diff --git a/backend/documents_parser/elmhurst_extractor.py b/backend/documents_parser/elmhurst_extractor.py
index 78a86d97..f682e665 100644
--- a/backend/documents_parser/elmhurst_extractor.py
+++ b/backend/documents_parser/elmhurst_extractor.py
@@ -235,6 +235,19 @@ class ElmhurstSiteNotesExtractor:
         thickness_mm = (
             int(thickness_raw.split()[0]) if thickness_raw else None
         )
+        # Composite / retrofit insulation thickness — Summary §7.0
+        # writes the value on the line pair "Insulation Thickness" /
+        # "100 mm" when a composite filled-cavity-plus-external (or
+        # equivalent) wall is lodged. The "Insulation Thickness" label
+        # is local-scoped inside the §7 block so it does not collide
+        # with the §8 Roofs / §9 Floors blocks. None when the PDF
+        # omits the line (no retrofit lodged).
+        ins_thickness_raw = self._local_val(lines, "Insulation Thickness")
+        insulation_thickness_mm = (
+            int(ins_thickness_raw.split()[0])
+            if ins_thickness_raw and ins_thickness_raw.split()[0].isdigit()
+            else None
+        )
         return WallDetails(
             wall_type=self._local_str(lines, "Type"),
             insulation=self._local_str(lines, "Insulation"),
@@ -242,6 +255,7 @@ class ElmhurstSiteNotesExtractor:
             u_value_known=self._local_bool(lines, "U-value Known"),
             party_wall_type=self._local_str(lines, "Party Wall Type"),
             thickness_mm=thickness_mm,
+            insulation_thickness_mm=insulation_thickness_mm,
             alternative_walls=self._alternative_walls_from_lines(lines),
         )
 
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 118ea9e9..173c34d3 100644
--- a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
+++ b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
@@ -538,6 +538,32 @@ def test_summary_0380_filled_cavity_plus_external_insulation_routes_to_code_6()
     assert main.wall_insulation_type == 6  # 6 = filled cavity + external
 
 
+def test_summary_0380_surfaces_wall_insulation_thickness_100mm() -> None:
+    # Arrange — cert 0380's Summary §7.0 Walls block lodges the
+    # composite-wall insulation thickness on the line pair
+    # "Insulation Thickness" / "100 mm". Without surfacing this to
+    # `wall_insulation_thickness`, the heat-transmission cascade
+    # falls through `_parse_thickness_mm(None) → None` and the
+    # composite filled-cavity-plus-external U-value calc uses its
+    # default thickness rather than the lodged 100 mm — leaving cert
+    # 0380's `walls_w_per_k` at 24.62 vs API's 11.62 even with
+    # `wall_insulation_type=6` set (Slice S0380.3). Mirror of the
+    # existing `_roof_details_from_lines` reader that surfaces roof
+    # `insulation_thickness_mm` from the same "Insulation Thickness"
+    # label.
+    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 — match the API mapper's "100mm" string (the EPC schema
+    # type is `Optional[str]`; the cascade's `_parse_thickness_mm`
+    # strips non-digit trailers).
+    main = epc.sap_building_parts[0]
+    assert main.wall_insulation_thickness == "100mm"
+
+
 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 12c60a00..a19c1fd7 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2824,6 +2824,15 @@ def _map_elmhurst_building_part(
         party_wall_construction=_elmhurst_party_wall_construction_int(walls.party_wall_type),
         sap_floor_dimensions=floor_dims,
         wall_thickness_mm=walls.thickness_mm,
+        # API mapper lodges wall_insulation_thickness as the string
+        # "100mm"; the cascade's `_parse_thickness_mm` accepts the
+        # digit-prefix form. Mirror the API shape so cert-to-cert
+        # parity tests (Summary EPC ≡ API EPC) compare equal.
+        wall_insulation_thickness=(
+            f"{walls.insulation_thickness_mm}mm"
+            if walls.insulation_thickness_mm is not None
+            else None
+        ),
         roof_construction_type=_strip_code(roof.roof_type),
         roof_insulation_location=_strip_code(roof.insulation),
         roof_insulation_thickness=_resolve_sloping_ceiling_thickness(
diff --git a/datatypes/epc/surveys/elmhurst_site_notes.py b/datatypes/epc/surveys/elmhurst_site_notes.py
index a110517b..bc70ffbb 100644
--- a/datatypes/epc/surveys/elmhurst_site_notes.py
+++ b/datatypes/epc/surveys/elmhurst_site_notes.py
@@ -79,6 +79,10 @@ class WallDetails:
         default_factory=lambda: []  # type: ignore[reportUnknownLambdaType]
     )
     thickness_mm: Optional[int] = None
+    # Insulation thickness in mm — Summary §7.0 lodges this on the
+    # "Insulation Thickness" / "100 mm" line pair when a composite or
+    # retrofit insulation is recorded. None when the PDF omits the line.
+    insulation_thickness_mm: Optional[int] = None
 
 
 @dataclass