diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 3d3915b9b..239ab9115 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -1913,6 +1913,10 @@ class EpcPropertyDataMapper:
             insulated_door_count=schema.insulated_door_count,
             draughtproofed_door_count=None,
             percent_draughtproofed=schema.percent_draughtproofed,
+            # 20.0.0 lodges insulated_door_u_value but the mapper dropped it,
+            # unlike 21.0.1. heat_transmission reads it as the measured door
+            # U-value override; without it every door falls to the RdSAP default.
+            insulated_door_u_value=schema.insulated_door_u_value,
             # ADR-0027: 20.0.0 has no flue/fan/vent counts (calculator defaults
             # them via RdSAP Table 5), but sheltered_sides must come from
             # built_form — else the calculator assumes mid-terrace (2) for all.
@@ -2187,6 +2191,10 @@ class EpcPropertyDataMapper:
                 schema.mechanical_ventilation_index_number
             ),
             mechanical_vent_duct_type=schema.mechanical_vent_duct_type,
+            # 21.0.0 lodges insulated_door_u_value but the mapper dropped it,
+            # unlike 21.0.1. heat_transmission reads it as the measured door
+            # U-value override; without it every door falls to the RdSAP default.
+            insulated_door_u_value=schema.insulated_door_u_value,
             led_fixed_lighting_bulbs_count=schema.led_fixed_lighting_bulbs_count or 0,
             cfl_fixed_lighting_bulbs_count=schema.cfl_fixed_lighting_bulbs_count or 0,
             incandescent_fixed_lighting_bulbs_count=schema.incandescent_fixed_lighting_bulbs_count,
diff --git a/tests/datatypes/epc/domain/test_mapper_insulated_door_u_value.py b/tests/datatypes/epc/domain/test_mapper_insulated_door_u_value.py
new file mode 100644
index 000000000..80b33aa4c
--- /dev/null
+++ b/tests/datatypes/epc/domain/test_mapper_insulated_door_u_value.py
@@ -0,0 +1,49 @@
+"""Regression: the 20.0.0 and 21.0.0 mappers must carry the lodged
+insulated_door_u_value, like the 21.0.1 path.
+
+PRD #1385 phase 2. Both dropped `insulated_door_u_value` (top-level Optional
+float). heat_transmission reads it as the measured door U-value override
+(`cert_to_inputs` → `heat_transmission`), so dropping it forced every door onto
+the RdSAP default U-value.
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from typing import Any
+
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+
+# 21.0.0 sample lodges insulated_door_u_value=3.
+_FIXTURE_21_0_0 = Path("backend/epc_api/json_samples/RdSAP-Schema-21.0.0/epc.json")
+# 20.0.0 certs don't lodge it in the corpus (latent) — inject to prove the path.
+_FIXTURE_20_0_0 = Path("tests/fixtures/epc_prediction/PE71NT/cert-460c2d58495c.json")
+
+
+def test_21_0_0_carries_insulated_door_u_value() -> None:
+    data: dict[str, Any] = json.loads(_FIXTURE_21_0_0.read_text())
+    assert data["schema_type"] == "RdSAP-Schema-21.0.0"
+    assert data["insulated_door_u_value"] == 3
+
+    epc = EpcPropertyDataMapper.from_api_response(data)
+    assert epc.insulated_door_u_value == 3
+
+
+def test_20_0_0_carries_injected_insulated_door_u_value() -> None:
+    data: dict[str, Any] = json.loads(_FIXTURE_20_0_0.read_text())
+    assert data["schema_type"] == "RdSAP-Schema-20.0.0"
+    data["insulated_door_u_value"] = 1.5
+
+    epc = EpcPropertyDataMapper.from_api_response(data)
+    # Was hardcoded away (None); now carried through.
+    assert epc.insulated_door_u_value == 1.5
+
+
+def test_20_0_0_absent_insulated_door_u_value_maps_to_none() -> None:
+    # The real cert lodges no value → None (no phantom U-value).
+    data: dict[str, Any] = json.loads(_FIXTURE_20_0_0.read_text())
+    assert data.get("insulated_door_u_value") is None
+
+    epc = EpcPropertyDataMapper.from_api_response(data)
+    assert epc.insulated_door_u_value is None