What Climate Zone Am I In? IECC Zone Lookup

What a climate zone is

The International Energy Conservation Code (IECC) splits the US into climate zones numbered 1 (hottest) to 8 (coldest). A letter after the number describes moisture: A is humid, B is dry and C is marine (the mild, damp Pacific coast). Zones 7 and 8 have no letter. Each county sits in exactly one zone, and the zone decides how much insulation a new home or addition needs.

The zones come from long-term weather data, mostly heating and cooling degree days. The 2021 code redrew the map using newer data, and about 1 in 8 counties (12%) moved. More than 9 in 10 of those moved one zone warmer; a few moved colder.

Looking for your gardening zone instead? That's the USDA plant hardiness zone, a different map based only on winter lows; use our planting zone lookup for that.

Worked example: Collin County, Texas

• Collin County (Plano, Frisco, McKinney) is in zone 3A.
• The 2021 code calls for R-49 in the attic and R-20, or 13+5ci, or R-15ci in wood-frame walls.
• An attic with 6 inches of old blown fiberglass has roughly R-15. Reaching R-49 means adding about R-34, or roughly 14 inches of blown fiberglass.
• Next door, Dallas County moved from zone 3 to zone 2 in the 2021 map. Texas still enforces the 2015 code statewide, so for a Dallas permit the older zone 3 applies unless your city has adopted a newer edition. Always check your own county and your city's code.

How to read the R-values

• R-value is resistance to heat flow. Higher is better.
• "ci" means continuous insulation: foam board or similar running unbroken over the outside of the studs. It stops heat leaking through the wood framing.
• "13+5ci" means R-13 in the stud cavities plus R-5 of continuous insulation.
• Mass walls (concrete block, brick, ICF) show two numbers: the first if the insulation is on the outside, the second if it's on the inside.
• Slab edge insulation runs down the edge of the slab (or under it) for the depth shown.

A standard 2×4 wall holds about R-13 to R-15, and a 2×6 wall about R-19 to R-21. In zone 3 a 2×6 wall (or a 2×4 with R-5 foam outside) meets the code. In zones 4 to 8 you need foam board outside as well: R-5 over a 2×6, or R-10 over a 2×4.

Which code applies to you?

The 2021 IECC is the model code, but states and cities adopt editions on their own schedule, often with changes. In 2026 some states enforce the 2024 IECC, many are on 2021 or 2018, Texas is on the 2015 IECC statewide, and a few states leave it to each city. We show the 2021 and 2018 values side by side. For a permit, your local building department's adopted edition is what counts.

Topping up an attic

• Measure first. Check depth in several places; loose fill settles and gets pushed around. Use the average.
• Air seal before you add insulation. Seal gaps around light fixtures, plumbing stacks and the attic hatch. Insulation doesn't stop air leaks.
• Fit a baffle in each rafter bay above the soffit so the new insulation doesn't block the intake vents.
• Use the bag's coverage chart to count bags. Every loose-fill product prints bags per 1,000 sq ft for each R-value. Our depth is an estimate from typical R-per-inch values, and the chart for your product wins.
• Don't compress batts or pile insulation against recessed lights that aren't IC-rated.

Frequently asked questions

Last reviewed 2026-10-21. See how we calculate for our sources and assumptions.