its installed thickness. For example, 12 inches of loose-fill cellulose at R-3.5 per inch gives about R-42. When layers stack, such as batt plus continuous foam board, you add each layer’s R-value together to get the assembly total, which is how “+5” wall code lines work.
Related reading: our full attic insulation guide covers types and cost by zone, blown-in insulation breaks down loose-fill cost and DIY versus pro, and the best insulation for an attic helps you match material to your situation.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.