Field-level bias scan (scripts/corpus_1000/field_bias.py) flagged solid-brick +
internal-insulation certs (wall_con+ins=(3,3)) systematically under-rating
(median -0.40, n=18). Root cause: _insulation_bucket routed insulation that is
KNOWN PRESENT but with a thin lodged thickness (5-24mm, below the smallest
tabulated 50mm column) to bucket 0 (uninsulated, U 1.7) - WORSE than the
present-but-unknown case, which the RdSAP Table 6 footnote correctly routes to
the 50mm row. A known 10mm shouldn't read as higher-U than not knowing at all.
Now present insulation never falls to bucket 0; thin/unknown thickness both
round up to the 50mm row. Only genuinely absent insulation maps to 0.
Corpus cert 5117280 (10mm internal, no wall thickness -> bucket path): Δ -4.44
-> +1.08. Corpus MAE 0.641 -> 0.637, PE MAE 3.0 -> 2.9. Gauge 77.7% holds; 44
RealCert pins + 342 fabric tests green. TDD: 2 new unit tests.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
