diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 8f55db80..efacc71b 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -1698,6 +1698,19 @@ class EpcPropertyDataMapper:
                 if getattr(bp, "glazed_perimeter", None) is None
             ],
             sap_conservatory=_api_sap_conservatory(schema.sap_building_parts),
+            # ADR-0037 follow-up: 19.0 previously passed no `sap_ventilation=`,
+            # dropping the whole §2 block. The schema only lodges the top-level
+            # `mechanical_ventilation` enum + `built_form` (no fan/flue/vent
+            # counts — RdSAP uses Table-5 age defaults), so mirror just those
+            # two fields from 21.0.1. An MEV/MVHR cert that defaulted to NATURAL
+            # under-stated its ventilation heat loss; `sheltered_sides` (§S5)
+            # avoids the cascade's shelter default of 2.
+            sap_ventilation=SapVentilation(
+                sheltered_sides=_api_sheltered_sides(schema.built_form),
+                mechanical_ventilation_kind=_api_mechanical_ventilation_kind(
+                    schema.mechanical_ventilation
+                ),
+            ),
         )
 
     @staticmethod
@@ -2274,6 +2287,19 @@ class EpcPropertyDataMapper:
                     else None
                 ),
             ),
+            # ADR-0037 follow-up: 21.0.0 previously passed no `sap_ventilation=`,
+            # dropping the whole §2 block. Unlike 21.0.1 it does not lodge the
+            # fan/flue/vent counts, so mirror only the two fields it does carry:
+            # the top-level `mechanical_ventilation` enum (→ §2 MV-kind dispatch;
+            # an MEV/MVHR cert otherwise defaulted to NATURAL and under-stated
+            # ventilation heat loss) and `sheltered_sides` from built_form (§S5,
+            # vs the cascade's shelter default of 2).
+            sap_ventilation=SapVentilation(
+                sheltered_sides=_api_sheltered_sides(schema.built_form),
+                mechanical_ventilation_kind=_api_mechanical_ventilation_kind(
+                    schema.mechanical_ventilation
+                ),
+            ),
         )
 
     @staticmethod