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 94a3b927..c3a1f4df 100644
--- a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
+++ b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
@@ -4330,3 +4330,32 @@ def test_from_elmhurst_site_notes_matches_hand_built_000516() -> None:
         f"hand-built EpcPropertyData for cohort cert 000516:\n  " +
         "\n  ".join(diffs)
     )
+
+
+def test_elmhurst_jacket_cylinder_insulation_maps_to_loose_jacket_code_2() -> None:
+    # Arrange — an Elmhurst §15.1 "Cylinder Insulation Type: Jacket"
+    # lodging is a loose jacket, which SAP 10.2 Table 2 Note 1 gives a
+    # separate (higher) storage-loss factor than factory foam. The SAP10
+    # `cylinder_insulation_type` enum uses 2 for loose jacket (1 = factory
+    # foam), matching the GOV.UK API path — so the Summary "Jacket" label
+    # must resolve to 2 for cross-mapper parity, and so the
+    # loose-jacket storage-loss branch (S0380.224) fires. Observed on the
+    # simulated-case-19 worksheet (210 L jacket cylinder + storage heaters).
+    from datatypes.epc.domain.mapper import _elmhurst_cylinder_insulation_code  # pyright: ignore[reportPrivateUsage]
+
+    # Act
+    code = _elmhurst_cylinder_insulation_code("Jacket", cylinder_present=True)
+
+    # Assert
+    assert code == 2
+
+
+def test_elmhurst_foam_cylinder_insulation_still_maps_to_factory_code_1() -> None:
+    # Arrange — regression guard: the factory-foam label is unchanged.
+    from datatypes.epc.domain.mapper import _elmhurst_cylinder_insulation_code  # pyright: ignore[reportPrivateUsage]
+
+    # Act
+    code = _elmhurst_cylinder_insulation_code("Foam", cylinder_present=True)
+
+    # Assert
+    assert code == 1
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index caf09b6f..98a341b9 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -4710,8 +4710,14 @@ _ELMHURST_CYLINDER_SIZE_LABEL_TO_SAP10: Dict[str, int] = {
 # which SAP 10.2 Table 2 Note 2 treats as factory-applied PU foam).
 # Other labels (Loose Jacket, None) raise `UnmappedElmhurstLabel`
 # until a fixture exercises them.
+# SAP10 cylinder_insulation_type enum: 1 = factory-applied foam,
+# 2 = loose jacket (matching the GOV.UK API codes). SAP 10.2 Table 2
+# Note 1 gives loose jacket a separate, ~2× higher storage-loss factor;
+# the cascade's loose-jacket branch is wired (S0380.224), so "Jacket"
+# resolves to 2 for cross-mapper parity with the API path.
 _ELMHURST_CYLINDER_INSULATION_LABEL_TO_SAP10: Dict[str, int] = {
     "Foam": 1,
+    "Jacket": 2,
 }