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docs: roof-8 lead closed as data-fidelity (not a bug) + description-vs-code audit

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Records the session-4 audit: walls/heating/controls clean; roof_construction=8
("Pitched, insulated", no measured thickness) computing U=2.30 is CORRECT, not a
bug — confirmed by user worksheet sim-case-29 (band C → Elmhurst SAP 55 ≡ our
56.75; lodged 80 is data-fidelity artifact). Lesson: "insulated (assumed)" = the
age-band default insulation level, not "well insulated". DO NOT re-chase roof-8.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Khalim Conn-Kowlessar 2026-06-08 12:43:02 +00:00
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docs/HANDOVER_API_PROFILING.md
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@ -65,6 +65,31 @@ profile-surfaced buckets below.
`roof_codes=1` broad bucket is mean −0.15 (the −1.78 was top-floor-electric-flat outliers
−29/−25). Remaining gains need per-cert worksheets (start code-3) or the unsupported-schema ticket.
## SESSION-4 AUDIT — description-vs-code cross-check (other element types)
After floor-3, audited every API field that carries BOTH an integer code AND an
independent `…[].description` (roofs, walls, floors, main_heating, controls), by joining
code↔description on **single-element certs** (the multi-element `[]` summary is lossy).
- **Walls / heating / controls: CLEAN** — codes map 1:1 to their description families;
residual error is per-cert scatter, not mis-mapping.
- **Roof `roof_construction=8`: investigated hard, NOT a bug — DO NOT re-chase.** It looked
like a huge lead (n=57, ~189 |err|, mean −2.43) because code-8 sloping ceilings described
"Pitched, insulated" with no numeric thickness compute U=2.30 (e.g. 7921-0052 roof_w 241
W/K, SAP 56.75 vs lodged 80). **User worksheet (simulated case 29) settled it: at band C,
RdSAP/Elmhurst assumes UNINSULATED for an "insulated (assumed)" roof+wall with no measured
thickness → Elmhurst SAP 55, which MATCHES our 56.75.** The lodged 80 needed real insulation
the API record doesn't carry = data-fidelity artifact (meter-3 class). KEY LESSON (user):
**"insulated (assumed)" = the age-band DEFAULT insulation level, NOT "well insulated"** — at
old bands that default is ~uninsulated. Re-audit by band confirmed: old-band no-numeric =
artifacts (we ≡ Elmhurst); newer-band code-8 already gets correct insulated U (2031 band I
U≈0.18, 1436 band E 0.17, 0536 band B 0.09) — their under-rates are elsewhere; numeric-
thickness = accurate (9884 +0.06). So the force-uninsulated-at-pre-1950 rule is CORRECT.
A roof-8 "fix" would push us AWAY from Elmhurst's faithful 55 toward the unreproducible 80.
- **Roof `roof_construction=3`: latent mis-map (inert).** gov desc "(another dwelling above)"
(party roof) but we map to "Pitched, no access to loft"; masked by dwelling exposure
(mean −0.06). Correct for robustness only if touching the roof mapper; not worth chasing.
- **Worksheet-gen constraints (user, for future repros):** Elmhurst no longer lets you pick
build form for a flat; and a band-C repro defaults to uninsulated walls+roof.
## KEY INSIGHT (load-bearing, from the user)
**The gov EPC API JSON is the published OUTPUT of RdSAP software (Elmhurst), not its input.**
So any API field Elmhurst doesn't expose as an *input* is register metadata the RdSAP10 method