S0380.210: cert 0390 cavity "partial insulation (assumed)" → as-built row, not filled

Golden cert 0390-2954-3640 (detached, TFA 360, age F) carried a +7 SAP /
-28 kWh/m² PE residual the audit attributed to a demand-side fabric gap.
Walking the §3 cascade localised it to the Main wall: lodged
wall_construction=4 (cavity), wall_insulation_type=4 (as-built / assumed),
description "Cavity wall, as built, partial insulation (assumed)". The
cascade mis-routed it to the Table 6 "Filled cavity" row (band F = 0.40)
because `_described_as_insulated` matches the "partial insulation"
substring.

RdSAP 10 Specification (10-06-2025) Table 6 — Wall U-values, England
distinguishes two cavity rows:
  "Cavity as built"  A-E 1.5, F 1.0, G 0.60, H 0.60, I 0.45, J 0.35, ...
  "Filled cavity"    A-E 0.7, F 0.40, G 0.35, H 0.35, I 0.45†, J 0.35†, ...
An "as built ... partial insulation (assumed)" cavity is the as-built
partial fill of the age band, NOT a retrofit cavity fill (a genuine fill
lodges the distinct "Cavity wall, filled cavity", wall_insulation_type=2).
It therefore routes to "Cavity as built" (band F = 1.0), mirroring the
worksheet-validated solid-brick rule in S0380.209 (cases 9/10: "as built,
insulated (assumed)" → as-built age-band row, not retrofit).

New `_cavity_described_as_filled` predicate is used only in u_wall's
cavity filled-row branch; it excludes the "partial insulation" substring
while keeping "insulated (assumed)" → filled (the unrelated, separately
asserted test_cavity_as_built_insulated_assumed_uses_filled_cavity_row is
unchanged). The shared `_described_as_insulated` (also consumed by the
roof/floor paths) is left untouched.

Wall HLC +53.6 W/K (U 0.40 → 1.0 over ~268 m²) lifts all four metrics
together — the signature of a real fabric bug, not a tuned offset:
  SAP  +7      → +0
  PE   -27.9745 → +0.5281 kWh/m²
  CO2  -2.7134  → -0.1189 t/yr
Bands I-M are unaffected (the two rows coincide per the † footnote), so
golden certs 0535 (band M) / 7536 (band L) with "insulated (assumed)"
cavities continue to pin at 0. Full suite 2384 passed, 1 skipped.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

domain/sap10_ml/rdsap_uvalues.py

@@ -66,6 +66,41 @@ def _described_as_insulated(description: Optional[str]) -> bool:
     return "insulated" in desc or "partial insulation" in desc
 
 
+def _cavity_described_as_filled(description: Optional[str]) -> bool:
+    """True when an as-built cavity wall's description asserts the cavity is
+    insulated/filled, routing it to the Table 6 "Filled cavity" row.
+
+    Distinguishes the three as-built cavity states the EPC renders by age
+    band when wall_insulation_type=4 ("as-built / assumed"):
+
+      - "...insulated (assumed)"          → Filled cavity (assessor judges
+                                            the cavity filled but lodges no
+                                            thickness)
+      - "...partial insulation (assumed)" → "Cavity as built" row (the
+                                            as-built partial fill of the age
+                                            band, NOT a retrofit cavity fill)
+      - "...no insulation (assumed)"      → "Cavity as built" row
+
+    Narrower than `_described_as_insulated`: it excludes the "partial
+    insulation" substring so a "partial insulation (assumed)" cavity stays on
+    the as-built row. RdSAP 10 Table 6 (England) "Cavity as built" band F =
+    1.0 vs "Filled cavity" band F = 0.40 — for an as-built band-F cavity the
+    filled row understates heat loss by 2.5x. A genuine retrofit fill is
+    lodged distinctly as "Cavity wall, filled cavity"
+    (wall_insulation_type=2), handled by the explicit-code branch.
+
+    Real-cert evidence: golden cert 0390-2954-3640 (detached, band F, cavity
+    type 4, "partial insulation (assumed)") closes all four SAP metrics on
+    the as-built 1.0 row; the filled 0.40 row under-counts PE by ~28 kWh/m².
+    """
+    if description is None:
+        return False
+    desc = description.lower()
+    if "no insulation" in desc:
+        return False
+    return "insulated" in desc
+
+
 # ---------------------------------------------------------------------------
 # Country
 # ---------------------------------------------------------------------------
@@ -597,7 +632,7 @@ def u_wall(
         )
     if wall_type == WALL_CAVITY and (
         wall_insulation_type == WALL_INSULATION_FILLED_CAVITY
-        or _described_as_insulated(description)
+        or _cavity_described_as_filled(description)
     ):
         return _CAVITY_FILLED_ENG[age_idx]
     bucket = _insulation_bucket(insulation_thickness_mm, insulation_present)

tests/domain/sap10_calculator/rdsap/test_golden_fixtures.py

@@ -236,9 +236,9 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
     _GoldenExpectation(
         cert_number="0390-2954-3640-2196-4175",
         actual_sap=60,
-        expected_sap_resid=+7,
-        expected_pe_resid_kwh_per_m2=-27.9745,
-        expected_co2_resid_tonnes_per_yr=-2.7134,
+        expected_sap_resid=+0,
+        expected_pe_resid_kwh_per_m2=+0.5281,
+        expected_co2_resid_tonnes_per_yr=-0.1189,
         notes=(
             "Detached, TFA 360, age F, Firebird oil combi PCDF 9005 "
             "(winter eff 86.4%). PCDB record lodges separate_dhw_tests=0 + "
@@ -269,7 +269,18 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
             "Slice S0380.151 wired RdSAP 10 §4.1 Table 5 (PDF p.28) "
             "extract-fans default (age F → 1 fan). Cascade ventilation "
             "HLC rises ~0.03 ACH × volume; PE -28.0830 → -27.9745 "
-            "(closer to zero), CO2 -2.7342 → -2.7134."
+            "(closer to zero), CO2 -2.7342 → -2.7134. "
+            "Slice S0380.210 CLOSED the residual: the Main cavity wall lodges "
+            "wall_insulation_type=4 (as-built/assumed) + description "
+            "'Cavity wall, as built, partial insulation (assumed)'. The "
+            "cascade mis-routed it to the Table 6 'Filled cavity' row "
+            "(band F = 0.40) via the 'partial insulation' substring; "
+            "RdSAP 10 Table 6 (England) routes an as-built partial-fill "
+            "cavity to the 'Cavity as built' row (band F = 1.0). New "
+            "`_cavity_described_as_filled` excludes 'partial insulation' "
+            "(keeping 'insulated (assumed)' → filled). Wall HLC +53.6 W/K "
+            "(0.40 → 1.0 over 268 m²) lifted all four metrics together: "
+            "SAP +7 → +0, PE -27.9745 → +0.5281, CO2 -2.7134 → -0.1189."
         ),
     ),
     _GoldenExpectation(

tests/domain/sap10_calculator/worksheet/test_heat_transmission.py

@@ -307,6 +307,58 @@ def test_cavity_as_built_insulated_assumed_uses_filled_cavity_row() -> None:
     assert result.walls_w_per_k == pytest.approx(70.0, abs=1.0)
 
 
+def test_cavity_as_built_partial_insulation_assumed_uses_as_built_row() -> None:
+    # Arrange — the EPC renders a cavity wall lodged wall_insulation_type=4
+    # (as-built / assumed) with description "Cavity wall, as built, partial
+    # insulation (assumed)" for age bands where the as-built construction
+    # carries only partial cavity fill. "Partial insulation" is the as-built
+    # thermal state of the age band, NOT a retrofit cavity fill — the spec
+    # routes it to the "Cavity as built" row, not "Filled cavity":
+    # RdSAP 10 Table 6 (England) "Cavity as built" band F = 1.0 vs
+    # "Filled cavity" band F = 0.40. A genuine fill renders the distinct
+    # "Cavity wall, filled cavity" description (wall_insulation_type=2),
+    # caught separately. Contrast the "insulated (assumed)" variant above,
+    # which the assessor judges as filled.
+    #
+    # Real-cert evidence: golden cert 0390-2954-3640 (detached, band F,
+    # cavity type 4, "partial insulation (assumed)") closes all four SAP
+    # metrics (PE/SAP/CO2/cost) on the as-built 1.0 row — at the filled
+    # 0.40 row its PE under-counts by ~28 kWh/m².
+    # Geometry: 100 m² floor, 40 m perimeter, 2.5 m height, single storey
+    # → gross_wall = 100 m². walls_w_per_k expected = 1.0 × 100 = 100 W/K.
+    main = make_building_part(
+        construction_age_band="F",
+        wall_construction=4,
+        wall_insulation_type=4,
+        party_wall_construction=1,
+        roof_construction=4,
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=100.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=40.0, floor=0,
+            ),
+        ],
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=100.0,
+        country_code="ENG",
+        sap_building_parts=[main],
+    )
+    epc.walls = [
+        EnergyElement(
+            description="Cavity wall, as built, partial insulation (assumed)",
+            energy_efficiency_rating=3,
+            environmental_efficiency_rating=3,
+        ),
+    ]
+
+    # Act
+    result = heat_transmission_from_cert(epc)
+
+    # Assert — U=1.0 × 100 m² gross wall = 100 W/K (as-built, not filled 70).
+    assert abs(result.walls_w_per_k - 100.0) <= 1.0
+
+
 def test_walls_description_measured_transmittance_overrides_construction_cascade() -> None:
     # Arrange — a full-SAP (not RdSAP) cert lodges the wall U-value
     # directly in walls[i].description ("Average thermal transmittance