diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index e17b43033..cb118b083 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -596,9 +596,13 @@ class EpcPropertyDataMapper:
             heated_rooms_count=schema.heated_room_count,
             wet_rooms_count=0,
             extensions_count=schema.extensions_count,
-            open_chimneys_count=0,
+            # Both lodged by the schema and read by the 17.1/18.0/20.0 paths.
+            # open_chimneys_count was hardcoded 0 (understating infiltration);
+            # percent_draughtproofed was omitted entirely.
+            open_chimneys_count=schema.open_fireplaces_count,
             insulated_door_count=schema.insulated_door_count,
             draughtproofed_door_count=None,
+            percent_draughtproofed=schema.percent_draughtproofed,
             led_fixed_lighting_bulbs_count=0,
             cfl_fixed_lighting_bulbs_count=0,
             incandescent_fixed_lighting_bulbs_count=0,
@@ -728,6 +732,19 @@ class EpcPropertyDataMapper:
                 if getattr(bp, "glazed_perimeter", None) is None
             ],
             sap_conservatory=_api_sap_conservatory(schema.sap_building_parts),
+            # 17.0 lodges `mechanical_ventilation` + `built_form` but passed no
+            # sap_ventilation, so the §2 cascade fell back to NATURAL and its
+            # default sheltered_sides=2. Mirror the 19.0/17.1 path exactly (same
+            # gov code lists — verified in-set across the corpus, strict-coverage
+            # raises on any divergence). For the current corpus the score-mover
+            # is sheltered_sides from built_form (every 17.0 cert lodges
+            # mechanical_ventilation=0 → NATURAL).
+            sap_ventilation=SapVentilation(
+                sheltered_sides=_api_sheltered_sides(schema.built_form),
+                mechanical_ventilation_kind=_api_mechanical_ventilation_kind(
+                    schema.mechanical_ventilation
+                ),
+            ),
         )
 
     @staticmethod
@@ -1540,9 +1557,13 @@ class EpcPropertyDataMapper:
             heated_rooms_count=schema.heated_room_count,
             wet_rooms_count=0,
             extensions_count=schema.extensions_count,
-            open_chimneys_count=0,
+            # Both lodged by the schema and read by the 17.1/18.0/20.0 paths.
+            # open_chimneys_count was hardcoded 0 (understating infiltration);
+            # percent_draughtproofed was omitted entirely.
+            open_chimneys_count=schema.open_fireplaces_count,
             insulated_door_count=schema.insulated_door_count,
             draughtproofed_door_count=None,
+            percent_draughtproofed=schema.percent_draughtproofed,
             led_fixed_lighting_bulbs_count=0,
             cfl_fixed_lighting_bulbs_count=0,
             incandescent_fixed_lighting_bulbs_count=0,
diff --git a/tests/datatypes/epc/domain/test_mapper_ventilation_parity.py b/tests/datatypes/epc/domain/test_mapper_ventilation_parity.py
new file mode 100644
index 000000000..fb31fdfea
--- /dev/null
+++ b/tests/datatypes/epc/domain/test_mapper_ventilation_parity.py
@@ -0,0 +1,67 @@
+"""Regression: the degraded RdSAP paths (17.0, 19.0) must carry the lodged
+ventilation fields, like the reference-complete 17.1/18.0/20.0 paths.
+
+PRD #1385 (mapper normalization). Both paths dropped ventilation lodgements the
+schema carries and the SAP §2 cascade reads:
+
+  * `open_fireplaces_count` → `open_chimneys_count` — was hardcoded 0 on both,
+    understating infiltration.
+  * `percent_draughtproofed` — was omitted on both.
+  * 17.0 additionally passed no `sap_ventilation` block at all, so the cascade
+    fell back to NATURAL + its default `sheltered_sides=2`. Restored from the
+    lodged `mechanical_ventilation` + `built_form` (same gov code lists the
+    19.0/17.1 paths already use; strict-coverage raises on any divergence). 19.0
+    already carried `sap_ventilation`, so only its two counts are restored here.
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from typing import Any
+
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+
+# 17.0 cert: open_fireplaces=1, percent_draughtproofed=100, built_form=2
+# (Semi-Detached → sheltered_sides=1, i.e. NOT the cascade default of 2),
+# mechanical_ventilation=0 (natural → kind None).
+_FIXTURE_17_0 = Path("tests/fixtures/epc_prediction/BD24JG/cert-f326c2524ab3.json")
+# 19.0 cert: open_fireplaces=1, percent_draughtproofed=100.
+_FIXTURE_19_0 = Path("tests/fixtures/epc_prediction/RM143YU/cert-720d5771bfd7.json")
+
+
+def _load(path: Path, schema_type: str) -> dict[str, Any]:
+    data: dict[str, Any] = json.loads(path.read_text())
+    assert data["schema_type"] == schema_type
+    return data
+
+
+def test_17_0_path_carries_ventilation_lodgements() -> None:
+    epc = EpcPropertyDataMapper.from_api_response(
+        _load(_FIXTURE_17_0, "RdSAP-Schema-17.0")
+    )
+
+    # Counts that were hardcoded 0 / omitted.
+    assert epc.open_chimneys_count == 1
+    assert epc.percent_draughtproofed == 100
+
+    # sap_ventilation block was absent entirely — now built from built_form +
+    # mechanical_ventilation. built_form 2 (Semi-Detached) → sheltered_sides 1,
+    # which differs from the cascade's default of 2 (the real score-mover here).
+    assert epc.sap_ventilation is not None
+    assert epc.sap_ventilation.sheltered_sides == 1
+    # mechanical_ventilation 0 is natural → no mechanical kind.
+    assert epc.sap_ventilation.mechanical_ventilation_kind is None
+
+
+def test_19_0_path_carries_ventilation_counts() -> None:
+    epc = EpcPropertyDataMapper.from_api_response(
+        _load(_FIXTURE_19_0, "RdSAP-Schema-19.0")
+    )
+
+    # The two counts 19.0 previously dropped.
+    assert epc.open_chimneys_count == 1
+    assert epc.percent_draughtproofed == 100
+    # 19.0 already carried sap_ventilation; confirm it still resolves built_form.
+    assert epc.sap_ventilation is not None
+    assert epc.sap_ventilation.sheltered_sides == 1