4.2 Insulation Minimum R-Values — Table R402.1.3

4.2 Insulation Minimum R-Values — Table R402.1.3

Quick Answer: Table R402.1.3 is the 2021 IECC R-value alternative to the U-factor table. Assemblies with insulation-material R-values equal to or greater than the table may be used instead of meeting Table R402.1.2 U-factors. R-values are minimums; fenestration U-factors and SHGC in the same table are maximums. Climate Zone 4 except Marine and Climate Zones 5–8 require R-60 ceilings and wood-frame walls of R-30, or 20&5ci, or 13&10ci, or 0&20ci. Climate Zone 5 floors are R-30. Do not accept 2018 R-49 ceilings or R-13&5ci walls in CZ5.

Which table you are in

Section R402.1.3 states that assemblies with R-values of insulation materials equal to or greater than Table R402.1.3 shall be an alternative to the U-factor in Table R402.1.2. That sentence does three inspector jobs:

1. It names Table R402.1.3 as the component R-value table — cavity and continuous insulation listed by assembly and climate zone.
2. It reminds you that Table R402.1.2 is the U-factor table, not the R-value table. Swapping those numbers is the most common 2018-to-2021 paperwork error.
3. It tells you the R-value path is optional. A project may instead document assembly U-factors (Section 4.3) or Total UA (R402.1.5).

The table also repeats fenestration U-factor and glazed fenestration SHGC. Those columns remain maximums. A Climate Zone 3 window labeled U-0.32 fails the 0.30 maximum even if every opaque assembly beats the R-value cell.

Climate zone comes from Chapter 3, including Marine and Warm-Humid overlays. Using the wrong row is a plans failure before you ever look at a batt. ci means continuous insulation. NR means not required.

2021 rows you must have at hand (CZ3–CZ6)

Wood-frame wall cells list equivalent options; any one listed option satisfies the cell.

Surrounding rows, in brief. CZ0–1: fenestration U NR, skylight U-0.75, SHGC 0.25, ceiling R-30, wood-frame 13 or 0&10ci, mass 3/4, floor R-13, basement/slab/crawl 0. CZ2: fenestration U-0.40, skylight U-0.65, SHGC 0.25, ceiling R-49, wood-frame the same as CZ0–1, mass 4/6, floor R-13, basement/slab/crawl 0. CZ7 and 8: fenestration U-0.30, skylight U-0.55, SHGC NR, ceiling R-60, the same wood-frame wall options, mass 19/21, and floors R-38.

The 2018 hangover that fails in 2021

Ceiling R-values jumped. The 2018 IECC used R-49 in Climate Zones 4–8. 2021 uses R-60 in Climate Zone 4 except Marine and in Climate Zones 5–8. A specification that still says "R-49 blown attic, CZ5" fails Table R402.1.3 unless the project is on the U-factor or Total UA path — and even then the opaque-ceiling U-factor target is U-0.024, not a casual R-49 claim. R402.2.1 later allows uncompressed R-49 over 100 percent of the ceiling including the top plates to satisfy an R-60 R-value-table requirement (and R-38 to satisfy R-49). That raised-heel trade does not apply to Section R402.1.2 or to Total UA. Learn the construction details in the assemblies chapter; do not grant the trade from Table R402.1.3 alone.

Wood-frame walls also jumped in the cold climates. CZ3 still allows R-20, or 13&5ci, or 0&15ci. CZ4 except Marine and CZ5–8 require R-30, or 20&5ci, or 13&10ci, or 0&20ci.

Worked inspector scenario — Climate Zone 5 walls claimed as R-13+5ci

A Climate Zone 5 house is submitted with 2×4 walls, R-13 cavity batts, and R-5 rigid foam (ci) over the sheathing. The energy notes say "R-13+5ci wood walls per IECC."

Result: the assembly fails Table R402.1.3. R-13&5ci is a listed option in Climate Zone 3, and it was a listed cold-climate option in 2018. It is not a listed 2021 option in Climate Zone 5. The 2021 CZ5 wood-frame cell is R-30 (typically a 2×6 cavity), or R-20+5ci, or R-13+10ci, or R-0+20ci. R-13+5ci is one continuous-insulation step too thin on the ci side, and it is one cavity step too thin if the applicant meant the R-20+5ci option.

How the builder can fix it without leaving the R-value table:

• Upgrade the foam to R-10ci and keep R-13 cavity (13&10ci).
• Keep R-5ci and upgrade the cavity to R-20 (20&5ci), usually a 2×6 wall.
• Fill a 2×6 or deeper cavity to R-30 with no ci.
• Use R-20ci over an empty or uncounted cavity (0&20ci).

Cavity insulation cannot be added to ci to invent a hybrid that is not listed. That mixing ban is R402.1.4, taught in Section 4.3. If the applicant instead documents a tested or calculated wood-frame assembly U-factor equal to or less than 0.045, they have left Table R402.1.3 and are on Table R402.1.2. Do not average R-13+5ci into a U-0.045 claim without a real U-factor calculation that includes framing.

Footnotes that change pass or fail

Footnote a. R-values are minimums. U-factors and SHGC are maximums. Where insulation is installed in a cavity thinner than the label or design thickness, the installed R-value shall be not less than the table R-value. Compressed R-19 stuffed into a 5.5-inch stud bay is not R-19.

Footnote b. The fenestration U-factor column excludes skylights — use the skylight U-factor column. The SHGC column applies to all glazed fenestration. Exception: in Climate Zones 0 through 3, skylights may be excluded from the glazed-fenestration SHGC requirement provided the SHGC for those skylights does not exceed 0.30.

Footnote c. "5ci or 13" means R-5 continuous insulation on the interior or exterior surface of the wall or R-13 cavity insulation on the interior side of the wall. "10ci or 13" follows the same pattern with R-10ci. "15ci or 19 or 13&5ci" means R-15ci on the interior or exterior; or R-19 cavity on the interior; or R-13 interior cavity plus R-5ci on the interior or exterior.

Footnote d. Heated slabs. R-5 insulation shall be provided under the full slab area in addition to the required slab-edge insulation R-value. The slab-edge insulation for heated slabs shall not be required to extend below the slab. Unheated slabs: R-10ci to 2 feet in CZ3; R-10ci to 4 feet in CZ4 except Marine and CZ5–8. CZ0–2 slabs are 0 on this table.

Footnote e. There are no SHGC requirements in the Marine Zone. Table R402.1.3 still prints 0.40 on the combined "Climate Zone 5 and Marine 4" row. Teach the table you are using. Climate Zone 5 (not Marine) on the R-value table has SHGC 0.40. Marine 4 on that same table is covered by footnote e. Do not import the U-factor table's NR cell until you are actually on Table R402.1.2.

Footnote f. Basement wall insulation is not required in Warm Humid locations as defined by Figure R301.1 and Table R301.1. That is why a CZ3 basement cell of 5ci or 13 can become "not required" in a Warm Humid county.

Footnote g. The first value is cavity insulation; the second is continuous insulation. As an example, "13&5" means R-13 cavity plus R-5 continuous insulation.

Footnote h. Mass walls shall be in accordance with Section R402.2.5. The second R-value applies where more than half of the insulation is on the interior of the mass wall. Climate Zone 5 mass walls are 13/17: interior-majority insulation must meet R-17, not R-13. Climate Zone 3 and CZ4 except Marine are 8/13; CZ6 is 15/20; CZ7–8 are 19/21.

Footnote i. A maximum U-factor of 0.32 shall apply in Climate Zones 3 through 8 to vertical fenestration products installed in buildings located either above 4,000 feet in elevation or in windborne debris regions where protection of openings is required by IRC Section R301.2.1.2. That is a limited relaxation from 0.30 for those high-elevation or protected-opening products. It is not a license to use U-0.32 on every CZ3–8 window. On Table R402.1.2 the matching note is narrower (Marine 4 and CZ5–8 only) — compare footnotes when the applicant switches tables.

SHGC by climate on this table

• CZ0–3: SHGC 0.25, with the skylight exception above.
• CZ4 except Marine: SHGC 0.40.
• CZ5 and Marine 4 row: SHGC 0.40 printed; Marine Zone footnote e still applies to Marine 4.
• CZ6–8: SHGC NR.

A Climate Zone 6 project on the R-value table has no SHGC maximum in Table R402.1.3. A Climate Zone 5 project on the R-value table does (0.40), unless it is Marine 4 and you are applying footnote e. That is the opposite of assuming "cold climate means no SHGC."

When you walk a CZ5 house, look at wood walls for the 2018 R-13+5ci miss, attics for R-60 versus leftover R-49 notes, floors for R-30 versus an R-19 floor leftover from CZ4, and slabs for 4 feet of edge insulation plus R-5 under any heated slab.