R-30 vs R-38 insulation: the difference comes down to eight points of thermal resistance and roughly two extra inches of depth, but the more useful answer is that both sit below what current code wants in most attics. R-38 blocks conductive heat flow through a ceiling plane about 21 percent better than R-30, yet the 2021 IECC and ENERGY STAR now call for R-49 to R-60 in the attics of nearly every U.S. climate zone. R-30 meets attic recommendations only in Zone 1. Neither is a finish line for a modern attic. Both earn their place where rafter depth, budget, or a floor or wall cavity caps how much you can fit. This guide covers attic batts and blown fill, plus the rigid polyiso board used above the deck on low-slope roofs.
R-30 vs R-38 insulation at a glance
The difference between R-30 and R-38 insulation is simple on paper: R-38 delivers 8 more R-value points than R-30, needs about 2 to 3 more inches of depth depending on material, and costs roughly 20 to 30 percent more per square foot installed. R-30 is the practical ceiling for many 2×10 rafter bays and additions; R-38 is the higher bar for attic floors where depth is not limited. The table below sets the two side by side on the numbers that drive the decision.
| Factor | R-30 | R-38 |
|---|---|---|
| Thermal resistance | R-30 | R-38 (about 27% higher rated value) |
| Conductive heat-flow cut vs the other | Baseline | About 21% less loss through that plane |
| Fiberglass batt thickness | About 9.5 to 10 in (8.25 in high-density) | About 12 to 13 in (10.25 in high-density) |
| Blown cellulose thickness | About 8.5 to 9 in | About 10.5 to 11 in |
| Blown fiberglass thickness | About 12 to 13 in | About 15 to 16 in |
| Best fit | Cathedral or 2×10 bays, floors, mild zones | Attic floors, colder zones, add-over layers |
| Meets 2021 IECC attic minimum? | Zone 1 only | No zone in the IECC table (Zones 2 to 8 are R-49 or R-60); Florida’s own code sets R-38 in its Zone 2 |
How much better is R-38 than R-30?
R-38 carries 8 more R-value points, a 27 percent higher rated resistance, but that does not translate to a 27 percent smaller heating bill. Heat flow through an assembly falls in proportion to 1 divided by R-value, so upgrading a ceiling plane from R-30 to R-38 cuts conductive loss through that plane by about 21 percent (1 minus 30/38). The gain applies only to the surface you improved, not the whole house, and only to conduction, not the air leakage that often dominates real attic losses.
The vendor claim of “up to 25 percent energy savings” refers to the surface, not the utility bill. Once you account for windows, walls, air leaks, and ductwork, the whole-home effect of an R-30 to R-38 attic swap is usually a few percent. That is still real money over 20 years, but it is smaller than the headline suggests, and it shrinks as you climb the R-value ladder because each added inch returns less. That is the real answer to whether you can over insulate a house.
How thick is R-30 vs R-38 by material?
R-30 runs about 8.25 to 13 inches and R-38 about 10.25 to 16 inches, and the exact number depends entirely on the material. Denser products hit the same R-value in less depth, which is why the rafter cavity, not the climate, often decides which one you can install. A standard 2×10 rafter gives about 9.25 inches of usable depth, enough for a high-density R-30 batt but not a standard R-38. Cross-check any figure against a full insulation R-value chart before you buy.
- Fiberglass batt: R-30 about 9.5 to 10 in standard, 8.25 in high-density; R-38 about 12 to 13 in standard, 10.25 in high-density.
- Blown cellulose: about R-3.5 per settled inch, so R-30 about 8.5 to 9 in and R-38 about 10.5 to 11 in.
- Blown fiberglass: about R-2.5 per inch, so R-30 about 12 to 13 in and R-38 about 15 to 16 in.
- Closed-cell spray foam: about R-6.5 per inch, so R-30 about 4.6 in and R-38 about 5.8 in.
For blown insulation, the real question is how deep R-30 or R-38 has to be after it settles. Every bag answers it. The FTC R-value rule (16 CFR 460.12) requires the coverage chart on loose-fill packages to list the minimum settled thickness, initial installed thickness, bags per 1,000 square feet and minimum weight per square foot at R-13, R-19, R-22, R-30, R-38 and R-49. Blow to the initial installed thickness so the layer still meets the settled number years later. If you have not picked a material yet, compare the best insulation for an attic by type and climate before you size the depth.
How thick is R-30 or R-38 rigid roof insulation?
On a low-slope roof insulated above the deck, R-30 in polyisocyanurate (polyiso) board takes about 5.2 inches, typically two 2.6-inch layers rated R-15 each. R-38 takes about 6.5 to 7 inches: two layers of 3.0 and 3.5 inches reach R-37.9, and two 3.5-inch layers reach R-41. Polyiso is rated at roughly R-5.7 per inch, so it reaches either value in about half the depth of batts.
The figures below come from the MuleHide Poly ISO 2 product data sheet (rev. October 2025). They are LTTR (long-term thermal resistance) values under ASTM C1289 and CAN/ULC S770, a 15-year time-weighted average, so they already account for the aging that lowers polyiso’s R-value after manufacture. The rating is not perfectly linear with thickness, so read the board table rather than multiplying per-inch values.
| Polyiso board thickness | LTTR R-value | Layers to reach R-30 or R-38 |
|---|---|---|
| 2.0 in | R-11.4 | Too thin to stack efficiently for either target |
| 2.6 in | R-15 | Two layers (5.2 in) = R-30 |
| 3.0 in | R-17.4 | With one 3.5 in layer (6.5 in) = R-37.9 |
| 3.5 in | R-20.5 | Two layers (7.0 in) = R-41 |
| 4.0 in | R-23.6 | Add a thinner layer to reach R-30 or R-38 |
Installers lay rigid board in two or more layers with staggered joints so no seam runs straight through the assembly. Tapered polyiso, used to build slope to drains, carries lower average R-values per piece than flat board of the same top thickness, so a tapered system needs its own R-value calculation. Board thickness also changes edge details: the curb, parapet and edge-metal heights must clear the full stack.
Is R-30 good enough for an attic?
R-30 is good enough for an attic floor only in Climate Zone 1, which covers the tip of Florida, Hawaii, and the U.S. territories. Everywhere else, current guidance has moved past it. The 2021 IECC and ENERGY STAR set the attic target at R-49 in Zones 2 and 3 and R-60 in Zones 4 through 8. That makes R-30 a partial layer in most homes, not a finished attic, though it remains a sound choice for a cathedral ceiling or addition where depth is capped. For the full picture of attic insulation by R-value and climate zone, start with the zone target and work back to a material.
| Climate zone (examples) | 2021 IECC / ENERGY STAR attic target | Where R-30 or R-38 fits |
|---|---|---|
| Zone 1 (S. Florida, Hawaii) | R-30 | R-30 can meet minimum; R-38 exceeds it |
| Zone 2-3 (Gulf Coast, Southeast, S. California) | R-49 | Both fall short; use as a base layer |
| Zone 4-5 (Mid-Atlantic, Midwest) | R-60 | Both fall short; add-over needed |
| Zone 6-8 (Northern tier, mountain, Alaska) | R-60 | Both well short; go straight to R-49 to R-60 |
Florida is the main exception to the zone targets above, because it writes its own energy code. Table R301.1 of the Florida Building Code, Energy Conservation, 8th Edition (2023) puts only seven counties in Zone 1 (Broward, Collier, Hendry, Lee, Miami-Dade, Monroe and Palm Beach), and its Table R402.1.2 sets the new-construction ceiling minimum at R-30 there and R-38 in the other 60 counties. Section R402.2.1 then lets R-30 satisfy an R-38 requirement “wherever the full height of uncompressed R-30 insulation extends over the wall top plate at the eaves,” which usually takes a raised-heel truss. So in most of Florida, R-38 is the code minimum and R-30 can meet it with full depth at the eaves, while ENERGY STAR’s retrofit target for Zone 2 is still R-49. For county-by-county detail, see our guide to R-30 and R-38 under the Florida Building Code.
When R-38 is the right call, and when it still is not enough
R-38 is the right call for an attic floor in Zones 2 and 3 as a strong starting layer, for retrofits where you top an existing R-19 with an R-19 add-over to reach the 60s, and for any assembly where you have the depth to spare. It is not enough as a standalone attic in Zones 4 through 8, where the target is R-60. Treating R-38 as the destination in a cold climate leaves R-value on the table that a few more inches of blown cellulose would deliver cheaply.
Where R-38 genuinely shines is the constrained cathedral or vaulted ceiling built with 2×12 rafters, which give about 11.25 inches of usable depth. A high-density R-38 batt at 10.25 inches fits with room for a ventilation baffle, while a standard R-38 does not. In that specific assembly, R-38 high-density is often the most you can install without furring down the ceiling or switching to closed-cell foam.
Cost and payback: does the extra R-value pay off?
Upgrading from R-30 to R-38 typically adds about 15 to 30 cents per square foot in material and labor, or a few hundred dollars on an average attic, and the payback usually runs several years. The extra 8 points cut conductive loss through the ceiling by about 21 percent, but because attic insulation follows a law of diminishing returns, the R-30 to R-38 step returns less per dollar than the first R-13 did and more than the later R-49 to R-60 step.
The stronger financial move in most homes is not choosing R-38 over R-30, it is air sealing the attic first and then blowing to the full zone target in one pass. The federal energy efficient home improvement credit (section 25C), which covered 30 percent of qualifying insulation material cost, is not available for 2026 work: the IRS says it applies to improvements made through December 31, 2025, after Public Law 119-21 ended it. State and utility programs may still help, so check what your utility offers before you set the target.
Why installation quality can beat a higher R-value
A properly air-sealed attic with correctly installed R-30 can outperform a poorly installed R-38 riddled with gaps, compression, or blocked soffit vents. R-value is a laboratory rating measured at uniform conditions; real assemblies lose it to compressed batts, missed corners, and convective looping through gaps. Air leakage, not conduction, is often the largest single attic loss, and no R-value rating captures it.
- Do not compress batts. An R-38 batt crushed into a 9.25-inch bay performs closer to R-30, erasing the upgrade.
- Keep baffles clear. Insulation stuffed into the soffit blocks intake airflow and invites moisture and ice dams.
- Air seal first. Seal top plates, wire and pipe penetrations, and the attic hatch before adding depth.
- Mind the faced side. A vapor retarder faces the conditioned space; doubling faced batts can trap moisture.
How to choose between R-30 and R-38
Choose by depth first, then climate zone, then budget. The rafter cavity or attic space caps what physically fits, the zone sets the real target, and cost decides how you get there. Work the decision in this order rather than picking an R-value off a chart.
- Measure usable depth. A 2×10 caps you near R-30 high-density; a 2×12 opens R-38 high-density; an open attic floor has no cap.
- Find your climate zone. Zone 1 can stop at R-30; Zones 2-3 target R-49; Zones 4-8 target R-60.
- Compare to the target. If R-30 or R-38 falls short, plan the add-over layer now rather than reinsulating later.
- Air seal, then insulate. Seal penetrations first, then blow or lay to the full zone number in one pass.
- Check current incentives. Confirm any state or utility rebates that may offset the cost of hitting the higher target; the federal 25C credit ended for work after 2025.
Frequently asked questions
Is R-30 or R-38 better for an attic?
R-38 is better for an open attic floor because the extra 8 points cut conductive heat loss through the ceiling by about 21 percent. In most U.S. zones, though, current code targets R-49 to R-60, so treat either as a base layer rather than the finished attic. R-30 is the better fit only where rafter depth is limited or you are in Climate Zone 1.
How much thicker is R-38 than R-30?
R-38 is about 2 to 3 inches thicker for the same material. In fiberglass batt, R-30 runs about 9.5 to 10 inches and R-38 about 12 to 13 inches. In blown cellulose, R-30 is about 8.5 to 9 inches and R-38 about 10.5 to 11 inches. High-density batts hit each value in roughly an inch and a half less depth.
Does R-38 save 25 percent more energy than R-30?
No. The 8-point jump cuts conductive loss through that one ceiling plane by about 21 percent, not the whole-home bill. Once windows, walls, air leaks, and ducts are counted, the real utility effect of an R-30 to R-38 attic swap is usually a few percent. Air sealing often saves more than the R-value step itself.
Can I put R-38 in a 2×10 rafter cavity?
Not a standard R-38 batt. A 2×10 gives about 9.25 inches of usable depth, and a standard R-38 needs 12 to 13 inches. Compressing it into the bay drops its real performance closer to R-30. A high-density R-30 batt at about 8.25 inches fits with room for a ventilation baffle, which is why R-30 is common in 2×10 assemblies.
Is R-38 enough for a cold-climate attic?
Usually not on its own. Climate Zones 4 through 8 target R-60 for attic floors under the 2021 IECC, so R-38 leaves resistance on the table. It works as a base layer that you top with an add-over of blown cellulose or fiberglass to reach the 60s. In a depth-limited cathedral ceiling, R-38 high-density may be the practical maximum.
Reviewed by The Roofing Brief Team. Last reviewed: October 2026.