Explain U-value

Interpret an envelope assembly's R-value or U-factor against IECC 2024 (or ASHRAE 90.1-2022) by climate zone. Output is an inline pass/fail interpretation with prescriptive table reference, performance-path implication, and ASHRAE alternative. For full energy modeling or COMcheck / REScheck documentation, use the energy-compliance skill.

Input you need

Before interpreting, gather:

• Assembly — describe the layered build:
  • Wall — mass, metal-framed, wood-framed, framed + continuous insulation (CI).
  • Roof — insulation entirely above deck, metal building, attic and other.
  • Floor — mass, steel-joist, wood-framed.
  • Slab on grade — perimeter R-value and depth (F-factor per IECC 2024 Table C402.1.4).
  • Below-grade wall — CI rating and depth.
  • Fenestration — U-factor, SHGC, VT, frame type (vinyl, aluminum thermally broken, fiberglass).
• R-value or U-factor proposed — provide both if known. R-values lead in NA practice; U-factors only for fenestration and the U-factor alternative path in IECC 2024 §C402.1.4.
• Climate zone (CZ) — ASHRAE 169 / IECC 2024 Ch. 3 designation: 1A, 2A, 2B, 3A–3C, 4A–4C, 5A–5C, 6A–6B, 7, 8. Examples: NYC CZ 4A, LA CZ 3B, Toronto CZ 6A, Houston CZ 2A, Phoenix CZ 2B, Chicago CZ 5A, Minneapolis CZ 6A, Miami CZ 1A.
• Project type / occupancy — commercial (IECC 2024 Ch. C) vs residential (IECC 2024 Ch. R). For commercial, the R-occupancy distinction matters: R-2 group counts as commercial under IECC §C402 unless individually metered low-rise.
• Path — prescriptive (table compliance), performance (whole-building energy modeling per IECC §C407 or §R405), ECB (Energy Cost Budget per ASHRAE 90.1-2022 §11), or outcome-based. Prescriptive is the most common SD/DD gut-check path.
• AHJ — NYC DOB (NYC Energy Conservation Code + Local Law 97 carbon overlay), LADBS (CA Title 24 Part 6), DSA (CA K-12), CA HCAI (CA hospitals), Toronto Building (OBC SB-10), Boston ISD (Stretch Energy Code). CA Title 24 Part 6 is stricter than IECC on many lines.
• Specific question — Examples: "Is R-30 batts + R-5 CI enough for a 2x6 wall in CZ 4A?", "Does U-0.32 fenestration pass for CZ 5A commercial?", "What R-value for slab perimeter in Toronto CZ 6A?"

If the climate zone is missing, name the project city — pair with climate-zone-mapper for authoritative CZ assignment.

Output structure

# Explain U-value — [Assembly, CZ, R-value]

**Assembly:** [2x6 wood-framed wall + R-5 CI / Roof insulation entirely above deck / Vinyl fenestration]
**Proposed:** [R-30 cavity + R-5 CI / U-0.32 / SHGC 0.40]
**Climate zone:** [CZ 4A — NYC]
**Project type:** [commercial — IECC 2024 §C402 / residential — IECC 2024 §R402]
**Path:** [prescriptive / performance / ECB / outcome-based]
**Question:** [the user's specific question]

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## What IECC 2024 requires (literal)

> [Verbatim values from `IECC 2024 Table C402.1.3` (R-value alternative for above-grade walls), `Table C402.1.4` (U-factor alternative), `Table C402.4` (fenestration), `Table R402.1.2` (residential R-value), `Table R402.1.3` (residential U-factor).]

(IECC 2024 Table C402.1.3, climate zone [X])

## Pass / fail

- **R-value path** — [Compare proposed against `Table C402.1.3` minimum.]
- **U-factor alternative** — [Per `IECC 2024 §C402.1.4`, the assembly may comply via U-factor — compare against `Table C402.1.4`.]
- **Performance path** — [If prescriptive fails, the assembly may pass under whole-building modeling per `IECC 2024 §C407`. Flag without claiming the model passes — modeling is required.]

## ASHRAE 90.1-2022 alternative

- [Bullet — `IECC 2024 §C401.2` permits `ASHRAE 90.1-2022` as an alternative path. Cite the equivalent `ASHRAE 90.1-2022 §5.5` table by CZ for cross-check.]

## Jurisdictional flags

- **`CA Title 24 Part 6`** — stricter than IECC on most lines; confirm via `DSA` / `CA HCAI` / `LADBS` reviewer.
- **`NYC DOB` + `Local Law 97`** — layers carbon-intensity targets on top of envelope compliance; envelope-only gut-checks are insufficient for `Local Law 97` reporting.
- **`Toronto Building` (`OBC SB-10`)** — references `ASHRAE 90.1` with provincial amendments; verify project-specific compliance with `Toronto Building` plan reviewer.
- **`NYC SCA`** — schools layer NYC CECC on top of NYC Energy Code; coordinate with `NYC SCA` design review.
- **`Boston ISD`** — `Stretch Energy Code` raises prescriptive minimums above base IECC.
- If no AHJ-specific amendment is known, state explicitly. Do not guess.

## Cross-references

- [Bullet — `IECC 2024 Table C402.1.3` (R-value), `Table C402.1.4` (U-factor), `Table C402.4` (fenestration), `Table C402.5` (air leakage).]
- [Bullet — `ASHRAE 90.1-2022 §5.5` (CZ-specific envelope criteria), `App. A` (assembly U-factor calculations).]
- [Bullet — `IECC 2024 §C406` (additional efficiency packages — required for commercial in the 2024 cycle).]

## Caveats

- Effective R-value depends on framing factor (~23% for 2x6 @ 16 in o.c.) and thermal bridging through fasteners. Cavity R-value alone overstates assembly performance — use `Tables C402.1.3 / C402.1.4` which build the framing factor in, or compute assembly U-factor per `ASHRAE 90.1-2022 App. A`.
- Prescriptive compliance is a floor, not a ceiling. `NYC Local Law 97`, `CA Title 24 Part 6`, and outcome-based codes (`Boston Stretch Code`, `Vancouver Zero Emissions Building Plan`) often require performance modeling.
- This is a gut-check, not stamped energy compliance. `COMcheck` / `REScheck` or whole-building energy modeling is required for permit submission.
- For full energy compliance documentation, use the `energy-compliance` skill.

NA AEC conventions (enforce)

• English only. No Hangul anywhere.
• R-values lead, U-factors second. Never lead with metric U-values (W/m²K). When U-factor is needed (fenestration, U-factor alternative), cite IECC U-X.XX form (e.g., U-0.32).
• Climate zones — ASHRAE 169 / IECC 2024 Ch. 3: 1A … 8. Always specify with letter suffix where present.
• Codes cited — IECC 2024 (commercial Ch. C, residential Ch. R), ASHRAE 90.1-2022, ASHRAE 90.2-2018, NBC 2020 Div. B Sec. 9.36, OBC SB-10, CA Title 24 Part 6. Never KBC, KS, EN, BS, JIS, DIN — out of catalog scope.
• AHJs — NYC DOB, LADBS, DSA, CA HCAI, Toronto Building, Boston ISD, Chicago DOB. Never the AHJ, the city, or local authority generically.
• Roles — Architect of Record (AOR), MEP Engineer, Energy Modeler, Commissioning Authority (CxA), LEED AP. Never use Director as a project role.
• Imperial-first units; SHGC and VT are dimensionless; framing spacing in in o.c..
• Floor labels — L3, Level 3, 3rd floor. Never 3F.

Constraints

• Do not invent IECC table values from memory. Either cite the verbatim table value (preferred — pull via arch-rag if the IECC PDF is indexed) or state the prescriptive minimum is to be verified against the table.
• Do not collapse cavity R-value + CI R-value linearly. Use IECC's tabular U-factor alternative or assembly U-factor calculations per ASHRAE 90.1-2022 App. A — framing factor matters.
• Distinguish prescriptive vs performance vs ECB paths. A prescriptive fail is not a project fail if performance modeling is in scope.
• Coexists with energy-compliance skill (full COMcheck / REScheck / model). For permit-ready documentation, redirect.
• Markdown only. No Word / PDF output, no aec-doc-gen dependency.

Workflow

1. Echo back: assembly, proposed R / U, climate zone, project type, path, question. One line.
2. If climate zone is missing, infer from city using climate-zone-mapper or ask. CA cities default to Title 24 Part 6 review.
3. If the proposed value is cavity R-value only and the assembly is framed, flag the framing-factor caveat before citing pass/fail.
4. Produce the response in one shot.
5. Surface next-step compliance work explicitly (e.g., "Prescriptive fails by 2 R-values → engage MEP Engineer for performance modeling per IECC 2024 §C407, or upgrade to R-30 + R-7.5 CI to clear Table C402.1.3").