fix(mapper): drop only U=0 internal RR stud walls, keep positive-U ones

A Detailed room-in-roof lodges "Stud Wall" surfaces, but the cascade billed
every one through Table 17 from its insulation — over-counting fabric on
internal studs that carry no heat loss. sim case 20's two studs lodge §8.1
Default U-value 0.00 and the P960 worksheet omits them from BOTH fabric heat
loss (§3: (33)=285.9847) and total exposed area (31)=239.68; the cascade
computed ~0.52 each → (33) +4.16 W/K and continuous SAP 43.05 vs 43.6322.

Gate the drop on the lodged Default U-value: 0.00 → internal knee wall,
return None (no heat loss, no area); positive → a real exposed knee wall
(cert 000565 Ext2 Detailed: 0.31 / 0.10) that still falls through to the
Table-17 path. The earlier over-broad "drop all studs" zeroed 000565's
genuine studs — this keeps them.

Pins test_summary_001431_case20_fabric_heat_loss_matches_worksheet_line_33
((33)=285.9847 at 1e-4); case 20 continuous SAP now EXACT (43.6322). 2850
pass (the lone test_total_floor_area failure is pre-existing on base);
pyright strict net-zero (32=32).

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

backend/documents_parser/tests/test_summary_pdf_mapper_chain.py

@@ -45,7 +45,11 @@ from datatypes.epc.domain.mapper import (
     _elmhurst_glazing_type_code,  # pyright: ignore[reportPrivateUsage]
 )
 from domain.sap10_calculator.calculator import calculate_sap_from_inputs
-from domain.sap10_calculator.rdsap.cert_to_inputs import SAP_10_2_SPEC_PRICES, cert_to_inputs
+from domain.sap10_calculator.rdsap.cert_to_inputs import (
+    SAP_10_2_SPEC_PRICES,
+    cert_to_inputs,
+    heat_transmission_section_from_cert,
+)
 from domain.sap10_ml.rdsap_uvalues import u_party_wall
 from tests.domain.sap10_calculator.worksheet import (
     _elmhurst_worksheet_000474 as _w000474,
@@ -142,6 +146,22 @@ def test_summary_001431_case20_extracts_all_five_section11_windows() -> None:
     assert len(survey.windows) == 5
 
 
+def test_summary_001431_case20_fabric_heat_loss_matches_worksheet_line_33() -> None:
+    # Arrange — sim case 20's room-in-roof (type 2, Detailed) lodges two
+    # "Stud Wall" surfaces at §8.1 Default U-value 0.00, which the P960
+    # worksheet §3 excludes from fabric heat loss: (33) = 285.9847 W/K.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_001431_CASE20_PDF)
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(
+        ElmhurstSiteNotesExtractor(pages).extract()
+    )
+
+    # Act
+    ht = heat_transmission_section_from_cert(epc)
+
+    # Assert
+    assert abs(ht.fabric_heat_loss_w_per_k - 285.9847) <= 1e-4
+
+
 def test_summary_000474_mapper_produces_three_building_parts() -> None:
     # Arrange — cert U985-0001-000474 is a mid-terrace with 3 building
     # parts (Main + 2 extensions) per the hand-built worksheet fixture

datatypes/epc/domain/mapper.py

@@ -3832,6 +3832,16 @@ def _map_elmhurst_rir_surface(
     # the same Simplified RR (scalar gable fields, no roof-going
     # detailed_surfaces; cert 6035) and the gables-only cert 000565.
     # Detailed (§3.10) assessments DO measure these surfaces — keep them.
+    # An RR stud wall (internal knee wall below the slope) is a heat-loss
+    # surface ONLY when Elmhurst lodges a positive §8.1 Default U-value
+    # (cert 000565 Ext2 Detailed: 0.31 / 0.10 — real exposed knee walls).
+    # A Default U-value of 0.00 marks an internal stud wall the P960
+    # worksheet excludes from BOTH fabric heat loss (§3) and total exposed
+    # area (31): sim case 20's (33)=285.9847 and (31)=239.68 both omit its
+    # 2×4 m² studs. Drop only the U=0 (internal) ones; positive-U studs
+    # fall through to the Table-17 path like slopes/ceilings.
+    if kind == "stud_wall" and surface.default_u_value == 0.0:
+        return None
     if is_simplified and kind in ("slope", "flat_ceiling", "stud_wall"):
         return None
     u_value_override: Optional[float] = None