By The Roofing Brief Team. Last reviewed: October 2026. Per-inch values come from manufacturer data sheets and coverage charts; code values from the 2021 IECC as extracted by PNNL and ENERGY STAR; costs from HomeGuide’s 2026 insulation cost guide.
On an insulation R-value chart, fiberglass batt insulation rates about R-3.1 to R-4.3 per inch, settled blown cellulose about R-3.5 to R-3.6, blown fiberglass R-2.7 to R-2.9, and closed-cell spray foam up to R-7.4. R-value measures resistance to heat flow, and total R-value equals R-per-inch multiplied by installed thickness: a 12-inch Owens Corning R-38 batt works out to about R-3.2 per inch, while the same maker’s 3.5-inch R-15 high-density batt works out to about R-4.3.
Code sets the target and the chart sets the depth. Under the 2021 IECC, attics need R-30 in climate zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8, per the ENERGY STAR recommended R-values table based on Table R402.1.3. Reaching R-60 then takes 16.53 inches of settled cellulose or 20.25 to 21.75 inches of blown fiberglass on the maker charts cited below.
Insulation R-value chart per inch by material
R-value per inch is the number that lets you compare materials fairly, and the most reliable version comes from the product’s own label: the FTC R-value Rule, 16 CFR 460.12, requires batt labels to show R-value and thickness and loose-fill bags to show the settled thickness for each R-value. The per-inch figures below are computed from those labels and data sheets, mostly tested per ASTM C518 at a 75°F mean temperature.
| Material | R-value per inch | How the figure is derived | Common use |
|---|---|---|---|
| Closed-cell spray foam | R-5.0 to R-7.4 | Huntsman Heatlok HFO Pro R-7.4 per in; HomeGuide range R-5.0 to R-7.0 | Roof decks, cathedral ceilings, tight cavities |
| Polyisocyanurate (polyiso) board | R-5.7 to R-5.9 (LTTR) | JM ENRGY 3 LTTR R-5.7 at 1 in, R-23.6 at 4.0 in | Low-slope commercial roofs, continuous exterior |
| XPS rigid foam board | R-5.0 | Owens Corning FOAMULAR NGX 250 data sheet | Below grade, continuous exterior sheathing |
| EPS rigid foam board | R-3.6 to R-4.4 | Varies by ASTM C578 type; check the product sheet | Continuous exterior, roof cover board |
| Mineral wool (rockwool) batt | R-4.0 to R-4.3 | ROCKWOOL Comfortbatt R-15 at 3.5 in, R-30 at 7.25 in, R-38 at 9.5 in | Walls, fire-rated assemblies, sound control |
| High-density fiberglass batt | R-3.7 to R-4.3 | Owens Corning R-15 at 3.5 in, R-21 at 5.5 in, R-38C at 10.25 in | 2×4 and 2×6 walls, shallow rafters |
| Open-cell spray foam | R-3.7 to R-4.0 | Huntsman Icynene Classic R-3.7; Classic Plus R-4.0 | Roof decks, interior walls, sound control |
| Loose-fill cellulose (settled) | R-3.5 to R-3.6 | Greenfiber INS541LD: R-30 at 8.62 in, R-60 at 16.53 in | Attic floors, blown application |
| Standard fiberglass batt | R-3.1 to R-3.7 | Owens Corning R-11 and R-13 at 3.5 in, R-30 at 9.5 in, R-38 at 12 in | Walls, floors, attic floors |
| Blown-in (loose-fill) fiberglass | R-2.7 to R-2.9 | AttiCat R-49 at 17 in; Climate Pro R-49 at 17 in; InsulSafe SP R-49 at 18.25 in | Attic floors over drywall |
Two numbers stand out for roofers and homeowners. Closed-cell foam at R-7.4 per inch carries about 2.6 times the R-per-inch of blown fiberglass at R-2.8, so it reaches R-49 in about 6.6 inches where AttiCat needs 17. Blown fiberglass sits at the bottom of the chart, which is why an attic that looks full at 10 inches is still short of R-30: Owens Corning’s AttiCat chart lists R-30 at 10.75 inches.
Batt and fiberglass insulation R-value per inch
Batt insulation R-value per inch runs R-3.1 to R-3.7 for standard fiberglass batts and R-3.7 to R-4.3 for high-density batts, based on the label thicknesses in the Owens Corning PINK Next Gen line. Blown-in fiberglass settles at R-2.7 to R-2.9 per inch on current charts. That spread is why the same brand sells R-11, R-13 and R-15 batts all at 3.5 inches thick.
| Fiberglass batt R-value | Label thickness (Owens Corning) | R per inch | Typical framing |
|---|---|---|---|
| R-11 | 3.5 in | 3.1 | 2×4 wall (low density) |
| R-13 | 3.5 in | 3.7 | 2×4 wall (standard) |
| R-15 | 3.5 in | 4.3 | 2×4 wall (high density) |
| R-19 | 6.5 in | 2.9 | 2×6 wall, floors |
| R-21 | 5.5 in | 3.8 | 2×6 wall (high density) |
| R-30C | 8.25 in | 3.6 | Cathedral ceiling, 2×10 rafter |
| R-30 | 9.5 in | 3.2 | Attic floor |
| R-38C | 10.25 in | 3.7 | Cathedral ceiling, 2×12 rafter |
| R-38 | 12 in | 3.2 | Attic floor |
| R-49 | 14 in | 3.5 | Cold-climate attic |
Note the R-19 to R-21 detail: an R-19 batt is labeled 6.25 to 6.5 inches thick, so a 2×6 wall (5.5 inches) compresses it. The Insulation Institute (NAIMA) compressed R-value chart rates an R-19 batt in a 5.5-inch cavity at about R-18, while a purpose-made R-21 batt fills the same cavity at its full R-21. The same chart rates a 12-inch R-38 batt squeezed into a 2×10 (9.25 inches) at R-32.
How thick does insulation need to be for each R-value?
Insulation thickness by R-value is the target divided by the material’s R-per-inch, and the maker charts give the exact figure. R-49 takes 14 inches of Owens Corning batt, 13.68 inches of settled Greenfiber cellulose, 17 to 18.25 inches of blown fiberglass, or about 6.6 inches of closed-cell foam at R-7.4. If your zone calls for more, our guides to R-49 insulation thickness and R-60 insulation cost and depth break each target down by product.
| Target R-value | Blown fiberglass (AttiCat, Climate Pro, InsulSafe SP) | Fiberglass batt (Owens Corning) | Settled cellulose (Greenfiber INS541LD) | Open-cell foam (R-3.7 to R-4.0) | Closed-cell foam (R-7.4) |
|---|---|---|---|---|---|
| R-30 | 10.75 to 11.5 in | 9.5 in (R-30C 8.25 in) | 8.62 in | 7.5 to 8.1 in | 4.1 in |
| R-38 | 13.5 to 14.5 in | 12 in (R-38C 10.25 in) | 10.77 in | 9.5 to 10.3 in | 5.1 in |
| R-49 | 17 to 18.25 in | 14 in | 13.68 in | 12.25 to 13.2 in | 6.6 in |
| R-60 | 20.25 to 21.75 in | 17.5 in stacked (R-49 plus R-13) | 16.53 in | 15 to 16.2 in | 8.1 in |
Depth matters on a roof where rafter depth is fixed. A 2×10 rafter gives 9.25 inches of cavity: open-cell foam at R-3.7 per inch fills it to about R-34, while closed-cell foam at R-7.4 reaches R-49 in 6.6 inches and leaves room for a vent channel. Huntsman lists a 6.5-inch maximum lift per pass for Heatlok HFO Pro, so anything over that is a two-pass job.
Which insulation fits your job?
| If your job is… | Material to price first | Why, in numbers | Source |
|---|---|---|---|
| Open attic floor with room for 14 in or more | Blown cellulose or blown fiberglass | R-60 in 16.53 in (cellulose) or 20.25 in (Climate Pro) | Maker charts |
| 2×6 wall, 5.5 in cavity | R-21 high-density batt, not R-19 | R-19 compressed to 5.5 in rates about R-18 | NAIMA compressed R-value chart |
| 2×10 cathedral ceiling, 9.25 in | Closed-cell foam or R-30C batt | R-49 in 6.6 in of foam; R-30C fits at 8.25 in | Huntsman; Owens Corning |
| Low-slope roof above the deck | Polyiso board, staggered layers | R-23.6 per 4.0 in board (LTTR) | JM ENRGY 3 data sheet |
| Below grade or wet exposure | XPS board | R-5.0 per inch at 75°F | Owens Corning FOAMULAR NGX 250 |
| Wall needing fire or sound rating | Mineral wool batt | R-15 in 3.5 in; R-23 in 5.5 in | ROCKWOOL Comfortbatt data sheet |
Ceiling and attic insulation R-value by climate zone
The 2021 International Energy Conservation Code (IECC) sets minimum ceiling, wall and floor R-values by climate zone in Table R402.1.3. The values below follow the PNNL Building America Solution Center extract of that table, which also shows the 2012 to 2018 values many states still enforce. Find your zone on the IECC climate map, then read across.
| Climate zone | Example regions | Attic / ceiling R-value (2021) | Ceiling (2012 to 2018) | Wood-frame wall (2021) | Floor |
|---|---|---|---|---|---|
| Zone 1 | South Florida, Hawaii | R-30 | R-30 | R-13 or R-0+10ci | R-13 |
| Zone 2 | Gulf Coast, Phoenix | R-49 | R-38 | R-13 or R-0+10ci | R-13 |
| Zone 3 | Atlanta, Los Angeles | R-49 | R-38 | R-20 or R-13+5ci or R-0+15 | R-19 |
| Zone 4 (except marine) | Virginia, Kansas | R-60 | R-49 | R-30 or R-20+5ci or R-13+10ci or R-0+20ci | R-19 |
| Zone 5 and marine 4 | Chicago, Denver | R-60 | R-49 | R-30 or R-20+5ci or R-13+10ci or R-0+20ci | R-30 |
| Zone 6 | Minneapolis, Vermont | R-60 | R-49 | R-30 or R-20+5ci or R-13+10ci or R-0+20ci | R-30 |
| Zones 7 and 8 | Northern Minnesota, interior Alaska | R-60 | R-49 | R-30 or R-20+5ci or R-13+10ci or R-0+20ci | R-38 |
The jump from zone 1 to zone 2 is the one people miss: under the 2021 IECC it goes straight from R-30 to R-49, skipping R-38, so a Gulf Coast attic topped out at R-38 meets the 2018 code but sits a full step below 2021. Zones 4 through 8 all share the R-60 ceiling target under the 2021 IECC, up from R-49 in the 2012 to 2018 editions.
How to read the chart for a real roof or attic job
Start with your climate zone and code edition, read the attic target, then pick a material and the thickness that hits it. Existing insulation counts toward the total, so measure what is already in the attic before you add. Worked example: a zone 5 attic with 4 inches of old blown fiberglass holds about R-11 (4 inches at the R-2.8 per inch implied by AttiCat’s R-30 at 10.75 inches). Reaching R-60 needs about R-49 more, which matches ENERGY STAR’s retrofit advice to add R-49 where 3 to 4 inches is already down in zones 4 through 8: 17 inches of AttiCat or 13.68 inches of settled cellulose.
- Locate your climate zone on the IECC map and read the attic/ceiling R-value from the zone table above (2021 or 2012 to 2018 column, whichever your jurisdiction enforces).
- Measure existing insulation depth at 3 or more spots about 10 feet apart and multiply by that material’s R-per-inch: about R-2.8 for old blown fiberglass, R-3.5 for settled cellulose, R-3.2 for a standard batt.
- Subtract the existing R-value from your target to find the R-value still needed, for example R-60 minus R-11 = R-49.
- Pick a material and use the thickness table to convert the remaining R-value into inches, then check the bag chart for bags per 1,000 sq ft (25 bags of AttiCat at R-49).
- Check clearances: keep loose-fill at least 3 inches from recessed lights that are not IC-rated (Owens Corning and CertainTeed bag instructions), and keep baffles open at every soffit vent.
R-value assumes the insulation is dry, uncompressed and installed without gaps. The NAIMA chart shows a 14-inch R-49 batt compressed into a 2×12 (11.25 inches) rates about R-42, and wind-washed loose-fill near eaves loses depth the same way. 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 high R-per-inch and are used as continuous insulation over sheathing or roof decks, where they also cut thermal bridging through framing. Polyiso leads at about R-5.7 per inch LTTR on the Johns Manville ENRGY 3 data sheet, XPS follows at R-5.0 on the Owens Corning FOAMULAR NGX 250 data sheet, and EPS runs R-3.6 to R-4.4 by ASTM C578 type. A “+5ci” in a wall code line, such as R-20+5ci, means R-20 in the cavity plus R-5 of continuous insulation.
| Rigid foam | R-value per inch | R-value at 2 in | Notes |
|---|---|---|---|
| Polyiso | R-5.7 to R-5.9 (LTTR) | about R-11.4 to R-11.8 | R drops in cold; verify aged R-value |
| XPS | R-5.0 | R-10 | Moisture resistant, below grade |
| EPS | R-3.6 to R-4.4 | R-7.2 to R-8.8 | Lowest cost per R, drainable |
Polyiso carries one caveat the label rarely shows: the JM ENRGY 3 values are tested at a 75°F mean temperature, and polyiso R-value can fall in colder conditions, so some designers derate it in zone 6 and above. On low-slope commercial roofs, polyiso is still the dominant board because of its R-5.7 LTTR per inch and code acceptance, a pattern covered in our roofing material carbon report.
Insulation cost per R-value by material
R-value per inch tells you how thin a material can be, not how cheap the R is. HomeGuide’s 2026 insulation cost guide lists cavity batts and rolls at $0.80 to $2.60 per sq ft installed, blown-in at $1.00 to $2.80, spray foam at $1.00 to $4.50 and rigid foam board at $1.20 to $3.70. Its material prices put open-cell foam at $0.30 to $0.50 per board foot and closed-cell at $0.90 to $1.00.
| Material | Installed cost per sq ft (HomeGuide, 2026) | R-value per inch | Material cost per R per sq ft |
|---|---|---|---|
| Cavity batt and roll | $0.80 to $2.60 | R-3.1 to R-4.3 | not calculable from HomeGuide’s per-sq-ft figure |
| Blown-in (cellulose or fiberglass) | $1.00 to $2.80 | R-2.7 to R-3.6 | not calculable from HomeGuide’s per-sq-ft figure |
| Open-cell spray foam | within $1.00 to $4.50 (spray foam) | R-3.5 to R-3.7 (HomeGuide) | $0.08 to $0.14 ($0.30 / 3.7 to $0.50 / 3.5) |
| Closed-cell spray foam | within $1.00 to $4.50 (spray foam) | R-5.0 to R-7.0 (HomeGuide) | $0.13 to $0.20 ($0.90 / 7.0 to $1.00 / 5.0) |
| Rigid foam board | $1.20 to $3.70 | R-3.6 to R-5.9 | check the board’s price per sheet |
A board foot of spray foam is 1 sq ft at 1 inch thick, which is why dividing its price by R-per-inch gives a cost per R. On HomeGuide’s material prices, closed-cell foam costs about 1.4 to 1.6 times as much per R as open-cell ($0.13 to $0.20 against $0.08 to $0.14), so it earns its premium mainly where depth is fixed, such as a 2×10 roof deck or cathedral ceiling, because it reaches R-49 in 6.6 inches against 13.2 inches for R-3.7 open-cell. Labor, attic access and regional rates move installed prices, so get quotes priced per R-value and depth, not per bag.
Aged R-value: why polyiso and XPS lose R over time
Foam boards do not hold their label number forever. Polyiso and XPS use blowing agents that slowly diffuse out, a process called thermal drift, so their long-term R-value settles below the initial figure. Polyiso is rated by LTTR, a 15-year time-weighted value under ASTM C1303 and CAN/ULC-S770, which is why roof designers spec the aged number rather than the fresh-off-the-line one.
- Polyiso: an initial R-6.0 per inch drifts to a design LTTR near R-5.7 per inch, measured per ASTM C1289 using the LTTR protocol (ASTM C1303 and CAN/ULC-S770).
- XPS: ASTM C578 is the product specification and does not set a long-term aging decline; Owens Corning describes its 180-day real-time aging as the conditioning C578 mandates. Owens Corning’s October 2025 FOAMULAR NGX 150 data sheet lists a minimum long-term thermal resistance (LTTR, accelerated aging per CAN/ULC S770-03) of R-5.0 per inch at 75°F mean temperature. Aged values for other brands and formulations can differ, so check the product’s own data sheet.
- EPS and mineral wool: no blowing-agent drift, so their R-value stays stable over the life of the assembly.
- Cold-temperature derate: polyiso R-value also falls as temperature drops, so some designers derate it in zone 6 and colder. ASTM C518 tests at a single mean temperature (75°F on the JM and Owens Corning sheets) and does not capture that swing.
Frequently asked questions
What R-value do I need for my attic?
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 2012 to 2018 editions, still enforced in many states, required R-38 in zones 2 and 3 and R-49 in zones 4 through 8. ENERGY STAR’s retrofit table recommends R-60 for uninsulated attics in zones 4 through 8.
What is the R-value of batt insulation per inch?
Standard fiberglass batts run about R-3.1 to R-3.7 per inch and high-density batts R-3.7 to R-4.3, based on Owens Corning label thicknesses (R-13 and R-15 at 3.5 inches, R-38 at 12 inches). ROCKWOOL mineral wool batts run R-4.0 to R-4.3 per inch. Blown-in fiberglass is lower, R-2.7 to R-2.9 per inch on current maker charts.
How many inches of insulation do I need for R-49?
Reaching R-49 takes 14 inches of Owens Corning fiberglass batt, 13.68 inches of settled Greenfiber cellulose, 17 to 18.25 inches of blown fiberglass (AttiCat, Climate Pro, InsulSafe SP), or about 6.6 inches of closed-cell spray foam at R-7.4 per inch.
Which insulation has the highest R-value per inch?
Closed-cell spray foam has the highest R-value per inch of common insulations, up to R-7.4 for Huntsman Heatlok HFO Pro, followed by polyiso board at about R-5.7 LTTR and XPS at R-5.0. These materials hit code targets in the least thickness, which makes them the usual choice for shallow roof decks and cathedral ceilings.
Does more R-value always mean better performance?
Not always. R-value measures conductive heat resistance in a dry, uncompressed, gap-free layer. Compression cuts it: the Insulation Institute (NAIMA) chart rates a 12-inch R-38 batt squeezed into a 9.25-inch 2×10 at R-32. Air leaks and moisture also reduce real performance, so air seal before adding depth.
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 settled cellulose at R-3.5 per inch gives about R-42. When layers stack, such as cavity batt plus continuous foam board, add each layer’s R-value, which is how R-20+5ci wall code lines work.
Which insulation is cheapest per R-value?
Batts and blown-in insulation are the cheapest per R-value; HomeGuide’s 2026 guide lists them at $0.80 to $2.60 and $1.00 to $2.80 per sq ft installed. Among foams, open-cell costs about $0.08 to $0.14 per R per sq ft in material against $0.13 to $0.20 for closed-cell, based on HomeGuide’s board-foot prices.
Does insulation R-value change over time?
Fiberglass, cellulose, mineral wool, and EPS hold their R-value for the life of the assembly. Polyiso and XPS lose some R as their blowing agents diffuse out, a process called thermal drift. Polyiso is rated by LTTR, a 15-year time-weighted R-value near R-5.7 per inch measured per ASTM C1289, rather than its higher initial number.
Related guides
- Our full attic insulation guide covers types and cost by zone.
- Blown-in insulation breaks down loose-fill cost and DIY versus pro.
- The best insulation for an attic matches material to attic type.