slice S-B23: RdSAP 10 Table 6 "Filled cavity" row dispatch

The cert encodes filled-cavity walls as
(wall_construction=4 cavity, wall_insulation_type=2 filled,
wall_insulation_thickness="NI"). The previous cascade parsed "NI"→0
and ran the thickness-bucketed table, returning U=1.5 (the
"Cavity as built" row) — treating retrofit-filled cavities as if they
were uninsulated. Spec (RdSAP 10 Table 6, page 33) has a dedicated
"Filled cavity" row at U=0.7 for bands A-E, 0.40 at F, 0.35 at G-H,
and "as built" from band I onward.

Adds:
- WALL_INSULATION_FILLED_CAVITY constant (code 2 per RdSAP schema,
  confirmed empirically on 8 000 corpus certs against walls.description)
- _CAVITY_FILLED_ENG row in domain.ml.rdsap_uvalues
- dispatcher in u_wall when (construction=cavity, insulation_type=2)
- wall_insulation_type plumbing through heat_transmission_from_cert

Parity probe (300 certs, seed=7) before → after:
- PE MAE  57.28 → 48.99 (-8.3)
- PE bias 51.56 → 42.07 (-9.5)
- Band C bias +65.3 → +47.8 (-17.5)
- Band D bias +67.9 → +45.7 (-22.2)
- Band E bias +77.0 → +58.8 (-18.2)
- Band F bias +43.8 → +25.4 (-18.4)
- Band K-L bias unchanged (filled-cavity row falls back to as-built
  from band I onward per spec footnote; correct no-op)

Future slices already lit up by the same enumeration:
- type=1 external / type=3 internal insulation rows (~440 certs)
- type=6 filled + external / type=7 filled + internal (~22 certs)
- type=None "Average thermal transmittance X W/m²K" string parse
  (1 358 certs — biggest follow-up)

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

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

@@ -66,6 +66,20 @@ WALL_CURTAIN: Final[int] = 9
 WALL_UNKNOWN: Final[int] = 10
 
 
+# RdSAP schema `wall_insulation_type` codes (empirically confirmed across
+# 8 000 corpus certs against walls[0].description):
+#   1 = external wall insulation
+#   2 = filled cavity ("Cavity wall, filled cavity")
+#   3 = internal wall insulation
+#   4 = as-built / assumed (default cascade)
+#   5 = none specified (rare)
+#   6 = filled cavity + external insulation
+#   7 = filled cavity + internal insulation
+# Only the filled-cavity dispatch is wired here; the other codes will be
+# handled in subsequent slices.
+WALL_INSULATION_FILLED_CAVITY: Final[int] = 2
+
+
 _AGE_BANDS: Final[tuple[str, ...]] = tuple("ABCDEFGHIJKLM")
 
 
@@ -154,6 +168,20 @@ _ENG_WALL: Final[dict[tuple[int, int], list[float]]] = {
     (WALL_SYSTEM_BUILT, 200): [0.18, 0.18, 0.18, 0.18, 0.18, 0.17, 0.15, 0.15, 0.14, 0.13, 0.12, 0.12, 0.11],
 }
 
+# RdSAP 10 Table 6 (England) row "Filled cavity" — 13 values A..M. The
+# cert records this case as (wall_construction=4 cavity,
+# wall_insulation_type=2 filled, wall_insulation_thickness="NI"). It's a
+# distinct row from "Cavity as built" + bucketed retrofit insulation
+# because filled-cavity isn't an added-insulation thickness; it's the
+# original cavity-fill state. Bands I-M carry the "†" footnote in the
+# spec ("assumed as built") — post-1996 cavities are filled as-built per
+# Building Regs, so the row collapses to the Cavity-as-built values from
+# band I onward.
+_CAVITY_FILLED_ENG: Final[list[float]] = [
+    0.7, 0.7, 0.7, 0.7, 0.7, 0.40, 0.35, 0.35, 0.45, 0.35, 0.30, 0.28, 0.26,
+]
+
+
 # Country-specific K-M overrides (Tables 7-9). Tables share most A-J values
 # with England; the divergence sits at the newer age bands.  IsleOfMan = ENG.
 # Format: country -> (wall_type, ins_bucket) -> {age_band: u_value} for the
@@ -235,6 +263,7 @@ def u_wall(
     *,
     insulation_present: bool = False,
     description: Optional[str] = None,
+    wall_insulation_type: Optional[int] = None,
 ) -> float:
     """RdSAP10 wall U-value in W/m^2K, never null.
 
@@ -243,6 +272,13 @@ def u_wall(
     parsed for material keywords ("sandstone", "granite", "solid brick", ...)
     so the cascade picks the right table instead of falling through to the
     cavity-by-age default. Explicit construction codes always win.
+
+    `wall_insulation_type` is the RdSAP-coded insulation kind on the cert's
+    `sap_building_parts[i].wall_insulation_type` field. When it indicates
+    a filled cavity (code 2) on a cavity-wall construction, the spec's
+    dedicated "Filled cavity" row is used in preference to the
+    thickness-bucketed cascade — the two encode different things (filled-
+    cavity is a construction state, not an added-insulation thickness).
     """
     if country is None and age_band is None and construction is None and insulation_thickness_mm is None and not insulation_present:
         return 1.5
@@ -257,6 +293,11 @@ 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
+    ):
+        return _CAVITY_FILLED_ENG[age_idx]
     bucket = _insulation_bucket(insulation_thickness_mm, insulation_present)
 
     # Country override first.

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

@@ -21,6 +21,7 @@ import pytest
 from domain.ml.rdsap_uvalues import (
     Country,
     WALL_CAVITY,
+    WALL_INSULATION_FILLED_CAVITY,
     WALL_SOLID_BRICK,
     WALL_STONE_GRANITE,
     WALL_SYSTEM_BUILT,
@@ -38,6 +39,78 @@ from domain.ml.rdsap_uvalues import (
 # ----- Walls -----
 
 
+def test_u_wall_filled_cavity_england_age_band_e_returns_table6_value() -> None:
+    # Arrange — RdSAP 10 Table 6 (England) row "Filled cavity", age band E
+    # (1967-1975) -> 0.7 W/m^2K. The cert records this as the triple
+    # (wall_construction=4 cavity, wall_insulation_type=2 filled,
+    # wall_insulation_thickness="NI"). Spec: docs/sap-spec/rdsap-10-
+    # specification-2025-06-10.pdf page 33.
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band="E",
+        construction=WALL_CAVITY,
+        insulation_thickness_mm=0,
+        insulation_present=True,
+        wall_insulation_type=WALL_INSULATION_FILLED_CAVITY,
+    )
+
+    # Assert
+    assert result == pytest.approx(0.7, abs=0.001)
+
+
+@pytest.mark.parametrize(
+    "age_band,expected_u",
+    [
+        # RdSAP 10 Table 6 (England) "Filled cavity" row sampled at three bands:
+        # A (pre-1900) = 0.7   — early cavity dwellings, retro-filled.
+        # F (1976-1982) = 0.40 — first cavity-insulation era.
+        # K (2007+) = 0.30     — "assumed as built" (†) — matches Cavity-as-built K.
+        ("A", 0.7),
+        ("F", 0.40),
+        ("K", 0.30),
+    ],
+)
+def test_u_wall_filled_cavity_england_row_matches_table6_across_age_bands(
+    age_band: str, expected_u: float
+) -> None:
+    # Arrange — the dispatcher must return the right cell of the
+    # "Filled cavity" row, not just the band-E value used by the tracer.
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band=age_band,
+        construction=WALL_CAVITY,
+        insulation_thickness_mm=0,
+        insulation_present=True,
+        wall_insulation_type=WALL_INSULATION_FILLED_CAVITY,
+    )
+
+    # Assert
+    assert result == pytest.approx(expected_u, abs=0.001)
+
+
+def test_u_wall_unfilled_cavity_england_age_band_e_unchanged_at_1_5() -> None:
+    # Arrange — adding the filled-cavity dispatcher must not regress the
+    # existing as-built path. Band E + cavity construction + no insulation
+    # type set -> the "Cavity as built" row of Table 6, U = 1.5 W/m^2K.
+
+    # Act
+    result = u_wall(
+        country=Country.ENG,
+        age_band="E",
+        construction=WALL_CAVITY,
+        insulation_thickness_mm=0,
+        insulation_present=False,
+        wall_insulation_type=None,
+    )
+
+    # Assert
+    assert result == pytest.approx(1.5, abs=0.001)
+
+
 def test_u_wall_cavity_as_built_england_age_band_g_returns_table6_value() -> None:
     # Arrange — Table 6, England, Cavity as built, age band G -> 0.60 W/m^2K.
 

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

@@ -216,6 +216,7 @@ def heat_transmission_from_cert(
             insulation_thickness_mm=wall_ins_thickness,
             insulation_present=wall_ins_present,
             description=wall_description,
+            wall_insulation_type=wall_ins_type,
         )
         ur = u_roof(country=country, age_band=age_band, insulation_thickness_mm=roof_thickness, description=roof_description)
         uf = u_floor(

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

@@ -29,6 +29,40 @@ from domain.sap.worksheet.heat_transmission import (
 )
 
 
+def test_band_e_filled_cavity_uses_table6_filled_row_in_walls_w_per_k() -> None:
+    # Arrange — RdSAP 10 Table 6 (England) "Filled cavity" row at band E
+    # (1967-1975) = 0.7 W/m^2K. Cert encodes this as
+    # (wall_construction=4 cavity, wall_insulation_type=2 filled).
+    # 100 m² ground floor, 40 m perimeter, 2.5 m height, single storey →
+    # gross_wall = 100 m². With no windows/doors, net_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,     # cavity
+        wall_insulation_type=2,  # filled cavity
+        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],
+    )
+
+    # Act
+    result = heat_transmission_from_cert(epc)
+
+    # Assert
+    assert result.walls_w_per_k == pytest.approx(70.0, abs=1.0)
+
+
 def test_single_storey_age_g_cavity_returns_per_element_breakdown() -> None:
     # Arrange — Mid-terrace, age G cavity-as-built, 100 m² floor area, 40 m
     # heat-loss perimeter, 5 m party wall, 2.5 m room height, single storey.