diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 722e92c2..14ac33ec 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -1564,6 +1564,10 @@ class EpcPropertyDataMapper:
             multiple_glazed_proportion=schema.multiple_glazed_proportion,
             extract_fans_count=schema.extract_fans_count,
             blocked_chimneys_count=schema.blocked_chimneys_count,
+            # cert_to_inputs reads epc.percent_draughtproofed for the §2
+            # ventilation cascade (window draught loss). Without this the
+            # cascade defaults to 0 — treats every cert as fully draughty.
+            percent_draughtproofed=schema.percent_draughtproofed,
             insulated_door_u_value=schema.insulated_door_u_value,
             mechanical_vent_duct_placement=schema.mechanical_vent_duct_placement,
             mechanical_vent_duct_insulation=schema.mechanical_vent_duct_insulation,
diff --git a/datatypes/epc/domain/tests/test_from_rdsap_schema.py b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
index ce83ddce..428f4b8a 100644
--- a/datatypes/epc/domain/tests/test_from_rdsap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
@@ -622,6 +622,21 @@ class TestFromRdSapSchema21_0_1:
         assert sv is not None
         assert sv.extract_fans_count == 2
 
+    def test_percent_draughtproofed_flows_through_to_calculator_input(
+        self, result: EpcPropertyData
+    ) -> None:
+        # Arrange — fixture lodges `percent_draughtproofed: 100` at the
+        # cert root. cert_to_inputs reads it via epc.percent_draughtproofed
+        # for the §2 ventilation cascade (window draught loss). Without
+        # this the cascade defaults to 0 — treats every cert as fully
+        # draughty, over-counting infiltration.
+
+        # Act
+        v = result.percent_draughtproofed
+
+        # Assert
+        assert v == 100
+
     def test_ventilation_completeness_all_seven_vent_fields_flow_through(
         self, result: EpcPropertyData
     ) -> None:
diff --git a/packages/domain/src/domain/sap/rdsap/tests/test_golden_fixtures.py b/packages/domain/src/domain/sap/rdsap/tests/test_golden_fixtures.py
index 83bd83a2..7c7ccb17 100644
--- a/packages/domain/src/domain/sap/rdsap/tests/test_golden_fixtures.py
+++ b/packages/domain/src/domain/sap/rdsap/tests/test_golden_fixtures.py
@@ -74,9 +74,9 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
     _GoldenExpectation(
         cert_number="0240-0200-5706-2365-8010",
         actual_sap=73,
-        expected_sap_resid=-12,
-        expected_pe_resid_kwh_per_m2=+5.5809,
-        expected_co2_resid_tonnes_per_yr=+0.3436,
+        expected_sap_resid=-10,
+        expected_pe_resid_kwh_per_m2=-2.0499,
+        expected_co2_resid_tonnes_per_yr=-0.0447,
         notes=(
             "Detached house, TFA 202, age J, oil boiler, Table 4b code 130. "
             "API response lodges sap_room_in_roof.room_in_roof_type_1 with "
@@ -94,9 +94,9 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
     _GoldenExpectation(
         cert_number="0300-2747-7640-2526-2135",
         actual_sap=78,
-        expected_sap_resid=-9,
-        expected_pe_resid_kwh_per_m2=+4.4546,
-        expected_co2_resid_tonnes_per_yr=-0.5359,
+        expected_sap_resid=-7,
+        expected_pe_resid_kwh_per_m2=-1.9082,
+        expected_co2_resid_tonnes_per_yr=-1.0829,
         notes=(
             "Large semi-detached, TFA 526, age D, gas boiler PCDB-listed "
             "(no Table 4b code). Cert lodges open_flues_count=1 + "
@@ -106,41 +106,41 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
     _GoldenExpectation(
         cert_number="0390-2954-3640-2196-4175",
         actual_sap=60,
-        expected_sap_resid=-7,
-        expected_pe_resid_kwh_per_m2=-26.6760,
-        expected_co2_resid_tonnes_per_yr=-2.5816,
+        expected_sap_resid=-4,
+        expected_pe_resid_kwh_per_m2=-31.5482,
+        expected_co2_resid_tonnes_per_yr=-3.0251,
         notes="Large detached, TFA 360, age F, oil PCDB-listed. Cert lodges has_draught_lobby=true.",
     ),
     _GoldenExpectation(
         cert_number="6035-7729-2309-0879-2296",
         actual_sap=70,
-        expected_sap_resid=-6,
-        expected_pe_resid_kwh_per_m2=+45.0454,
-        expected_co2_resid_tonnes_per_yr=+1.0245,
+        expected_sap_resid=-5,
+        expected_pe_resid_kwh_per_m2=+36.1487,
+        expected_co2_resid_tonnes_per_yr=+0.8134,
         notes="Mid-terrace, TFA 128, age A, gas combi Table 4b code 104.",
     ),
     _GoldenExpectation(
         cert_number="7536-3827-0600-0600-0276",
         actual_sap=68,
-        expected_sap_resid=+3,
-        expected_pe_resid_kwh_per_m2=-17.9833,
-        expected_co2_resid_tonnes_per_yr=-0.4781,
+        expected_sap_resid=+4,
+        expected_pe_resid_kwh_per_m2=-27.1721,
+        expected_co2_resid_tonnes_per_yr=-0.7230,
         notes="Detached + 2 extensions, TFA 152, age D, gas PCDB.",
     ),
     _GoldenExpectation(
         cert_number="8135-1728-8500-0511-3296",
         actual_sap=72,
         expected_sap_resid=+1,
-        expected_pe_resid_kwh_per_m2=-9.5017,
-        expected_co2_resid_tonnes_per_yr=-0.1537,
+        expected_pe_resid_kwh_per_m2=-16.9775,
+        expected_co2_resid_tonnes_per_yr=-0.2951,
         notes="Semi-detached, TFA 102, age C, gas PCDB-listed. Cert lodges blocked_chimneys_count=1.",
     ),
     _GoldenExpectation(
         cert_number="2130-1033-4050-5007-8395",
         actual_sap=82,
-        expected_sap_resid=+8,
-        expected_pe_resid_kwh_per_m2=-65.8945,
-        expected_co2_resid_tonnes_per_yr=+0.1856,
+        expected_sap_resid=+9,
+        expected_pe_resid_kwh_per_m2=-69.5678,
+        expected_co2_resid_tonnes_per_yr=+0.1422,
         notes=(
             "End-terrace + 1 extension, TFA 64, gas combi PCDB index 17505, "
             "postcode DE22 (PCDB Table 172 match), PV: 2× 2.04 kWp arrays "
@@ -158,9 +158,9 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
     _GoldenExpectation(
         cert_number="0390-2254-6420-2126-5561",
         actual_sap=65,
-        expected_sap_resid=0,
-        expected_pe_resid_kwh_per_m2=+0.1797,
-        expected_co2_resid_tonnes_per_yr=+0.0413,
+        expected_sap_resid=+1,
+        expected_pe_resid_kwh_per_m2=-8.9202,
+        expected_co2_resid_tonnes_per_yr=-0.0942,
         notes=(
             "End-terrace + 1 extension, TFA 80, gas combi PCDB index 18119, "
             "no PV, no secondary, postcode LN12 (PCDB Table 172 match). "