diff --git a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
index a4fb4229..118ea9e9 100644
--- a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
+++ b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
@@ -514,6 +514,30 @@ def test_summary_0380_main_heating_category_is_heat_pump() -> None:
     assert main.main_heating_category == 4
 
 
+def test_summary_0380_filled_cavity_plus_external_insulation_routes_to_code_6() -> None:
+    # Arrange — cert 0380's Summary lodges main walls as
+    # `wall_type = "CA Cavity"` and `insulation = "FE Filled Cavity +
+    # External"` (a cavity wall with subsequent external-insulation
+    # upgrade). The cascade enum `wall_insulation_type=6` is
+    # "filled cavity + external insulation" (per
+    # `domain.sap10_ml.rdsap_uvalues` lines 120-131); without it the
+    # cascade defaults to the as-built routing and overstates walls
+    # heat loss by +58 W/K on cert 0380 (Summary 69.69 vs API 11.62
+    # at HEAD before this slice). API path EPC for cert 0380 surfaces
+    # `wall_insulation_type=6` and is the ground-truth pin here.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000899_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert
+    assert epc.sap_building_parts, "no building parts surfaced"
+    main = epc.sap_building_parts[0]
+    assert main.wall_construction == 4  # 4 = Cavity ('CA')
+    assert main.wall_insulation_type == 6  # 6 = filled cavity + external
+
+
 def test_summary_0380_full_chain_sap_matches_worksheet_pdf_exactly() -> None:
     # Arrange — cert 0380-2471-3250-2596-8761 (Summary_000899.pdf /
     # dr87-0001-000899.pdf) is the first heat-pump cert under per-cert
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 05af513f..12c60a00 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2040,12 +2040,21 @@ _ELMHURST_WALL_CODE_TO_SAP10: Dict[str, int] = {
 
 # Elmhurst wall-insulation-type codes mapped to the SAP10 integer enum
 # documented at domain.sap10_ml.rdsap_uvalues.WALL_INSULATION_FILLED_CAVITY.
+# Two-letter dual codes ("FE", "FI") encode a cavity-wall that received a
+# subsequent external- or internal-insulation upgrade — these map to the
+# SAP10 composite codes 6 and 7, which the cascade routes through the
+# series-resistance composite-U calc rather than the filled-cavity table
+# default. Cert 0380's `walls.insulation = "FE Filled Cavity + External"`
+# is the cohort fixture pinning the FE branch.
 _ELMHURST_INSULATION_CODE_TO_SAP10: Dict[str, int] = {
     "E": 1,    # External wall insulation
     "F": 2,    # Filled cavity
     "I": 3,    # Internal wall insulation
     "A": 4,    # As built / assumed (default cascade)
     "N": 5,    # None specified
+    "FE": 6,   # Filled cavity + external insulation (cohort: cert 0380)
+    "FI": 7,   # Filled cavity + internal insulation (no fixture yet —
+               # mirror of FE for the internal-upgrade variant)
 }