Fiberglass Attic Insulation: How Thick Is R-38 and What It Covers
Fiberglass attic insulation is the most common insulation product installed in American homes, and R-38 is one of the most frequently specified values for attic floors in the middle climate zones. But before you order materials or schedule an installation, you need to know how thick is R38 insulation and whether your existing attic framing depth can accommodate it.
This guide covers r-38 insulation thickness for both batt and blown-in products, explains what depth you need to achieve the right r 38 insulation thickness, and helps you choose between fiberglass batts and r-38 blown insulation thickness options.
What R-38 means
R-value measures thermal resistance—the higher the number, the more the insulation resists heat flow. R-38 is the Department of Energy’s recommended attic floor R-value for climate zones 3–4, covering much of the Southeast, Mid-Atlantic, and lower Midwest. Climate zones 5–7 (Midwest, Northeast, Mountain West) call for R-49 to R-60, so R-38 may be a starting point rather than a final target depending on where you live.
The actual r-38 insulation thickness you need depends on which insulation product you choose. Different materials deliver different R-values per inch, so the physical depth required varies.
R-38 fiberglass batt thickness
Standard R-38 fiberglass batts are manufactured at 10.25 to 12 inches thick. Most product lines designed for R-38 compress slightly during installation. Kraft-faced R-38 batts are typically 10.25 inches; some high-density R-38 batts compress to fit into deeper framing cavities and are nominally 12 inches unfaced.
For attic floors with 2×10 or 2×12 joists (actual depths of 9.25 and 11.25 inches respectively), R-38 batts fit snugly without compression that would reduce their rated value. If your attic joists are shallower—2×8 or 2×6 framing—you’ll need to either stack layers or use blown insulation to achieve R-38 without overfilling the joist bays.
Stacking batts to achieve R-38
A common approach in older homes with shallow attic framing is to install one layer of batts in the joist bays (R-13 or R-19, depending on joist depth), then run a perpendicular layer of unfaced R-19 or R-21 batts across the top of the joists. This cross-hatch pattern covers the thermal bridging at the joists and allows you to reach R-38 without any framing modification.
R-38 blown insulation thickness
The r-38 blown insulation thickness depends on the material used:
- Blown fiberglass: Approximately 10–12 inches of settled depth to achieve R-38, at R-2.5 to R-4.3 per inch depending on density.
- Blown cellulose: Approximately 10–12 inches of settled depth at R-3.2 to R-3.8 per inch.
- Mineral wool (blown): Approximately 8–10 inches at R-3.7 to R-4.2 per inch.
Blown insulation settles after installation—fiberglass settles roughly 5–10%, cellulose up to 20%. Installers account for settling by blowing in more than the target depth. Depth rulers (simple stake gauges) placed across the attic floor let you verify that the installed depth meets your target after settling.
Climate zone requirements
R-38 fiberglass attic insulation is the minimum for zone 3 and a common upgrade target for zone 4 homes that are under-insulated. If you’re in zone 5 or higher, the DOE recommends R-49 to R-60 for attic floors. Installing R-38 in a zone 5 home still improves performance over uninsulated or under-insulated conditions, but the marginal return from going to R-49 is measurable and the cost difference is often modest when you’re already paying for labor.
Check your state and local energy code for minimum requirements if you’re doing a permitted renovation—some jurisdictions have adopted codes that require higher R-values than the DOE minimum recommendations.
Installation tips for attics
Before installing R-38 fiberglass attic insulation, complete these steps:
- Air seal all top-plate penetrations, can lights, and plumbing stacks. Air sealing before adding insulation prevents warm interior air from bypassing the insulation layer through gaps.
- Install baffles at each rafter bay at the eaves to maintain a ventilation channel from soffit vents to the attic air space. Blocking this channel causes moisture buildup and reduces shingle life.
- If installing batts, cut them to length without gaps at walls and framing members. Gaps—even small ones—disproportionately reduce effective R-value.
- For blown insulation, verify that the attic access is sealed and weatherstripped after installation to prevent heat loss through the hatch.
Pro tips recap: R-38 fiberglass batt thickness runs 10.25–12 inches; blown fiberglass or cellulose needs 10–12 inches of settled depth for the same R-value. Air seal before you insulate—it’s the highest-return energy improvement you can make in most attics. If you’re in climate zones 5–7, consider going to R-49 while you’re already working in the attic.
