diff --git a/docs/migrations/epc-property-round-trip-fidelity.md b/docs/migrations/epc-property-round-trip-fidelity.md
index d9ed6557..53590a2f 100644
--- a/docs/migrations/epc-property-round-trip-fidelity.md
+++ b/docs/migrations/epc-property-round-trip-fidelity.md
@@ -32,8 +32,9 @@ and **still require the matching DB migration** wherever the physical tables liv
   `heating_secondary_heating_type`, `heating_shower_outlet_type`, `energy_pv_connection`;
   `epc_main_heating_detail`: `main_fuel_type`, `heat_emitter_type`, `emitter_temperature`,
   `main_heating_control`; `epc_building_part`: `wall_construction`, `wall_insulation_type`,
-  `party_wall_construction`, `flat_roof_insulation_thickness`, `roof_insulation_location`,
-  `roof_insulation_thickness`; `epc_window`: `glazing_gap`, `orientation`, `window_type`,
+  `party_wall_construction`, `wall_insulation_thickness`, `flat_roof_insulation_thickness`,
+  `roof_insulation_location`, `roof_insulation_thickness`; `epc_window`: `glazing_gap`,
+  `orientation`, `window_type`,
   `glazing_type`, `window_location`, `window_wall_type`, `draught_proofed`,
   `permanent_shutters_present`, `transmission_data_source`).
 - **New scalar columns** — `epc_property`: `heating_number_baths`, `heating_number_baths_wwhrs`,
@@ -68,7 +69,7 @@ in the forward mapper so the Python type round-trips exactly (JSON scalars prese
 |---|---|
 | `epc_property` | `heating_cylinder_size`, `heating_immersion_heating_type`, `heating_cylinder_insulation_type`, `heating_secondary_heating_type`, `heating_shower_outlet_type`, `energy_pv_connection` |
 | `epc_main_heating_detail` | `main_fuel_type`, `heat_emitter_type`, `emitter_temperature`, `main_heating_control` |
-| `epc_building_part` | `wall_construction`, `wall_insulation_type`, `party_wall_construction`, `flat_roof_insulation_thickness`, `roof_insulation_location`, `roof_insulation_thickness` |
+| `epc_building_part` | `wall_construction`, `wall_insulation_type`, `party_wall_construction`, `wall_insulation_thickness`, `flat_roof_insulation_thickness`, `roof_insulation_location`, `roof_insulation_thickness` |
 | `epc_window` | `glazing_gap`, `orientation`, `window_type`, `glazing_type`, `window_location`, `window_wall_type`, `draught_proofed`, `permanent_shutters_present` |
 
 (`energy_meter_type` and `energy_wind_turbines_terrain_type` are `str` in the domain — leave as
diff --git a/infrastructure/postgres/epc_property_table.py b/infrastructure/postgres/epc_property_table.py
index 539628bd..4b235d34 100644
--- a/infrastructure/postgres/epc_property_table.py
+++ b/infrastructure/postgres/epc_property_table.py
@@ -549,7 +549,15 @@ class EpcBuildingPartModel(SQLModel, table=True):
     building_part_number: Optional[int] = Field(default=None)
     wall_dry_lined: Optional[bool] = Field(default=None)
     wall_thickness_mm: Optional[int] = Field(default=None)
-    wall_insulation_thickness: Optional[str] = Field(default=None)
+    # Union[str, int] — int mm when the API lodges
+    # `wall_insulation_thickness == "measured"` (resolved by
+    # `_api_resolve_wall_insulation_thickness`), else the lodged string
+    # ("NI", a numeric string, ...). JSONB to preserve int vs str on
+    # round-trip, exactly like the sibling `roof_insulation_thickness` /
+    # `flat_roof_insulation_thickness`.
+    wall_insulation_thickness: Optional[Union[str, int]] = Field(
+        default=None, sa_column=Column(JSONB, nullable=True)
+    )
     floor_heat_loss: Optional[int] = Field(default=None)
     floor_insulation_thickness: Optional[str] = Field(default=None)
     flat_roof_insulation_thickness: Optional[Union[str, int]] = Field(
diff --git a/tests/repositories/epc/test_epc_round_trip.py b/tests/repositories/epc/test_epc_round_trip.py
index 192027f7..8233bda3 100644
--- a/tests/repositories/epc/test_epc_round_trip.py
+++ b/tests/repositories/epc/test_epc_round_trip.py
@@ -48,3 +48,37 @@ def test_epc_property_data_round_trips(schema_dir: str, db_engine: Engine) -> No
 
     # Assert
     assert reloaded == original
+
+
+def test_building_part_wall_insulation_thickness_preserves_int(
+    db_engine: Engine,
+) -> None:
+    # SAP 10.2 §5.7/Table 8: when the API lodges
+    # `wall_insulation_thickness == "measured"`, the mapper resolves the
+    # value to an int mm. The `epc_building_part.wall_insulation_thickness`
+    # column must therefore preserve int vs str on round-trip (JSONB), like
+    # its `roof_insulation_thickness` sibling — a plain str column would
+    # round-trip the int 100 back as "100" and corrupt the Table 8 lookup.
+    from dataclasses import replace
+
+    # Arrange — take a green fixture and force the measured-int case.
+    original = _load_epc("RdSAP-Schema-21.0.0")
+    assert original.sap_building_parts, "fixture must have a building part"
+    bp0 = replace(original.sap_building_parts[0], wall_insulation_thickness=100)
+    original = replace(
+        original,
+        sap_building_parts=[bp0, *original.sap_building_parts[1:]],
+    )
+
+    # Act
+    with Session(db_engine) as session:
+        epc_property_id = EpcPostgresRepository(session).save(original)
+        session.commit()
+    with Session(db_engine) as session:
+        reloaded = EpcPostgresRepository(session).get(epc_property_id)
+
+    # Assert — the int survives as an int, not the string "100".
+    assert reloaded is not None
+    value = reloaded.sap_building_parts[0].wall_insulation_thickness
+    assert value == 100
+    assert isinstance(value, int)