Insulation R-Value by Climate Zone: The Contractor's Reference

Published July 3, 2026 · Prestige Insulation

One of the most common questions we get: "What R-value do I need for this job?" The answer depends entirely on your climate zone and the assembly type. This is the reference we wish existed when we started — zone-by-zone, assembly-by-assembly.

How climate zones work

The IECC (International Energy Conservation Code) divides the U.S. into 8 climate zones based on heating and cooling degree days. Zone 1 is the hottest (Miami), Zone 8 is the coldest (Fairbanks). Your local building department enforces the zone-specific requirements for your jurisdiction.

Wall R-values by climate zone (IECC 2021)

Zone	States (sample)	2×4 Wall	2×6 Wall
1	Hawaii, S. Florida	R-13	R-19
2	FL, S. TX, Puerto Rico	R-13	R-19
3	TX, GA, AL, AZ, S. CA	R-13 + R-3.75 ci (or R-15)	R-19 or R-21
4	VA, NC, TN, CO, WA	R-15 + R-3.8 ci (or 2×6 R-19)	R-21
5	OH, IL, PA, NY, OR	R-15 + R-7.5 ci (or 2×6 R-21)	R-21 + R-3.8 ci
6	MN, WI, MI, MT, northern NE	R-15 + R-7.5 ci	R-21 + R-5 ci
7–8	AK, far northern MN	R-15 + R-10 ci	R-21 + R-10 ci

ci = continuous insulation (exterior foam board). Without ci, 2×6 framing with R-21 HD is the practical alternative for zones 4–6.

Attic R-values by climate zone (IECC 2021)

Zone	Attic Floor Minimum	Recommended
1–2	R-30	R-38
3	R-38	R-49
4	R-38 to R-49	R-49
5	R-49	R-49 to R-60
6–8	R-49 to R-60	R-60

Floor R-values by climate zone

Zone	Floor Over Crawl / Unconditioned
1–3	R-13 to R-19
4–5	R-19 to R-30
6–8	R-30

Basement wall R-values

Zone	Below-Grade Wall
1–3	R-0 (not required)
4–8	R-10 to R-15 continuous OR R-13 cavity + R-5 ci

For below-grade applications, mineral wool (R-15) is the preferred product — it's non-combustible, moisture-resistant, and handles the humidity conditions common in basement assemblies. See our mineral wool page →

Quick reference: what to order by zone

• Zone 1–2: R-13 walls · R-30 attic · R-13 floor
• Zone 3: R-15 HD walls (2×4) · R-21 (2×6) · R-38 attic · R-19 floor
• Zone 4: R-15 HD or 2×6 R-21 · R-38–R-49 attic · R-19–R-30 floor
• Zone 5: 2×6 R-21 + R-3.8 ci · R-49 attic · R-30 floor
• Zone 6–8: 2×6 R-21 + R-5–10 ci · R-49–R-60 attic · R-30 floor