IECC R402 / C402 · ASHRAE 90.1 envelope

How to read insulation R-value & U-factor tables by climate zone

Climate zones, cavity vs continuous insulation, window U-factors, and prescriptive vs performance compliance — how the energy code envelope tables work, with a worked example and a link to the free ICC viewer.

Need the actual tables? Read IECC Chapter 4 in the free ICC code viewer →

How to read the envelope tables, step by step

Step	What you do	Watch out for
1	Find the climate zone for the county (IECC Chapter 3 table/map)	A number (thermal) plus a letter (A moist, B dry, C marine); some tables split on the letter or on "marine 4"
2	Decide residential vs commercial: R-chapter for low-rise dwellings, C-chapter (or ASHRAE 90.1) for everything else	Apartments over three stories fall under the commercial provisions
3	Pick the compliance path: prescriptive R-value table, U-factor/UA alternative, or performance	Mixing paths component by component isn’t allowed unless the path permits trade-offs
4	Find the component row (ceiling, wood-frame wall, mass wall, floor, slab, basement wall) and read the climate-zone column	Commercial tables also split by construction type (metal building, steel-framed, mass) and by roof type
5	Read the notation: first number is cavity, "ci" is continuous insulation	"20+5ci" means R-20 in the cavity plus R-5 continuous — not R-25 anywhere you like
6	Read fenestration separately: maximum U-factor (lower is better) and SHGC where it applies	U-factor is the whole NFRC-rated unit, not the glass center
7	Read the table footnotes	Footnotes carry the exceptions (e.g., reduced attic R-value at raised-heel eaves, log walls, slab depth)

Our own summary, not the code text. Read IECC Chapter 4 (residential and commercial) in the free ICC viewer (codes.iccsafe.org); ASHRAE 90.1 is published by ASHRAE. Verify against the energy code edition — and any state amendments — adopted by your AHJ.

Which compliance path fits?

Path	How it works	When contractors use it
Prescriptive R-value	Meet or beat every component R-value in the table	Simple jobs; the inspector checks labels and cert cards
U-factor alternative	Meet the assembly U-factor for each component	Nonstandard assemblies (SIPs, advanced framing, mass walls)
Total UA trade-off (REScheck / COMcheck)	The whole envelope’s area-weighted UA must not exceed the code-reference house/building	Trading more glass for better walls or attic
Performance (R405 / C407) or ERI (R406)	Energy model shows proposed design uses no more energy than the standard reference	Unusual designs; mandatory provisions still apply
ASHRAE 90.1 (commercial alternative)	Section 5 prescriptive tables by climate zone, or trade-off/modeling methods	Owners or engineers who design to 90.1 instead of IECC C-chapter

Worked example — new house in climate zone 5A, 2021 IECC prescriptive path

Input / step	Value	Source
County lookup	Climate zone 5A	IECC climate zone table
Path chosen	Prescriptive R-value	Builder choice
Ceiling / attic insulation	R-60	R-value table, ceiling row, zone 5
Windows (whole-unit NFRC label)	U-0.30 or lower	Fenestration U-factor, zone 5
Submitted window: U-0.28	Passes (0.28 ≤ 0.30)	Lower U-factor is better
Attic with full-height R-49 over the top plate at the eaves	May satisfy the R-60 row	Table footnote exception

Worked example values only (2021 IECC). Look up the full tables in the ICC viewer and check state amendments.

Common field mistakes

Mistake	What to do instead
Reading a higher U-factor as better	U-factor is heat flow — lower is better; R-value is resistance — higher is better
Adding cavity and ci together and installing all of it in the cavity	Continuous insulation must be continuous across framing to count as "ci"
Using the county from the job name instead of the site	Check the county the building sits in; a few counties straddle a zone line in newer editions
Assuming the model code is what’s enforced	Many states amend the IECC or stay on an older edition — energycodes.gov tracks adoption
Compressing batts at the eaves	Use raised-heel trusses or baffles so the full depth reaches the wall line

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Reading the envelope tables like an inspector

Read the tables in the free ICC code viewer. IECC Chapter 4 holds the residential (R402) and commercial (C402) envelope tables. ASHRAE 90.1 Section 5 is the commercial alternative, sold by ASHRAE. This page explains how to use them; it does not reproduce them. Edition referenced: 2021 IECC, with notes on 2018 and 2024 — verify against the edition adopted by your AHJ, and check energycodes.gov for state adoption status.

Climate zone first. Every envelope number hangs on the zone: a thermal number (0 hottest to 8 coldest) and a moisture letter (A moist, B dry, C marine). Zones come from a county table, not your gut — two jobs an hour apart can land in different zones. The residential tables mostly key off the number; commercial tables also split by construction type, so a metal building and a wood-framed building in the same zone read different rows.

R-values vs U-factors. The R-value table lists minimum insulation you install, written as cavity plus continuous ("ci"). The U-factor table lists maximum heat flow for the whole assembly, which lets engineered assemblies comply even if they don’t look like the R-value row. Fenestration is always a U-factor — lower is better — and in warmer zones an SHGC limit applies too.

Worked example. A new house in zone 5A under the 2021 IECC prescriptive path: the ceiling row calls for R-60 and the fenestration column for U-0.30 or lower. A window labeled U-0.28 passes. A footnote lets full-depth R-49 that extends over the wall top plate at the eaves stand in for R-60, which is why raised-heel trusses matter. If the builder wants more glass than the prescriptive path tolerates, REScheck’s UA trade-off or the performance path is the way out — not skipping insulation.

Related references: the duct construction and Manual J/S/D guide (loads depend on this envelope) and the IBC fire-resistance ratings guide (foam plastic and exterior-wall assemblies carry fire rules too). Field PM keeps insulation certificates, photos before cover, and inspection results with the job so the energy inspection has its paperwork.

FAQ

How do I find my IECC climate zone?+

Look up the county in the climate zone table in IECC Chapter 3 (residential R301 or commercial C301). The zone is a number from 0 to 8 plus a moisture letter: A (moist), B (dry), or C (marine). Read it free in the ICC viewer at codes.iccsafe.org.

What does 20+5ci mean in the IECC table?+

It means R-20 of cavity insulation between the studs plus R-5 of continuous insulation installed across the framing, usually as foam or mineral-wool board on the exterior. The continuous layer cuts thermal bridging through the studs.

Is a lower U-factor better?+

Yes. U-factor measures how much heat flows through the window, so lower numbers mean better performance. Under the 2021 IECC in climate zone 5, residential windows must be U-0.30 or lower under the prescriptive path — check your adopted edition.

What changed for attic insulation in the 2021 IECC?+

The 2021 edition raised the prescriptive ceiling requirement in the colder zones — zone 5 went to R-60 from the R-49 in the 2018 edition — with a footnote that lets full-depth R-49 over the top plate satisfy it. The 2021 edition also added climate zone 0, which doesn’t occur in the U.S.

Do I have to use the prescriptive table?+

No. You can also comply with the U-factor alternative, a total UA trade-off (REScheck or COMcheck), or the performance path with an energy model. Mandatory provisions such as air sealing and duct testing apply on every path.

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