4.3 Maximum Assembly U-Factors — Table R402.1.2 & R-Value Computation (R402.1.2, R402.1.4)
Key Takeaways
- Table R402.1.2 lists maximum assembly U-factors; Climate Zone 4 except Marine uses ceiling U-0.024, wood-frame wall U-0.045, floor U-0.047, basement wall U-0.059, crawl wall U-0.065, fenestration U-0.30, and SHGC 0.40
- Climate Zone 5 and Marine 4 keep ceiling U-0.024 and wood-frame wall U-0.045 on Table R402.1.2, but that table lists SHGC as NR — while Table R402.1.3 lists SHGC 0.40 for the same climate row
- R402.1.4: cavity R-value counts only toward the cavity column; continuous insulation R-value counts only toward the ci column; gypsum, other building materials, and air films shall not be added
- When insulated siding is used to meet Table R402.1.3 continuous insulation, the manufacturer's labeled R-value is reduced by R-0.6; blown-in insulation uses the manufacturer's settled R-value
- Table R402.1.2 footnote f allows vertical fenestration U-0.32 in Marine 4 and Climate Zones 5–8 above 4,000 feet or in windborne debris regions requiring opening protection; Table R402.1.3 footnote i applies that 0.32 relief in Climate Zones 3–8
4.3 Maximum Assembly U-Factors — Table R402.1.2 & R-Value Computation (R402.1.2, R402.1.4)
Quick Answer: Table R402.1.2 lists maximum assembly U-factors and fenestration criteria. Assemblies must be equal to or less than the tabulated U; fenestration U and glazed SHGC must be equal to or less than the table. Climate Zone 4 except Marine uses ceiling U-0.024, wood-frame wall U-0.045, floor U-0.047, and SHGC 0.40. Climate Zone 5 and Marine 4 keep those ceiling and wood-frame U-factors but list SHGC as NR on this table — different from SHGC 0.40 on Table R402.1.3. Section R402.1.4 forbids adding gypsum and air films, forbids mixing cavity R into the ci column, requires settled R for blown-in, and reduces insulated-siding R by R-0.6 when that siding is used as continuous insulation.
R402.1.2 is a U-factor ceiling, not an R-value floor
R402.1.2 says the building thermal envelope shall meet Table R402.1.2 based on the climate zone specified in Chapter 3. Assemblies shall have a U-factor equal to or less than that specified. Fenestration shall have a U-factor and glazed fenestration SHGC equal to or less than the table.
U-factor is overall thermal transmittance of the assembly, in Btu/h·ft²·°F. Lower is better. A wood-frame wall U-0.045 is a whole-wall number: cavity insulation, framing fraction, sheathing, and continuous insulation if any. You cannot convert a batt label to a U-factor by taking 1/13 and calling the wall done. Footnote a requires nonfenestration U-factors from measurement, calculation, or an approved source.
Selected 2021 U-factor rows (CZ3–CZ6)
| Climate zone | Fen. U | Skylight U | SHGC | Ceiling U | Wood-frame wall U | Mass wall U | Floor U | Basement wall U | Crawl wall U |
|---|---|---|---|---|---|---|---|---|---|
| CZ3 | 0.30 | 0.55 | 0.25 | 0.026 | 0.060 | 0.098 | 0.047 | 0.091 | 0.136 |
| CZ4 except Marine | 0.30 | 0.55 | 0.40 | 0.024 | 0.045 | 0.098 | 0.047 | 0.059 | 0.065 |
| CZ5 and Marine 4 | 0.30 | 0.55 | NR | 0.024 | 0.045 | 0.082 | 0.033 | 0.050 | 0.055 |
| CZ6 | 0.30 | 0.55 | NR | 0.024 | 0.045 | 0.060 | 0.033 | 0.050 | 0.055 |
CZ0–1 use fenestration U-0.50, skylight 0.75, SHGC 0.25, ceiling U-0.035, wood-frame U-0.084. CZ2 uses fenestration U-0.40, skylight 0.65, ceiling U-0.026, wood-frame still 0.084. CZ7 and 8 keep fenestration U-0.30 and wood-frame U-0.045, with ceiling U-0.024, mass U-0.057, and floor U-0.028.
The SHGC table-switch trap
Item writers love this pair of cells.
- On Table R402.1.3 (R-value alternative), Climate Zone 5 and Marine 4 show SHGC 0.40.
- On Table R402.1.2 (U-factor), the same climate row shows SHGC NR.
- Both tables include a footnote that there are no SHGC requirements in the Marine Zone.
If the permit documents say "prescriptive U-factor, Table R402.1.2, Climate Zone 5," you do not fail windows for SHGC 0.50 against that table's SHGC column — the cell is NR. If the same house is documented on Table R402.1.3, Climate Zone 5 SHGC 0.40 applies (Marine 4 still has the Marine Zone footnote). Read the table the applicant used. Do not merge the two tables into one imaginary SHGC.
The Marine Zone footnote on Table R402.1.2 is consistent with the NR cell for "5 and Marine 4." On the R-value table, that footnote still matters because the printed cell is 0.40 for a combined row that includes Marine 4. Climate Zone 5 that is not Marine keeps SHGC 0.40 on Table R402.1.3 and NR on Table R402.1.2.
Other Table R402.1.2 footnotes
Footnote a. Nonfenestration U-factors from measurement, calculation, or an approved source — not from a batt bag.
Footnote b. Mass walls per R402.2.5. Where more than half the insulation is on the interior, mass-wall U-factors shall not exceed 0.17 in Climate Zones 0 and 1, 0.14 in Climate Zone 2, 0.12 in Climate Zone 3, 0.087 in Climate Zone 4 except Marine, 0.065 in Climate Zone 5 and Marine 4, and 0.057 in Climate Zones 6 through 8. Those interior-majority caps are tighter than the main mass-wall column (Climate Zone 5 main column 0.082 versus 0.065 if most insulation is inside).
Footnote c. In Warm Humid locations, basement wall U-factor shall not exceed 0.360 — the same number used in CZ0–2. The U-factor table is not requiring insulated basement walls in Warm Humid locations, matching Table R402.1.3 footnote f (basement wall insulation not required in Warm Humid).
Footnote d. The SHGC column applies to all glazed fenestration. Exception: in Climate Zones 0 through 3, skylights may be excluded from glazed-fenestration SHGC requirements provided skylight SHGC does not exceed 0.30.
Footnote e. There are no SHGC requirements in the Marine Zone.
Footnote f. A maximum U-factor of 0.32 shall apply in Marine Climate Zone 4 and Climate Zones 5 through 8 to vertical fenestration products installed in buildings located either above 4,000 feet in elevation above sea level, or in windborne debris regions where protection of openings is required by IRC Section R301.2.1.2. Compare that with Table R402.1.3 footnote i, which applies the 0.32 relief in Climate Zones 3 through 8. On the U-factor table, Climate Zone 3 and Climate Zone 4 except Marine do not get that 0.32 footnote. High-elevation CZ3 windows on a U-factor submittal still face U-0.30 unless another path applies.
R402.1.4 R-value computation
When someone claims Table R402.1.3, R402.1.4 is how you score the numbers.
Cavity column. Cavity insulation alone shall be used to determine compliance with the cavity insulation R-value requirements in Table R402.1.3. Where cavity insulation is installed in multiple layers, those cavity R-values shall be summed. The manufacturer's settled R-value shall be used for blown-in insulation. Cavity R-values shall not be used to determine compliance with the continuous insulation R-value requirements.
Continuous column. Continuous insulation alone shall be used to determine compliance with the ci R-value requirements. Multiple ci layers may be summed. You cannot add R-13 cavity to R-5ci and call the result "R-18ci," and you cannot treat extra cavity R as a substitute for a missing ci layer on a 13&10ci option.
No phantom R. Computed R-values shall not include an R-value for other building materials or air films. Gypsum board, plywood sheathing, interior and exterior air films, and cladding air spaces do not get added to turn R-13 into R-15.
Insulated siding used as ci. Where insulated siding is used for the purpose of complying with the continuous insulation requirements of Table R402.1.3, the manufacturer's labeled R-value for the insulated siding shall be reduced by R-0.6. Labeled R-2.6 siding counts as R-2.0 ci. Labeled R-5 siding counts as R-4.4 ci, which does not by itself meet a 5ci requirement.
Computation examples
- Climate Zone 5 wall claiming 13&10ci: R-13 fiberglass in the cavity (cavity column) plus R-10 foam that is continuous (ci column). OSB and gypsum are not added. The assembly passes that listed option if both products are actually installed at those R-values.
- Climate Zone 5 wall claiming 13&10ci but the "ci" is insulated siding labeled R-10: after the R-0.6 penalty the siding is R-9.4. That is not R-10ci. Fail, unless another ci layer makes up R-0.6.
- Climate Zone 4 ceiling claiming R-60 blown: use settled R at the installed thickness. If the installer places 16 inches and settled R is R-3.7 per inch, 16 × 3.7 = R-59.2, which is not R-60. Fail footnote a together with R402.1.4.
- A builder adds "R-0.45 gypsum plus R-0.68 interior air film" to R-13 to claim R-14 for a cavity-only cell. R402.1.4 forbids it.
The Climate Zone 5 house from Section 4.2 that claimed R-13+5ci does not get a second chance by adding gypsum and calling the wall R-19. Cavity stays R-13; ci stays R-5; the listed CZ5 options still start at 20&5ci or 13&10ci.
R402.1.5 in one paragraph
If neither component path is used, R402.1.5 allows a Total UA alternative: the sum of (U-factor × assembly area) for the proposed envelope must be less than or equal to the total UA from multiplying the U-factors in Table R402.1.2 by the same assembly areas. The calculation shall be consistent with the ASHRAE Handbook of Fundamentals and shall include the thermal bridging effects of framing materials. In addition to UA compliance, the SHGC requirements of Table R402.1.2 and the maximum fenestration U-factors of Section R402.5 shall be met. Total UA is its own documentation task in Chapter 3. The inspector takeaway here is that UA is judged against Table R402.1.2, not against Table R402.1.3 R-values, and that SHGC does not disappear just because UA passed.
Field and plans checklist
- Confirm which criterion is claimed: Table R402.1.2 U-factors, Table R402.1.3 R-values, or R402.1.5 UA.
- If U-factors, demand the source — NFRC labels for fenestration; measurement, calculation, or an approved source for opaque assemblies — and compare to the correct climate row. Remember Climate Zone 5 SHGC is NR on this table.
- If R-values, apply R402.1.4 before you compare to Table R402.1.3. Watch the CZ5 R-13+5ci miss, the R-60 ceiling, 2-foot versus 4-foot slab depth, and heated-slab R-5 under the full slab.
- If insulated siding is the ci, subtract R-0.6 from the label.
- Apply the vertical-fenestration U-0.32 relief only in the climate zones that footnote actually lists on the table in use: CZ3–8 on the R-value table; Marine 4 and CZ5–8 on the U-factor table; and only above 4,000 feet or in windborne debris regions requiring opening protection.
A Climate Zone 5 house that "almost" matches 2018 R-13+5ci walls is not a rounding error. It is the wrong code year on the wrong cell. Score it against 2021 Table R402.1.3 or require a wood-frame assembly U-factor ≤ 0.045.
Under the 2021 IECC, which statement correctly identifies Table R402.1.2?
A Climate Zone 4 except Marine house is submitted on the Table R402.1.2 U-factor path. What maximum wood-frame wall U-factor applies?
Insulated siding labeled R-5 is proposed as the continuous insulation on a Table R402.1.3 "5ci" wall option. What does 2021 IECC R402.1.4 require?
A Climate Zone 5 applicant claims Table R402.1.3 wood-frame walls using R-13 cavity batts, R-5 foam, R-0.45 gypsum, and an interior air film, and calls the cavity "R-19 equivalent." Which 2021 R402.1.4 result is correct?