3.3 Total UA Alternative & Fenestration Tradeoff Caps (R402.1.5, R402.5)

3.3 Total UA Alternative & Fenestration Tradeoff Caps (R402.1.5, R402.5)

Quick Answer: Total UA passes when proposed Σ(U × A) ≤ Table R402.1.2 U-factors × the same assembly areas. Use an ASHRAE Handbook of Fundamentals method and include framing thermal bridging. UA does not waive Table R402.1.2 SHGC. R402.5 still caps area-weighted fenestration U-factor at 0.48 (vertical, Climate Zones 4–5), 0.40 (vertical, Climate Zones 6–8), and 0.75 (skylights, Climate Zones 4–8). Storm shelters complying with ICC 500 are excepted from those U-factor and SHGC maxima.

Total UA is the prescriptive envelope's tradeoff language. Independent OpenExamPrep material for Exam 79 treats R402.1.5 as a documentation task with the same weight as reading Table R402.1.3. The house can have a slightly worse wall U-factor if another assembly more than pays for it. The house cannot invent area, ignore studs, or use UA as a blank check on solar heat gain.

What "UA" means

U is the assembly U-factor (Btu/h·ft²·°F). A is the area of that assembly (ft²). UA for one assembly is U times A. Total UA is the sum over the building thermal envelope assemblies in the calculation. R402.1.5 says: where the total building thermal envelope UA is less than or equal to the total UA resulting from multiplying the U-factors in Table R402.1.2 by the same assembly area as in the proposed building, the building shall be considered in compliance with Table R402.1.2.

Same assembly areas is the first fail point. Both columns use the proposed house's actual ceiling area, opaque wall area, floor area, basement wall area, and fenestration area. You do not shrink the proposed wall area, grow the reference ceiling area, or omit a band of rim joist because it is inconvenient. If the architectural plans show 1,800 ft² of wood-frame wall, both the proposed UA and the table UA for walls use 1,800 ft².

Method. The UA calculation shall be performed using a method consistent with the ASHRAE Handbook of Fundamentals and shall include the thermal bridging effects of framing materials. Cavity R-20 in a 2x6 wall at 16 inches on center is not an assembly U-factor of 1/20. Studs, plates, headers, and other framing paths must be in the U-factor. Fenestration U-factors come from NFRC labels or the code's unlabeled defaults, not from a sales brochure's center-of-glass number.

SHGC is not traded away by UA

The last sentence of R402.1.5 is the second fail point. 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.

UA is a U-factor times area sum. It has no SHGC term. A designer who specifies SHGC 0.50 glass in Climate Zone 4 except Marine because "the UA worksheet already passed" is applying the wrong rule. Table R402.1.2 lists SHGC 0.40 in Climate Zone 4 except Marine. That maximum still applies on the Total UA path.

Table R402.1.2 lists SHGC as NR for Climate Zone 5 and Marine 4, while Table R402.1.3 lists 0.40 for the R-value alternative in that same grouping. R402.1.5 points to Table R402.1.2, not Table R402.1.3. Do not import the R-value-table SHGC into a UA review, and do not import UA into an R-value-table review. Where Table R402.1.2 lists a numeric SHGC, UA does not waive it.

R402.5 fenestration tradeoff caps

R402.5 limits how far fenestration U-factor (and, on the performance path, SHGC) may be traded even when the rest of the building "pays" for it:

Vertical 0.48 in Climate Zones 4 and 5 is worse than the Table R402.1.2 maximum of 0.30, so some window tradeoff is allowed. It is not unlimited: an area-weighted vertical U-factor of 0.50 in Climate Zone 5 fails R402.5 even if extra ceiling insulation makes Total UA pass. Climate Zones 6 through 8 tighten that cap to 0.40. Skylights in Climate Zones 4 through 8 cannot be traded beyond 0.75 on an area-weighted basis.

The SHGC cap of 0.40 in Climate Zones 0 through 3 applies to tradeoffs from R405, not from R402.1.5. On Total UA, SHGC still comes from Table R402.1.2 (0.25 in Climate Zones 0 through 3).

Exception: The maximum U-factor and SHGC for fenestration shall not be required in storm shelters complying with ICC 500. That is a storm-shelter exception, not a sunroom exception and not a "rooms that feel like shelters" exception.

Worked Climate Zone 5 UA example

Use Table R402.1.2 Climate Zone 5 values: ceiling U-0.024, wood-frame wall U-0.045, floor U-0.033, fenestration U-0.30. A simplified house has 1,200 ft² of ceiling, 1,800 ft² of opaque wood-frame wall, 400 ft² of floor over a garage, and 240 ft² of vertical fenestration. (Basement and slab are omitted so the arithmetic stays on one page; a real worksheet includes every envelope assembly.)

Table UA (reference):

• Ceiling: 0.024 × 1,200 = 28.80
• Walls: 0.045 × 1,800 = 81.00
• Floor: 0.033 × 400 = 13.20
• Fenestration: 0.30 × 240 = 72.00
• Total table UA = 195.00

Proposed house A — extra ceiling insulation, slightly worse walls:

• Ceiling U-0.018: 0.018 × 1,200 = 21.60 (credit 7.20 versus table)
• Walls U-0.048: 0.048 × 1,800 = 86.40 (penalty 5.40)
• Floor U-0.033: 13.20 (even)
• Fenestration U-0.30: 72.00 (even)
• Total proposed UA = 193.20

193.20 ≤ 195.00, so UA passes. The worse wall U-factor is legal because the ceiling more than paid for it, and because the areas did not change. Fenestration U-0.30 is at the table value and well under the R402.5 Climate Zone 5 vertical cap of 0.48.

Now move the same areas to Climate Zone 4 except Marine, where Table R402.1.2 SHGC is 0.40. If house A specified SHGC 0.50 glass because the UA total already passed, the file fails R402.1.5. UA never bought the right to ignore SHGC.

Proposed house B — UA passes, R402.5 fails:

• Ceiling U-0.013: 0.013 × 1,200 = 15.60
• Walls U-0.026: 0.026 × 1,800 = 46.80
• Floor U-0.028: 0.028 × 400 = 11.20
• Vertical fenestration U-0.50: 0.50 × 240 = 120.00
• Total proposed UA = 193.60 ≤ 195.00, UA passes

Area-weighted vertical fenestration U-factor is 0.50, which exceeds the R402.5 maximum of 0.48 in Climate Zones 4 and 5. The extra insulation made UA work and still did not authorize 0.50 glass. That is the point of R402.5: it is a backstop on fenestration tradeoffs from R402.1.5 or R405.

Credits and exemptions that do not apply to Total UA

Several popular prescriptive relief valves are written out of R402.1.5:

• R402.2.1 raised-heel trade. Installing uncompressed R-49 over 100 percent of the ceiling, including over the top plates, may satisfy an R-60 R-value requirement. That reduction shall not apply to the U-factor criteria in R402.1.2 or to the Total UA alternative in R402.1.5. On a UA worksheet, the ceiling U-factor is the real assembly U-factor, not a substituted R-49 credit.
• R402.2.2 ceilings without attics. The limited R-30 reduction (500 ft² or 20 percent of insulated ceiling area, whichever is less) shall not apply to Total UA.
• R402.3.3 glazed exemption. Not greater than 15 ft² of glazed fenestration per dwelling unit may be exempt from U-factor and SHGC in R402.1.2. That exemption shall not apply to Total UA. Every square foot of glass stays in the UA sum.
• R402.3.4 opaque door exemption. One side-hinged opaque door not greater than 24 ft² may be exempt from the R402.1.2 U-factor. That exemption shall not apply to Total UA.

If a worksheet silently dropped 15 ft² of glass or a 20 ft² entry door from the UA sum, send it back.

Plans-examiner documentation

Exam 79 separately weights Total UA documentation. Require:

1. Declared envelope method. The energy sheet should state Total UA (R402.1.5) as the method of complying with Table R402.1.2, inside a declared R401.2.1 prescriptive path (or as the envelope input to another allowed showing). A UA worksheet attached to an ERI-only set of drawings is the wrong report.
2. Takeoff that matches the plans. Assembly areas on the worksheet must match the architectural floor, elevation, and roof plans. Spot-check ceiling area against the floor plan, window area against the schedules, and wall area against elevations minus openings.
3. U-factors with framing. Opaque U-factors should cite a calculation consistent with the ASHRAE Handbook of Fundamentals or an approved source, including framing. Fenestration U-factors should cite NFRC values from the window schedule.
4. SHGC and R402.5 lines, not just a UA total. The packet should show Table R402.1.2 SHGC (where the table lists a maximum) and the area-weighted fenestration U-factor against R402.5. A software report that prints only "Total UA: PASS" without those lines is incomplete.
5. Certificate. R401.3 still needs fenestration U-factor and SHGC, including the largest-area value and the area-weighted average when more than one value exists, plus the compliance path. UA is an envelope method; it does not hide fenestration from the certificate.

Scenario. A Climate Zone 5 worksheet uses 1,200 ft² of ceiling in the proposed column and 1,400 ft² of ceiling in the table column "because attics are bigger than floors." That violates the same-assembly-area rule. Both columns use 1,200 ft².

Scenario. A Climate Zone 6 project trades to an area-weighted vertical fenestration U-factor of 0.45. Table R402.1.2 allows 0.30, and UA might still pass with a much better wall. R402.5 still caps Climate Zones 6 through 8 vertical fenestration at 0.40. 0.45 fails the cap.

Inspectors in the field are not re-running the UA math. They confirm labeled fenestration U-factor and SHGC, insulation marks, and that the assemblies on site are the assemblies on the approved worksheet. A substituted window series with a worse NFRC U-factor is a new UA run, not a field discretion.