By The Roofing Brief Team. Last reviewed: October 2026.
R-38 insulation thickness runs from about 5.5 inches of closed-cell spray foam to about 13.5 inches of blown fiberglass, depending on the material. On current maker charts, a standard fiberglass batt hits R-38 at 12 inches and the high-density R-38C batt at 10.25 inches (Owens Corning PINK Next Gen fact sheet, May 2026), a stone wool batt at 9.5 inches (ROCKWOOL Comfortbatt data sheet, July 2026), blown cellulose at 11.2 inches installed and 10.4 inches settled (Greenfiber SANCTUARY chart, May 2026), and blown fiberglass at 13.25 to 13.5 inches. The number that matters is R-38 (the thermal resistance), not the depth.
R-38 is no longer the attic target in most of the US. The ENERGY STAR retrofit table, which follows 2021 IECC Table R402.1.3, recommends R-49 to R-60 for an uninsulated attic in Climate Zones 2 through 8 and R-30 in Zone 1. R-38 is ENERGY STAR’s top-up level in Zones 2 and 3 when the attic already has 3 to 4 inches, and its floor level over unheated space in Zones 7 and 8. Under the 2018 IRC, R-38 was the code ceiling minimum in Zones 2 and 3 only, with R-49 in Zones 4 through 8, per the ICC’s Significant Changes to the 2018 IRC.
R-38 insulation thickness by material (reference table)
Depth for R-38 equals 38 divided by the material’s R-value per inch: 11.9 inches at R-3.2 per inch, 6.2 inches at R-6.1. The table below uses the maker’s published thickness for a named product where one exists, so you can check the row against the bag or data sheet you are holding.
| Material | Product checked | R-value per inch (derived) | Thickness for R-38 | Source |
|---|---|---|---|---|
| Closed-cell spray foam | Typical 2.0 lb/cu ft foam; GVP 2.0 HFO | R-6.1 typical; R-7.1 aged on the GVP sheet | About 6.25 in (R-6.1) or 5.4 in (R-7.1) | DOE Building America spray foam guide; GVP 2.0 HFO data sheet, May 2025 |
| Stone wool batt | ROCKWOOL Comfortbatt R38 | About R-4.0 | 9.5 in | ROCKWOOL data sheet, issued July 2026 |
| Fiberglass batt, high-density | Owens Corning R-38C | About R-3.7 | 10.25 in | Owens Corning fact sheet 10025109-B, May 2026 |
| Open-cell spray foam | Typical 0.5 lb/cu ft foam | About R-3.6 | About 10.5 in | DOE Building America spray foam guide |
| Blown cellulose | Greenfiber SANCTUARY (25 lb bag) | About R-3.65 settled | 11.2 in installed, 10.4 in settled | Greenfiber DM-6.3-38 Rev F, May 2026 |
| Fiberglass batt, standard | Owens Corning R-38 | About R-3.2 | 12 in | Owens Corning fact sheet 10025109-B |
| Blown fiberglass | Owens Corning ProCat (33.5 lb bag) | About R-2.9 | 13.25 in | Owens Corning ProCat fact sheet, March 2026 |
| Blown fiberglass | Johns Manville Climate Pro | About R-2.8 | 13.4 in | JM Climate Pro attic card BIC-260 |
| Blown fiberglass | Owens Corning AttiCat (27.5 lb bag) | About R-2.8 | 13.5 in | AttiCat data sheet 10011273-L, May 2026 |
These depths assume the insulation is installed at its rated density and not compressed. The Insulation Institute’s compressed R-value chart (BI506, Oct 2020) shows a 12-inch R-38 fiberglass batt squeezed into an 11.25-inch 2×12 bay rates about R-37, and into a 9.25-inch 2×10 bay about R-32. For every material’s rating, see our insulation R-value chart by material, zone, and thickness.
Which R-38 product fits your space? (decision table)
| If your space is | Use | Depth needed for R-38 | Why |
|---|---|---|---|
| Open attic floor with 13.5 in or more of clear height at the eaves | Blown fiberglass or cellulose | 13.25 to 13.5 in (fiberglass); 11.2 in installed (cellulose) | Loose fill covers joist tops and wiring with no seams |
| Open attic floor with 11.5 to 13 in of clearance at the eaves | Cellulose, plus baffles at each rafter bay | 11.2 in installed, 10.4 in settled | About 3 in thinner than blown fiberglass for the same R-38 |
| 2×12 cathedral or vaulted rafter bay (11.25 in deep) | Owens Corning R-38C batt | 10.25 in | Fits the bay and leaves about 1 in for a vent channel |
| 2×10 rafter bay (9.25 in deep) | Closed-cell spray foam, or a batt plus rigid foam board under the rafters | About 6.25 in closed-cell | A 12 in batt compressed to 9.25 in drops to about R-32 |
| Unvented roof deck (conditioned attic) | Closed-cell spray foam, or foam plus blown fill | About 5.5 to 6.25 in closed-cell | Closed-cell is applied in 2 to 3 in passes and acts as a vapor retarder |
| Attic already has 3 to 4 in of old fill | Top up with blown fill to the chart depth | Use the bag chart row for the difference (R-38 minus existing R); over R-19, add 5.7 in of Greenfiber SANCTUARY | ENERGY STAR’s top-up target is R-38 in Zones 2 and 3, R-49 in Zones 4 to 8 |
R-38 insulation: how many inches for batts?
R-38 fiberglass batts are 12 inches thick in standard density and 10.25 inches in high-density R-38C, per the Owens Corning PINK Next Gen fact sheet (publication 10025109-B, May 2026). The standard R-38 batt ships in 16 in, 19.25 in and 24 in widths by 48 in long, and the R-38C batt in 15.5 in and 23.75 in widths. R-38C exists to fit R-38 into a 2×12 rafter bay, which is 11.25 inches deep.
Stone wool reaches R-38 thinner than fiberglass: ROCKWOOL lists Comfortbatt R38 at 9.5 inches for wood studs on its US data sheet (issued July 2026), about R-4.0 per inch. When you buy a batt labeled R-38, the maker has already sized the thickness to that rating; the FTC R-value Rule, 16 CFR Part 460 requires every batt package to list R-value, length, width, thickness and square feet. More on batt selection is in our guide to fiberglass attic insulation: batts vs blown, R-value, and cost.
Will R-38 batts fit standard attic joists?
Standard 2×10 attic joists are 9.25 inches deep, so a 12-inch R-38 batt sits about 2.75 inches proud of the framing on an open attic floor. That is normal and correct: the loft above the joists is where the R-value lives. Inside a closed 2×10 rafter bay the batt is compressed, and the Insulation Institute chart rates a 12-inch R-38 batt at about R-32 in a 9.25-inch cavity. In a 9.5-inch I-joist it rates about R-33. If you need R-38 inside a 2×10 rafter bay, use about 6.25 inches of closed-cell spray foam or add rigid board below the rafters.
R-38 blown insulation depth
Blown cellulose reaches R-38 at 11.2 inches installed and 10.4 inches settled on the Greenfiber SANCTUARY chart, at 52.6 bags per 1,000 sq ft and 1.32 lb per sq ft. Blown fiberglass needs 13.25 inches on the Owens Corning ProCat chart (16.4 bags per 1,000 sq ft, 0.550 lb per sq ft) and 13.4 inches on the Johns Manville Climate Pro card (17.7 bags, 0.53 lb per sq ft). The Owens Corning AttiCat sheet (27.5 lb bag) lists 13.5 inches at 19 bags and adds 1.5 inches in Minnesota to meet that state’s winter design temperature rule.
| Product (bag weight) | Installed depth for R-38 | Settled depth | Bags per 1,000 sq ft | Min. weight per sq ft | Source |
|---|---|---|---|---|---|
| Greenfiber SANCTUARY cellulose (25 lb) | 11.2 in | 10.4 in | 52.6 | 1.32 lb | Greenfiber DM-6.3-38 Rev F |
| Owens Corning ProCat fiberglass (33.5 lb) | 13.25 in | 13.25 in | 16.4 | 0.550 lb | Owens Corning 10019591-C |
| Johns Manville Climate Pro fiberglass | 13.4 in | 13.4 in | 17.7 | 0.53 lb | JM BIC-260 |
| Owens Corning AttiCat fiberglass (27.5 lb) | 13.5 in | 13.5 in | 19.0 | 0.523 lb | Owens Corning 10011273-L |
Under the FTC R-value Rule (16 CFR 460.12), every loose-fill bag must print the minimum settled thickness, initial installed thickness, maximum net coverage, bags per 1,000 sq ft and minimum weight per sq ft at R-13, 19, 22, 30, 38 and 49. Both Owens Corning and Greenfiber state that the installer must hit both the minimum bag count and the minimum depth, or the R-value is not met. IECC R303.1.1.1 requires depth markers (depth rulers) at least one per 300 sq ft, fixed to the joists or trusses, in new work, and they are just as useful on a retrofit, and blow cellulose to the installed depth, not the settled one. See our full guide to blown-in insulation: cost, R-value, and DIY vs pro.
Worked example: R-38 cellulose over a 1,200 sq ft attic
- Area: 1,200 sq ft of attic floor (net of the hatch).
- Chart row: Greenfiber SANCTUARY R-38 allows at most 19.0 sq ft per bag, or 52.6 bags per 1,000 sq ft.
- Bags: 1,200 x 52.6 / 1,000 = 63.1, so buy 64 bags.
- Depth check: blow to at least 11.2 inches so the settled depth is still 10.4 inches.
- Top-up case: if the attic already holds R-19 of material, the chart difference method needs only the R-19 row (5.7 in, 22.6 bags per 1,000 sq ft), or 28 bags for 1,200 sq ft.
R-38 fiberglass vs cellulose thickness
Cellulose reaches R-38 in less depth than blown fiberglass: 10.4 inches settled for Greenfiber SANCTUARY versus 13.25 to 13.5 inches for Owens Corning ProCat, Johns Manville Climate Pro and Owens Corning AttiCat, a gap of about 3 inches. In a low-clearance attic where the eave height is under 12 inches, that difference decides which material fits.
| Blown material | R per inch (derived) | Depth for R-38 | Weight at R-38 | Best when |
|---|---|---|---|---|
| Cellulose (Greenfiber SANCTUARY) | About R-3.65 settled | 11.2 in installed, 10.4 in settled | 1.32 lb per sq ft | Low headroom, air-sealing priority |
| Loose-fill fiberglass (OC ProCat, JM Climate Pro, OC AttiCat) | About R-2.8 to 2.9 | 13.25 to 13.5 in | 0.52 to 0.55 lb per sq ft | Open attics, weight limits on old ceilings |
Weight is the other tradeoff. At R-38, cellulose loads the ceiling with about 1.32 lb per sq ft against 0.52 to 0.55 lb for blown fiberglass on the charts above, roughly 2.4 times as much, which matters on old plaster or drywall ceilings; check the ceiling board maker’s insulation-weight limit before choosing cellulose. Greenfiber’s chart is based on settled thickness, while the Owens Corning and Johns Manville charts list the same installed and settled depth, so fiberglass needs no extra settling allowance.
How thick is R-38 spray foam?
Closed-cell spray foam reaches R-38 at about 6.25 inches at the R-6.1 per inch the DOE Building America spray foam guide lists for typical 2 lb foam, or about 5.4 inches for a newer HFO foam such as GVP 2.0 HFO, rated R-7.1 per inch aged at 3.5 inches and thicker. Open-cell foam at about R-3.6 per inch needs about 10.5 inches. Closed-cell is also a vapor retarder at typical thickness, which is why it is used under the roof deck in unvented (conditioned) attic assemblies.
The DOE guide notes that closed-cell foam is typically limited to 2 to 3 inch layers per pass, so R-38 takes 2 or 3 passes, while open-cell can go on up to 10 inches in one pass. Because closed-cell costs more per board foot, many retrofit jobs spray 2 to 3 inches for air sealing and vapor control, then top it with blown fiberglass or cellulose to reach R-38 at a lower total cost.
Do you need R-38, and where?
R-38 is a code minimum only in older or warmer cases. Under the 2018 IRC (N1102.1.2), the ceiling minimum was R-30 in Zone 1, R-38 in Zones 2 and 3 and R-49 in Zones 4 through 8. The 2021 IECC raised the attic level to R-49 in Zones 2 and 3 and R-60 in Zones 4 through 8, per Delaware DNREC’s summary of 2021 IECC residential changes.
| Climate zone | 2018 IRC ceiling minimum | ENERGY STAR, uninsulated attic | ENERGY STAR, attic with 3 to 4 in existing | Where R-38 fits |
|---|---|---|---|---|
| 1 | R-30 | R-30 | R-25 | Above both targets |
| 2 and 3 | R-38 | R-49 | R-38 | 2018 code minimum and ENERGY STAR top-up target |
| 4A and 4B | R-49 | R-60 | R-49 | Below both |
| 4C, 5 and 6 | R-49 | R-60 | R-49 | Below both |
| 7 and 8 | R-49 | R-60 | R-49 | ENERGY STAR floor level over unheated space |
Two code exceptions keep R-38 relevant. Under 2021 IECC R402.2.1, R-38 over 100 percent of the ceiling satisfies an R-49 requirement when full-height uncompressed insulation extends over the wall top plates at the eaves, as with raised-heel trusses. Under 2018 IRC N1102.2.2 (IECC R402.2.2), a cathedral ceiling without room for the required R-value can use R-30, limited to 500 sq ft or 20 percent of the insulated ceiling area, whichever is less. Your local adopted code edition controls.
If your attic already has some insulation, you add depth to reach R-38 rather than starting over: measure existing depth, identify the material, and top up using the R-value difference on the bag chart; over existing R-19 fill, the Greenfiber chart adds 5.7 inches of cellulose. If you are topping up with batts, our guide to attic insulation rolls covers sizes and how many to buy. To size the exact amount and cost for your attic, use our guide to how much it costs to insulate an attic and the broader attic insulation types, R-value by zone, and cost overview.
Frequently asked questions
How many inches is R-38 insulation?
R-38 insulation is about 5.5 to 13.5 inches thick for the products charted on this page. Owens Corning’s standard R-38 fiberglass batt is 12 inches, its R-38C cathedral batt 10.25 inches, ROCKWOOL Comfortbatt R-38 is 9.5 inches, Greenfiber SANCTUARY cellulose needs 11.2 inches installed (10.4 settled), blown fiberglass needs 13.25 to 13.5 inches, and closed-cell spray foam needs about 5.5 to 6.25 inches.
How thick is an R-38 fiberglass batt?
A standard R-38 fiberglass batt is 12 inches thick and a high-density R-38C batt is 10.25 inches thick, per Owens Corning’s PINK Next Gen fact sheet (May 2026). The R-38C batt is sized for 2×12 rafter bays (11.25 inches deep). Both deliver R-38 only when installed at full thickness without compression.
How deep does blown insulation need to be for R-38?
Blown cellulose needs 11.2 inches installed and 10.4 inches settled for R-38 on the Greenfiber SANCTUARY chart (52.6 bags per 1,000 sq ft). Blown fiberglass needs 13.25 inches on the Owens Corning ProCat chart (16.4 bags), 13.4 inches on the Johns Manville Climate Pro card (17.7 bags) and 13.5 inches on the Owens Corning AttiCat sheet (19 bags). Install both the minimum depth and the minimum bag count on your bag’s chart.
Is R-38 cellulose or fiberglass thinner?
Cellulose is thinner for R-38: 10.4 inches settled on the Greenfiber SANCTUARY chart versus 13.25 to 13.5 inches for blown fiberglass on the Owens Corning ProCat, Johns Manville Climate Pro and Owens Corning AttiCat charts, a difference of about 3 inches. Cellulose also weighs more, 1.32 lb per sq ft at R-38 versus 0.52 to 0.55 lb for blown fiberglass.
Will R-38 batts fit in 2×10 joists?
On an open attic floor, yes: a 12-inch R-38 batt sits about 2.75 inches above 2×10 joists (9.25 inches deep) and keeps its rating. In a closed 2×10 rafter bay it gets compressed, and the Insulation Institute’s compressed R-value chart puts a 12-inch R-38 batt squeezed into 9.25 inches at about R-32. For full R-38 in a 2×10 bay, use about 6 inches of closed-cell spray foam or add rigid foam below or above the rafters.
Why does R-38 thickness vary so much by material?
Each material has a different R-value per inch, so depth equals 38 divided by the R per inch. Closed-cell spray foam at R-6.1 per inch (DOE Building America guide) needs about 6.25 inches, cellulose at about R-3.65 per inch needs 10.4 inches settled, and blown fiberglass at about R-2.85 per inch needs 13.25 to 13.5 inches for the same R-38.