slice S-B25: description-based dispatch for as-built / assumed cavity

The RdSAP schema's `wall_insulation_type = 4` ("as-built / assumed")
covers two distinct cert populations that previously both routed to
the Cavity-as-built row (U=1.5 at band E):

  686 certs: "Cavity wall, as built, no insulation (assumed)" — U=1.5 ✓
 1171 certs: "Cavity wall, as built, insulated (assumed)" — should be 0.7
  147 certs: "Cavity wall, as built, partial insulation (assumed)" — 0.7

The description string disambiguates. The legacy production map at
recommendations/rdsap_tables.py:753 routes the latter two to "Filled
cavity" — we match that interpretation here for parity with the cert
assessor and the production recommendation engine.

`_cavity_described_as_filled` adds the description check; the existing
filled-cavity dispatcher in u_wall now fires on either signal:
- wall_insulation_type == 2 (S-B23 — explicit filled-cavity code)
- description contains "insulated" or "partial insulation" without
  the "no insulation" negation marker (S-B25 — assumed cavity-fill)

Parity probe at 300 certs, seed=7:
  PE MAE  46.78 → 45.74 (-1.04)
  PE bias 41.78 → 40.19 (-1.59)
  Band F bias +23.2 → +12.6 (-10.6)
  Band G bias +31.8 → +25.1 (-6.7)
  Band H bias +30.7 → +15.5 (-15.2)

Improvements localise to bands F-H (1976-1995), the era when Building
Regs mandated cavity insulation for new-builds — making "as built,
insulated (assumed)" the modal description. SAP MAE drifted up
+0.12 (cost-side residuals surfacing now that envelope is closer to
spec; tracked for follow-up).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/domain/src/domain/ml/rdsap_uvalues.py

@@ -45,6 +45,28 @@ def _measured_u_from_description(description: Optional[str]) -> Optional[float]:
         return None
 
 
+def _cavity_described_as_filled(description: Optional[str]) -> bool:
+    """RdSAP encodes "as-built / assumed" insulation as
+    `wall_insulation_type = 4`, which our cascade would otherwise treat
+    as uninsulated. The description disambiguates: "Cavity wall, as
+    built, insulated (assumed)" or "...partial insulation (assumed)"
+    means the assessor has determined the cavity is filled (or partly
+    filled), without lodging the thickness. Both route to the Filled-
+    cavity row of Table 6, matching the legacy production recommendation
+    engine's interpretation in `recommendations/rdsap_tables.py`.
+
+    "no insulation" markers take precedence to avoid the substring
+    "insulated" matching "no insulation" (it doesn't here, but the
+    explicit negative check makes the contract clear).
+    """
+    if description is None:
+        return False
+    desc = description.lower()
+    if "no insulation" in desc:
+        return False
+    return "insulated" in desc or "partial insulation" in desc
+
+
 # ---------------------------------------------------------------------------
 # Country
 # ---------------------------------------------------------------------------
@@ -323,9 +345,9 @@ def u_wall(
         wall_type = construction
     else:
         wall_type = _wall_type_from_description(description) or _DEFAULT_WALL_BY_AGE.get(band, WALL_CAVITY)
-    if (
-        wall_type == WALL_CAVITY
-        and wall_insulation_type == WALL_INSULATION_FILLED_CAVITY
+    if wall_type == WALL_CAVITY and (
+        wall_insulation_type == WALL_INSULATION_FILLED_CAVITY
+        or _cavity_described_as_filled(description)
     ):
         return _CAVITY_FILLED_ENG[age_idx]
     bucket = _insulation_bucket(insulation_thickness_mm, insulation_present)

packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py

@@ -80,6 +80,80 @@ def test_u_wall_description_with_malformed_transmittance_falls_through_to_cascad
     assert result == pytest.approx(0.60, abs=0.001)
 
 
+def test_u_wall_cavity_as_built_insulated_assumed_routes_to_filled_cavity_row() -> None:
+    # Arrange — 1 171 corpus certs (~4% of scanned bulk) lodge
+    # wall_insulation_type=4 ("as-built / assumed") together with the
+    # description "Cavity wall, as built, insulated (assumed)". The
+    # assessor is saying: this cavity is filled, but I haven't measured
+    # the thickness. Spec footnote on Table 6 covers this: "If a wall
+    # is known to have additional insulation but the insulation thickness
+    # is unknown, use the row in the table for 50 mm insulation" — but
+    # legacy convention (used by the production recommendation engine)
+    # is to route this to the Filled-cavity row, U = 0.7 at A-E. We
+    # follow the legacy convention here for parity with the cert assessor.
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band="E",
+        construction=WALL_CAVITY,
+        insulation_thickness_mm=None,
+        insulation_present=False,  # type=4 maps to wall_ins_present=False
+        wall_insulation_type=4,
+        description="Cavity wall, as built, insulated (assumed)",
+    )
+
+    # Assert
+    assert result == pytest.approx(0.7, abs=0.001)
+
+
+def test_u_wall_cavity_as_built_no_insulation_stays_at_table6_cavity_as_built_row() -> None:
+    # Arrange — the same wall_insulation_type=4 ("as-built / assumed")
+    # cert population also contains 686 "Cavity wall, as built, no
+    # insulation (assumed)" entries which must continue to route to the
+    # Cavity-as-built row of Table 6 (U=1.5 at band E). The "no
+    # insulation" substring marker takes precedence over the
+    # "insulated"-substring filled-cavity rule, so this case is
+    # disambiguated from "Cavity wall, as built, insulated (assumed)".
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band="E",
+        construction=WALL_CAVITY,
+        insulation_thickness_mm=None,
+        insulation_present=False,
+        wall_insulation_type=4,
+        description="Cavity wall, as built, no insulation (assumed)",
+    )
+
+    # Assert
+    assert result == pytest.approx(1.5, abs=0.001)
+
+
+def test_u_wall_cavity_as_built_partial_insulation_routes_to_filled_cavity_row() -> None:
+    # Arrange — 147 corpus certs lodge "Cavity wall, as built, partial
+    # insulation (assumed)" with wall_insulation_type=4. The legacy
+    # production map (recommendations/rdsap_tables.py:753) routes these
+    # to "Filled cavity" — same destination as the "insulated (assumed)"
+    # case. We match that interpretation for parity with the cert
+    # assessor and the production recommendation engine.
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band="D",  # 1950-1966 — typical partial-fill retrofit cohort
+        construction=WALL_CAVITY,
+        insulation_thickness_mm=None,
+        insulation_present=False,
+        wall_insulation_type=4,
+        description="Cavity wall, as built, partial insulation (assumed)",
+    )
+
+    # Assert — Filled-cavity row at band D = 0.7 W/m²K.
+    assert result == pytest.approx(0.7, abs=0.001)
+
+
 def test_u_wall_description_without_transmittance_phrase_routes_through_cascade() -> None:
     # Arrange — the measured-U dispatcher must only fire when the
     # description contains the "thermal transmittance" phrase. The

packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py

@@ -31,6 +31,50 @@ from domain.sap.worksheet.heat_transmission import (
 )
 
 
+def test_cavity_as_built_insulated_assumed_uses_filled_cavity_row() -> None:
+    # Arrange — the modal RdSAP encoding for a retrofitted-cavity dwelling:
+    # wall_construction=4 (cavity), wall_insulation_type=4 (as-built /
+    # assumed), and walls[0].description = "Cavity wall, as built,
+    # insulated (assumed)". The assessor has determined the cavity is
+    # filled but hasn't lodged a thickness. Without the description-based
+    # dispatcher, the cascade would return U=1.5; with it, the Filled-
+    # cavity row of Table 6 applies: U=0.7 at band E.
+    # Geometry: 100 m² floor, 40 m perimeter, 2.5 m height, single storey
+    # → gross_wall = 100 m². walls_w_per_k expected = 0.7 × 100 = 70 W/K.
+    main = make_building_part(
+        identifier="Main Dwelling",
+        construction_age_band="E",
+        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, insulated (assumed)",
+            energy_efficiency_rating=4,
+            environmental_efficiency_rating=4,
+        ),
+    ]
+
+    # Act
+    result = heat_transmission_from_cert(epc)
+
+    # Assert
+    assert result.walls_w_per_k == pytest.approx(70.0, abs=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