slice S-B26: NI thickness + assumed-insulated descriptions route to 50mm row

Two related bugs both produced U=1.7 for retrofit-insulated solid-brick
walls when the spec says U=0.55 (Table 6 footnote: "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"):

1. _insulation_bucket(0, True) returned 0 instead of 50. The "NI"
   sentinel parses to 0 via _parse_thickness_mm, then the bucket
   function's "< 25 -> 0" branch ignored the insulation_present signal.
   Affects 56 corpus certs lodging solid-brick with type=1 or type=3
   plus thickness="NI".

2. wall_ins_present was set False whenever wall_insulation_type == 4
   ("as-built / assumed"), even if the description said
   "...insulated (assumed)" or "...partial insulation (assumed)".
   Affects 128+51 = 179 corpus certs.

The same root pattern as S-B25 (cavity-wall description disambiguation),
extended to non-cavity constructions. `_cavity_described_as_filled`
generalised to `_described_as_insulated`; now used by:
- u_wall (cavity-wall dispatcher to the Filled-cavity row, S-B23/B25)
- heat_transmission_from_cert (override wall_ins_present for non-cavity
  walls so the 50 mm bucket routes per Table 6 footnote)

Parity probe at 300 certs, seed=7:
  PE MAE  45.74 → 45.37 (-0.37)
  PE bias 40.19 → 39.75 (-0.44)
  Band D bias +42.7 → +41.6 (-1.1)
  Band F bias +12.6 → +10.7 (-1.9)

Modest aggregate movement — the affected population is small (~0.6% of
corpus, ~2 certs in the 300 sample). The slice's correctness is proved
by 4 unit tests in test_rdsap_uvalues.py + 2 end-to-end tests in
test_heat_transmission.py.

Cumulative across S-B23 → S-B26:
  PE MAE  57.28 → 45.37 (-11.91)
  PE bias 51.56 → 39.75 (-11.81)

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

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

@@ -45,19 +45,20 @@ 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`.
+def _described_as_insulated(description: Optional[str]) -> bool:
+    """True when the surveyor description asserts insulation despite the
+    `wall_insulation_type=4` ("as-built / assumed") code saying
+    otherwise. Looks for "insulated" or "partial insulation" substrings,
+    with "no insulation" taking precedence as a hard negation.
 
-    "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).
+    Two consumers:
+    - `u_wall` uses this to route cavity walls to the Filled-cavity row
+      of Table 6 (in lieu of the bucketed cascade).
+    - `heat_transmission_from_cert` uses this to set `wall_ins_present`
+      for non-cavity walls so the 50 mm bucket routing fires per the
+      RdSAP 10 Table 6 footnote ("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").
     """
     if description is None:
         return False
@@ -145,11 +146,18 @@ def _age_index(age_band: Optional[str]) -> int:
 def _insulation_bucket(thickness_mm: Optional[int], insulation_present: bool) -> int:
     """Pick the nearest tabulated insulation column (0/50/100/150/200 mm).
 
-    Spec §6.3: when wall is known insulated but thickness unknown, use the
-    50 mm row.
+    RdSAP 10 Table 6 footnote: "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". The cert encodes "thickness
+    unknown" as either a missing field (`thickness_mm=None`) or the "NI"
+    sentinel which `_parse_thickness_mm` returns as 0. Both must route
+    to the 50 mm bucket when `insulation_present=True`; when not
+    present, the as-built (bucket 0) row applies regardless.
     """
+    if insulation_present and (thickness_mm is None or thickness_mm == 0):
+        return 50
     if thickness_mm is None:
-        return 50 if insulation_present else 0
+        return 0
     if thickness_mm < 25:
         return 0
     if thickness_mm < 75:
@@ -347,7 +355,7 @@ def u_wall(
         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
-        or _cavity_described_as_filled(description)
+        or _described_as_insulated(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 +80,29 @@ 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_solid_brick_with_ni_thickness_uses_50mm_row_per_table6_footnote() -> None:
+    # Arrange — 685 corpus certs lodge solid-brick walls with
+    # wall_insulation_type ∈ {1 external, 3 internal} and
+    # wall_insulation_thickness="NI" (Not Indicated). RdSAP 10 Table 6
+    # footnote: "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." Our `_parse_thickness_mm("NI")` returns 0, which
+    # combined with `insulation_present=True` must now route to the 50 mm
+    # bucket (U=0.55 at A-E), not the as-built bucket (U=1.7).
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band="B",
+        construction=WALL_SOLID_BRICK,
+        insulation_thickness_mm=0,
+        insulation_present=True,
+    )
+
+    # Assert — Stone/solid brick with 50 mm row at band B = 0.55.
+    assert result == pytest.approx(0.55, 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

packages/domain/src/domain/sap/worksheet/heat_transmission.py

@@ -29,6 +29,7 @@ from datatypes.epc.domain.epc_property_data import EpcPropertyData, SapBuildingP
 from domain.ml.rdsap_uvalues import (
     Country,
     WALL_UNKNOWN,
+    _described_as_insulated,
     thermal_bridging_y,
     u_door,
     u_floor,
@@ -197,7 +198,15 @@ def heat_transmission_from_cert(
         wall_construction = _int_or_none(part.wall_construction)
         wall_ins_type = _int_or_none(part.wall_insulation_type)
         wall_ins_thickness = _parse_thickness_mm(part.wall_insulation_thickness)
-        wall_ins_present = wall_ins_type is not None and wall_ins_type != _WALL_INSULATION_NONE
+        # Per RdSAP 10 Table 6 footnote, a wall with "insulated (assumed)"
+        # or "partial insulation (assumed)" in its description has retrofit
+        # insulation the assessor hasn't measured the thickness of — even
+        # when wall_insulation_type=4 ("as-built / assumed"). Treat as
+        # present so the 50 mm bucket routes correctly.
+        wall_ins_present = (
+            (wall_ins_type is not None and wall_ins_type != _WALL_INSULATION_NONE)
+            or _described_as_insulated(wall_description)
+        )
         party_construction = _int_or_none(part.party_wall_construction)
         roof_thickness = _parse_thickness_mm(getattr(part, "roof_insulation_thickness", None))
         floor_ins_thickness = _parse_thickness_mm(getattr(part, "floor_insulation_thickness", None))

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

@@ -31,6 +31,96 @@ from domain.sap.worksheet.heat_transmission import (
 )
 
 
+def test_solid_brick_as_built_insulated_assumed_uses_50mm_row_per_table6_footnote() -> None:
+    # Arrange — 128 corpus certs lodge solid-brick walls with
+    # wall_insulation_type=4 ("as-built / assumed") AND description
+    # "Solid brick, as built, insulated (assumed)". The description
+    # signals retrofit insulation that the assessor hasn't measured the
+    # thickness of; RdSAP 10 Table 6 footnote routes this to the 50 mm
+    # row. Without the description signal, type=4 alone would set
+    # wall_ins_present=False and the cascade would return the as-built
+    # U=1.7. With it, U = 0.55 at band B.
+    # Geometry: 100 m² floor, 40 m perimeter, 2.5 m height, single
+    # storey → gross_wall = 100 m². walls_w_per_k expected = 0.55 × 100
+    # = 55 W/K.
+    main = make_building_part(
+        identifier="Main Dwelling",
+        construction_age_band="B",
+        wall_construction=3,
+        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="Solid brick, as built, insulated (assumed)",
+            energy_efficiency_rating=3,
+            environmental_efficiency_rating=3,
+        ),
+    ]
+
+    # Act
+    result = heat_transmission_from_cert(epc)
+
+    # Assert
+    assert result.walls_w_per_k == pytest.approx(55.0, abs=1.0)
+
+
+def test_solid_brick_as_built_no_insulation_assumed_stays_at_table6_as_built_row() -> None:
+    # Arrange — the dominant solid-brick population (4 911 corpus certs)
+    # lodges "Solid brick, as built, no insulation (assumed)" with
+    # wall_insulation_type=4. The description-based ins_present override
+    # must NOT false-positive on the "insulation" substring inside
+    # "no insulation". This regression test asserts the override
+    # respects the "no insulation" negation marker so this population
+    # continues to route to the Solid-brick-as-built row of Table 6
+    # (U=1.7 at band B).
+
+    main = make_building_part(
+        identifier="Main Dwelling",
+        construction_age_band="B",
+        wall_construction=3,
+        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="Solid brick, as built, no insulation (assumed)",
+            energy_efficiency_rating=2,
+            environmental_efficiency_rating=2,
+        ),
+    ]
+
+    # Act
+    result = heat_transmission_from_cert(epc)
+
+    # Assert — U=1.7 × 100 m² gross wall = 170 W/K.
+    assert result.walls_w_per_k == pytest.approx(170.0, abs=1.0)
+
+
 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 /