R-30 vs R-38 Attic Insulation: Which Does Your Project Need?

Published July 3, 2026 · Prestige Insulation

Attic insulation is where most buildings lose the most energy — and where most code updates focus. The difference between R-30 and R-38 matters both for code compliance and for your client's energy bill. Here's how to choose.

Code minimums by climate zone

Climate Zone	States (examples)	Attic Minimum (IECC 2021)
Zone 1–2	FL, southern TX, HI	R-30
Zone 3	Most of TX, GA, AL, AZ, southern CA	R-38
Zone 4	VA, NC, TN, AR, inland CA, OR	R-38 to R-49
Zone 5	OH, IN, IL, PA, NY, WA	R-49
Zone 6+	MN, WI, MI, ME, VT, NH, MT	R-49 to R-60

R-30: where it still applies

R-30 satisfies the code minimum only in zones 1 and 2 — Florida, the Gulf Coast, Hawaii, and the very southern tip of Texas. For every other climate zone in the contiguous U.S., R-30 falls below the IECC 2021 minimum.

R-30 is also used in cathedral ceiling assemblies where rafter depth limits insulation thickness, and in floor assemblies over unconditioned crawl spaces in zones 3–4 where R-25 to R-30 floor values are code-minimum.

R-38: the national attic standard

R-38 is required in zones 3 through 4 for attic floors — that's the majority of the U.S. by population. It's the most commonly installed attic insulation value in new construction, and the spec most jurisdictions have adopted in their local amendments.

At approximately 12" thick in batt form, R-38 can be installed in a single layer in standard 2×12 attic framing, or cross-layered over existing insulation in retrofits.

R-49 and above: cold climates

Zone 5 and colder require R-49 to R-60. If your projects are in the Upper Midwest, New England, or mountain West, spec R-49 or higher. See our R-49 page →

Retrofit strategy

When adding attic insulation over existing product:

• Always use unfaced batts on top of existing insulation — never put a vapor retarder in the middle of an assembly.
• Cross-layer perpendicular to existing joists for maximum coverage and to eliminate thermal bridging at the framing.
• For R-38 retrofit: add R-19 or R-21 batts perpendicular over existing R-19 blown-in for a total of approximately R-38 to R-40.

Order from Prestige

We stock R-30, R-38, and R-49 in both 15" and 23" widths. Same-day quotes, next-day delivery. Call (929) 466-1426 or get a quote online.