6.2 Area-Weighted Averages, Dynamic Glazing & Exemptions (R402.3.1–R402.3.4)

6.2 Area-Weighted Averages, Dynamic Glazing & Exemptions (R402.3.1–R402.3.4)

Quick Answer: An area-weighted average of fenestration products may satisfy U-factor (R402.3.1). An area-weighted average of products more than 50 percent glazed may satisfy SHGC (R402.3.2). Dynamic glazing may satisfy Table R402.1.2 SHGC if the ratio of higher to lower labeled SHGC is ≥ 2.4 and the unit is automatically controlled to modulate solar gain in multiple steps; it is considered separately and shall not be averaged with non-dynamic glass. Up to 15 ft² of glazed fenestration per dwelling unit and one side-hinged opaque door ≤ 24 ft² are exempt from the stated R402.1.2 limits. Neither exemption applies to Total UA.

Section 6.1 put an NFRC number on each unit. This section is how a mixed window schedule still passes. Independent OpenExamPrep material for Exam 79 treats averaging as arithmetic with a code definition, not as a courtesy from the inspector. Use national 2021 text, not a local energy amendment.

R402.3.1 — area-weighted U-factor

An area-weighted average of fenestration products shall be permitted to satisfy the U-factor requirements. Every product in the average uses its NFRC 100 U-factor (or a Table R303.1.3 default if unlabeled) and its rough-opening or rated area as used on the schedule. The weighted U-factor is:

U_avg = Σ (U × A) ÷ Σ A

Compare U_avg to the Fenestration U-factor column of the table that applies (0.30 in Climate Zones 3–8 on both tables, 0.40 in Climate Zone 2, 0.50 or NR in Climate Zones 0–1 depending on the table). Skylights have a different maximum (0.75 / 0.65 / 0.55). Do not drop skylights into a vertical-only average and then claim the 0.30 column; keep skylight area against the skylight cap unless the energy packet clearly averages within the same requirement.

Opaque doors that are fenestration products may sit in the U-factor average. A poorly labeled entry door can be carried by better windows, or it can drag the average over the cap.

Worked example — three windows versus a 0.30 cap

Climate Zone 4 except Marine, Table R402.1.2 vertical fenestration maximum U-0.30. Three products:

U_avg = 11.12 ÷ 36 = 0.309. That exceeds 0.30, so the group fails R402.3.1 even though the largest window (16 ft²) is better than the cap. Individually, the 0.32 and 0.35 units already fail a unit-by-unit check; averaging does not save them when the poor units still have too much area.

Revise the 8 ft² stair unit from U-0.35 to U-0.28:

Σ(U × A) = 4.48 + 3.84 + (0.28 × 8) = 4.48 + 3.84 + 2.24 = 10.56
U_avg = 10.56 ÷ 36 = 0.293, which is ≤ 0.30. The group passes. The bedroom 0.32 unit is still worse than 0.30 by itself; R402.3.1 is what makes it legal inside this mix.

If the applicant instead claims the 8 ft² stair glass as part of the 15 ft² glazed exemption in R402.3.3, that area comes out of the R402.1.2 U-factor and SHGC check (on that path only). It does not come out of a Total UA worksheet. Do not exempt it from averaging and also leave it in UA as if it were 0.30 glass.

R402.3.2 — SHGC averaging and dynamic glazing

An area-weighted average of fenestration products more than 50-percent glazed shall be permitted to satisfy the SHGC requirements. A mostly opaque door with a slim lite is not in that SHGC average. Do not area-weight a 0.20 window against a 0.50 opaque-door “SHGC” that the product does not even have.

Dynamic glazing (electrochromic or other products that change SHGC in service) shall be permitted to satisfy the SHGC requirements of Table R402.1.2 provided both of the following are true:

1. The ratio of the higher labeled SHGC to the lower labeled SHGC is greater than or equal to 2.4, and
2. The dynamic glazing is automatically controlled to modulate the amount of solar gain into the space in multiple steps.

Dynamic glazing shall be considered separately from other fenestration, and area-weighted averaging with other fenestration that is not dynamic glazing shall be prohibited.

Exception: Dynamic glazing shall not be required to comply with this section where both the lower and higher labeled SHGC comply with Table R402.1.2.

Ratio example. Labels SHGC 0.12 (dark) and 0.36 (clear). Ratio = 0.36 ÷ 0.12 = 3.0, which is ≥ 2.4. In Climate Zone 3, Table R402.1.2 SHGC is 0.25, so the higher 0.36 value does not already meet the table. Automatic multi-step control is still required, and that 0.36 state cannot be averaged with the house’s ordinary SHGC 0.22 windows to “dilute” it.

Exception example. Labels SHGC 0.20 and 0.24 in Climate Zone 3. Both are ≤ 0.25, so the exception applies. The 2.4 ratio and the automatic-control rule in this section are not required for that product. The product still needs NFRC labels and still needs to meet U-factor.

Manual blinds, owner-operated shades, and a note that “occupants will close drapes” are not automatic multi-step control.

R402.3.3 — 15 square feet of glazed fenestration

Not greater than 15 square feet (1.4 m²) of glazed fenestration per dwelling unit shall be exempt from the U-factor and SHGC requirements in Section R402.1.2. This exemption shall not apply to the Total UA alternative in Section R402.1.5.

Memorize the edges:

• 15 ft², not 15 percent, not 15 ft² per floor, and not 15 ft² per elevation.
• Per dwelling unit, so a duplex gets 15 ft² per unit, not 15 ft² for the building.
• Glazed fenestration: windows, skylights, and glazed doors that you are parking in the exemption.
• Exempt from U-factor and SHGC in R402.1.2, not from air-leakage listing in R402.4.3, not from flashing, and not from the air-barrier seal at the jamb in Table R402.4.1.1.
• Does not apply to Total UA. Chapter 3 already flagged this; every square foot of glass stays in the UA sum at its real U-factor.

If the decorative stair glass is 20 ft², only 15 ft² is exempt. The remaining 5 ft² must meet U-factor and SHGC or ride in a legal R402.3.1 / R402.3.2 average.

R402.3.4 — one 24-square-foot opaque door

One side-hinged opaque door assembly not greater than 24 square feet (2.22 m²) in area shall be exempt from the U-factor requirement in Section R402.1.2. This exemption shall not apply to Total UA in R402.1.5.

Edges that fail exam items:

• One assembly, not one per elevation and not the front door plus the mudroom door.
• Side-hinged and opaque. A sliding glass door is glazed fenestration, not this exemption. A 36 ft² double-door pair is larger than 24 ft², so the assembly is not exempt.
• Exempt from U-factor in R402.1.2 only. If the door is glazed enough to have an SHGC, this section is not an SHGC holiday; use R402.3.2 or R402.3.3 as they actually apply.
• Not Total UA. Put the door’s U-factor and area in the UA worksheet.

An unlabeled wood opaque door defaults to U-0.50 under Table R303.1.3(2). On a Climate Zone 5 R402.1.2 path, that door fails 0.30 unless it is the one ≤ 24 ft² side-hinged opaque exemption (or it is carried by a U-factor average, or the project is on Total UA — in which case the exemption is gone and U-0.50 enters the sum).

Plans examiner workflow

1. List every fenestration product with area, NFRC U-factor, SHGC, and percent glazing.
2. Mark at most 15 ft² of glazed area as R402.3.3 exempt, and at most one ≤ 24 ft² side-hinged opaque door as R402.3.4 exempt, and only if the path is R402.1.2 — not R402.1.5.
3. Average remaining U-factors with R402.3.1. Average remaining SHGC only for products more than 50 percent glazed.
4. Hold dynamic glazing in its own column. Check the 2.4 ratio, automatic multi-step control, or the both-values-meet-the-table exception.
5. Compare skylights to the skylight U-factor column (and to SHGC rules in Section 6.1).

Scenario. A Climate Zone 5 Total UA worksheet omits a 20 ft² stained-glass panel “because of the 15 ft² rule.” Send it back. R402.3.3 does not apply to R402.1.5, and even on R402.1.2 only 15 ft² would have been exempt.

Scenario. Two 18 ft² opaque entry doors, both unlabeled wood (U-0.50). R402.3.4 saves one of them on the R402.1.2 path. The second door must meet 0.30, ride in an area-weighted U-factor average, or force a path that actually allows that U-factor under R402.5 tradeoff caps — it does not get a second 24 ft² holiday.

Keep averaging talk independent: this is OpenExamPrep teaching of 2021 IECC R402.3.1 through R402.3.4 for Exam 79 study, not an ICC interpretation service.