diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index d6d2443f6..037bf833a 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -6096,6 +6096,9 @@ def _map_main_building_part(
             main.walls_insulation_type
         ),
         wall_thickness_measured=main.wall_thickness_mm is not None,
+        # "Wall Dry-Lined? Yes" routes the wall U through the RdSAP10 §5.8 /
+        # Table 14 adjustment; None (not asked) leaves the cascade default.
+        wall_dry_lined=main.dry_lined,
         party_wall_construction=_pashub_party_wall_construction_int(
             main.party_wall_construction_type
         ),
@@ -6133,6 +6136,9 @@ def _map_extension_building_part(
             ext_c.walls_insulation_type
         ),
         wall_thickness_measured=ext_c.wall_thickness_mm is not None,
+        # "Wall Dry-Lined? Yes" routes the wall U through the RdSAP10 §5.8 /
+        # Table 14 adjustment; None (not asked) leaves the cascade default.
+        wall_dry_lined=ext_c.dry_lined,
         party_wall_construction=_pashub_party_wall_construction_int(
             ext_c.party_wall_construction_type
         ),