6.1 Fenestration U-Factor, SHGC & NFRC Labels (R402.3, R303.1.3)
Key Takeaways
- Windows, doors, and skylights take U-factors from NFRC 100; garage doors in the envelope may use NFRC 100 or ANSI/DASMA 105; SHGC and VT come from NFRC 200.
- Unlabeled products are assigned default U-factors and SHGC/VT from Tables R303.1.3(1) through R303.1.3(3) — metal double-pane windows default to U-0.80, far above a 0.30 climate-zone cap.
- Table R402.1.2 vertical fenestration U-factor is 0.50 in Climate Zones 0–1, 0.40 in Climate Zone 2, and 0.30 in Climate Zones 3–8; Table R402.1.3 lists Fen U as NR in Climate Zones 0–1.
- Skylight U-factor is 0.75 / 0.65 / 0.55 by the same zone groups on both tables. SHGC is 0.25 in Climate Zones 0–3; Table R402.1.2 lists NR for Climate Zone 5 and Marine 4 while Table R402.1.3 lists 0.40 on that row.
- Thermally isolated sunrooms and heated garages in Climate Zones 2–8 may use fenestration U-0.45 and skylight U-0.70; new glass between that room and the house must still meet full envelope fenestration numbers.
6.1 Fenestration U-Factor, SHGC & NFRC Labels (R402.3, R303.1.3)
Quick Answer: Determine U-factors for windows, doors, and skylights per NFRC 100. Garage doors that need a U-factor may use NFRC 100 or ANSI/DASMA 105. Determine SHGC and VT per NFRC 200. No label means Tables R303.1.3(1)–(3) defaults. Then read the fenestration columns of Table R402.1.2 or Table R402.1.3. Vertical U-factor on the U-factor table is 0.50 / 0.40 / 0.30 by zone group; the R-value table lists Fen U as NR in Climate Zones 0–1. Skylight U-factor is 0.75 / 0.65 / 0.55. SHGC is 0.25 in Climate Zones 0–3 — after that, read the table you are using.
Glazing and doors are 18 percent of the Exam 79 outline. Independent OpenExamPrep material for this exam treats every NFRC sticker as a code number, not as a sales claim. This chapter teaches 2021 IECC residential provisions (first printing). Confirm the code year on the live ICC Exam Catalog. Do not import a New Jersey energy-subcode overlay, a Colorado local amendment, or any other state edit as if it were the national 2021 text.
Chapter 2 already planted R303.1.3. This section applies those labels to R402.3 and to the two envelope tables. Until the product is rated, Table R402.1.2 has nothing legal to compare.
R303.1.3 — how a number gets on the schedule
U-factor. U-factors of fenestration products such as windows, doors, and skylights shall be determined in accordance with NFRC 100. Exception: where required, garage door U-factors shall be determined in accordance with either NFRC 100 or ANSI/DASMA 105. U-factors shall be determined by an accredited, independent laboratory and labeled and certified by the manufacturer. A builder letter, a center-of-glass brochure value, or a European Uw printout is not an NFRC 100 U-factor.
SHGC and VT. The solar heat gain coefficient (SHGC) and visible transmittance (VT) of glazed fenestration products such as windows, glazed doors, and skylights shall be determined in accordance with NFRC 200 by an accredited, independent laboratory, and labeled and certified by the manufacturer. The 2021 residential tables do not set a maximum VT. VT still appears on the NFRC label; inspectors use it as identification, not as a pass/fail SHGC substitute.
No label, no mercy. Products lacking a labeled U-factor shall be assigned a default from Table R303.1.3(1) (glazed windows, glass doors, skylights) or Table R303.1.3(2) (opaque doors). Products lacking a labeled SHGC or VT shall be assigned a default from Table R303.1.3(3). Those defaults are written to fail ordinary climate-zone caps:
| Product | Default table | Default value | Typical Table R402.1.2 cap in Climate Zones 3–8 |
|---|---|---|---|
| Metal-frame, double-pane window | R303.1.3(1) | U-0.80 | U-0.30 |
| Nonmetal or metal-clad, double-pane window | R303.1.3(1) | U-0.55 | U-0.30 |
| Metal-frame, single-pane skylight | R303.1.3(1) | U-2.00 | U-0.55 |
| Uninsulated metal opaque door | R303.1.3(2) | U-1.20 | U-0.30 |
| Wood opaque door | R303.1.3(2) | U-0.50 | U-0.30 |
| Single-glazed, clear SHGC | R303.1.3(3) | SHGC 0.8 | SHGC 0.25 in Climate Zones 0–3 |
A Climate Zone 5 house that installs unlabeled metal double-pane windows is not “close to 0.30.” It is U-0.80 until an NFRC 100 label appears. Insulated metal opaque doors default to U-0.60; even the best unlabeled opaque-door default (U-0.35, insulated nonmetal edge, not exceeding 45 percent glazing, double-pane) still misses a 0.30 cap unless an exemption in Section 6.2 applies.
When a garage door needs a rating. An unheated garage door on an unconditioned garage is usually not envelope fenestration. A garage door in the building thermal envelope of a heated garage (or another conditioned space) needs a U-factor determined to NFRC 100 or ANSI/DASMA 105, or it takes a default. Chapter 5 covers heated-garage insulation; this section covers the door as fenestration.
R402.3 — apply the rated numbers to Chapter 4
R402.3 is short: in addition to the requirements of Section R402, fenestration shall comply with Sections R402.3.1 through R402.3.5. Averaging, dynamic glazing, and the 15 ft² / 24 ft² exemptions are Section 6.2. Sunroom glass is below. First, the inspector must know which table the permit used.
R402.1.2 is the maximum assembly U-factor table. Assemblies shall have a U-factor equal to or less than the table. Fenestration shall have a U-factor and glazed fenestration SHGC equal to or less than the table.
R402.1.3 is the R-value alternative. Fenestration still uses U-factor and SHGC maxima in that table — they are not R-values. The Fenestration U-factor column excludes skylights; skylights have their own column. The SHGC column applies to all glazed fenestration unless a footnote exception applies.
Fenestration columns you must not mix
| Climate zone | Table R402.1.2 Fen U | Table R402.1.3 Fen U | Skylight U (both tables) | Table R402.1.2 SHGC | Table R402.1.3 SHGC |
|---|---|---|---|---|---|
| 0–1 | 0.50 | NR | 0.75 | 0.25 | 0.25 |
| 2 | 0.40 | 0.40 | 0.65 | 0.25 | 0.25 |
| 3 | 0.30 | 0.30 | 0.55 | 0.25 | 0.25 |
| 4 except Marine | 0.30 | 0.30 | 0.55 | 0.40 | 0.40 |
| 5 and Marine 4 | 0.30 | 0.30 | 0.55 | NR | 0.40 |
| 6 | 0.30 | 0.30 | 0.55 | NR | NR |
| 7 and 8 | 0.30 | 0.30 | 0.55 | NR | NR |
Trap 1 — Climate Zones 0–1 Fen U. On Table R402.1.2 the vertical maximum is 0.50. On Table R402.1.3 Fen U is NR (not required). Skylight U-factor 0.75 and SHGC 0.25 still apply on both tables. Do not tell a Miami-Dade R-value-path house that vertical windows must be U-0.50; do not tell a U-factor-path house that vertical windows are NR.
Trap 2 — Climate Zone 5 and Marine 4 SHGC. Table R402.1.2 prints NR. Table R402.1.3 prints 0.40. Both tables also footnote that there are no SHGC requirements in the Marine Zone. A 5A dwelling on the R-value alternative must meet SHGC 0.40. A 5A dwelling on the U-factor table has NR. A 4C Marine dwelling has no SHGC requirement under the Marine footnote, even though Table R402.1.3 prints 0.40 on the same row. Chapter 2 already warned you not to park Marine 4 on the 4-except-Marine row; this is the SHGC half of that warning.
Trap 3 — skylight SHGC in Climate Zones 0–3. The SHGC column applies to all glazed fenestration, except that in Climate Zones 0 through 3, skylights shall be permitted to be excluded from glazed fenestration SHGC requirements provided the SHGC for those skylights does not exceed 0.30. Vertical glass in Climate Zone 2 is still 0.25. A skylight at 0.30 can be excluded from that 0.25 column; a skylight at 0.35 cannot use the exception. Skylight U-factor (0.65 in Climate Zone 2) is a separate check.
Trap 4 — the 0.32 footnotes are not the same. Both tables allow a maximum U-factor of 0.32 for vertical fenestration (not skylights) in buildings located either above 4,000 feet in elevation or in windborne debris regions where protection of openings is required by IRC R301.2.1.2. That 0.32 figure is less stringent than 0.30; impact-rated and high-elevation products often cannot hit 0.30. Table R402.1.2 footnote f applies that 0.32 value in Marine Climate Zone 4 and Climate Zones 5 through 8. Table R402.1.3 footnote i applies it in Climate Zones 3 through 8. A Climate Zone 3 house at 4,500 feet on the R-value path may use 0.32. The same house on the U-factor table stays at 0.30, because footnote f does not list Climate Zone 3. Read the footnote on the table in the energy packet.
R402.3.5 — sunroom and heated-garage fenestration
Sunrooms and heated garages enclosing conditioned space shall comply with the fenestration requirements of this code. Exception: in Climate Zones 2 through 8, for sunrooms and heated garages with thermal isolation and enclosing conditioned space, the fenestration U-factor shall not exceed 0.45 and the skylight U-factor shall not exceed 0.70. Climate Zones 0 and 1 do not get that exception.
New fenestration separating a thermally isolated sunroom or heated garage from conditioned space shall comply with the building thermal envelope fenestration requirements — the full table values, not 0.45 / 0.70. The cheap glass is for the isolated room’s exterior. The door or window between the house and that room is envelope glass.
Plans review and field labels
R401.3 requires the posted certificate to list fenestration U-factors and SHGC. Where there is more than one value, the certificate shall indicate both the value covering the largest area and the area-weighted average if available. The window schedule on the drawings should already show NFRC U-factor, SHGC, VT, and the standard (NFRC 100 / 200, or DASMA 105 for envelope garage doors).
In the field, the temporary NFRC label is the inspection document. Match operator type, size group, and the U-factor / SHGC pair to the approved schedule. A substituted series with U-0.32 in a Climate Zone 5 house that declared Table R402.1.2 U-0.30 fails unless averaging in Section 6.2 saves the group. Peeling labels before inspection is not a courtesy; it is a failed identification under R303.1.3.
Scenario. Climate Zone 4 except Marine, Table R402.1.3 path, SHGC 0.40 on the schedule, NFRC labels read 0.37. That SHGC passes. The same labels with U-0.32 fail the 0.30 Fen U column unless R402.3.1 averaging (Section 6.2) or a qualifying 0.32 footnote condition is documented.
Scenario. Climate Zone 5, Table R402.1.2 path, SHGC 0.50 glass. Table R402.1.2 lists SHGC NR for Climate Zone 5 and Marine 4, so SHGC is not a Table R402.1.2 fail. Switch that same house to Table R402.1.3 and SHGC 0.40 is required. Always name the table before you fail the glass.
Keep the wording independent: this is OpenExamPrep teaching of 2021 IECC residential fenestration rules for Exam 79 study, not an ICC or NFRC endorsement.
A heated-garage overhead door is in the building thermal envelope. Which 2021 rating path may be used to determine its U-factor under R303.1.3?
A Climate Zone 1 dwelling uses the Table R402.1.3 R-value alternative. What is the 2021 vertical fenestration U-factor requirement in that table?
A Climate Zone 5A house is reviewed on Table R402.1.2. A second house on the same street is reviewed on Table R402.1.3. What is the glazed fenestration SHGC maximum on each table for that climate grouping?
Vertical fenestration is installed at 4,200 feet elevation in a windborne-debris region. Which statement matches the 2021 0.32 U-factor footnotes?