diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index d5b7e23a..f9fe566d 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -1524,7 +1524,11 @@ class EpcPropertyDataMapper:
                     glazing_gap=w.glazing_gap,
                     orientation=w.orientation,
                     window_type=w.window_type,
-                    frame_factor=w.frame_factor,
+                    frame_factor=(
+                        w.frame_factor
+                        if w.frame_factor is not None
+                        else _API_GLAZING_TYPE_TO_TRANSMISSION.get(w.glazing_type, (None, None, None))[2]
+                    ),
                     glazing_type=w.glazing_type,
                     window_width=_measurement_value(w.window_width),
                     window_height=_measurement_value(w.window_height),
@@ -1532,6 +1536,16 @@ class EpcPropertyDataMapper:
                     window_location=w.window_location,
                     window_wall_type=w.window_wall_type,
                     permanent_shutters_present=w.permanent_shutters_present == "Y",
+                    # When the API lodgement carries explicit
+                    # `window_transmission_details`, pass through verbatim
+                    # (Manufacturer-lodged U + solar takes precedence over
+                    # the cascade default). Otherwise derive from the
+                    # `glazing_type` integer code via the SAP10 lookup —
+                    # gives the cascade per-window U-values for the
+                    # `windows_have_per_window_u` fast path in
+                    # `heat_transmission.py`, matching the cohort
+                    # Elmhurst behaviour (which sets these per-window via
+                    # `make_window`).
                     window_transmission_details=(
                         WindowTransmissionDetails(
                             u_value=w.window_transmission_details.u_value,
@@ -1539,7 +1553,15 @@ class EpcPropertyDataMapper:
                             solar_transmittance=w.window_transmission_details.solar_transmittance,
                         )
                         if w.window_transmission_details is not None
-                        else None
+                        else (
+                            WindowTransmissionDetails(
+                                u_value=_API_GLAZING_TYPE_TO_TRANSMISSION[w.glazing_type][0],
+                                data_source="SAP10 lookup (glazing_type)",
+                                solar_transmittance=_API_GLAZING_TYPE_TO_TRANSMISSION[w.glazing_type][1],
+                            )
+                            if w.glazing_type in _API_GLAZING_TYPE_TO_TRANSMISSION
+                            else None
+                        )
                     ),
                     permanent_shutters_insulated=w.permanent_shutters_insulated,
                 )
@@ -2070,6 +2092,26 @@ def _api_roof_construction_str(value: Optional[int]) -> Optional[str]:
 _API_FLOOR_HEAT_LOSS_EXPOSED: Final[int] = 1
 
 
+# GOV.UK API `glazing_type` integer → (u_value W/m²K, g_perpendicular,
+# frame_factor) lookup the cascade reads via `window_transmission_
+# details` for per-window cascade fidelity. The cascade defaults to a
+# single global `u_window(None,None,None)=2.5` and `_G_PERPENDICULAR_
+# DEFAULT=0.76` when these are unset — close to right for typical
+# DG-pre-2002 dwellings but wildly off for newer DG (e.g. cert 001479
+# Ext1 lodges glazing_type=13 → manufacturer DG post-2022 Argon
+# U=1.4 / g=0.72, vs cascade default U=2.5).
+#
+# Codes observed across the 10 golden fixtures: 3 (Main DG pre-2002)
+# and 13 (Ext1 post-2022 Argon). The wider SAP10.2 glazing-type enum
+# (4-12, 14+) is not yet mapped — incremental coverage as new
+# fixtures surface them.
+_API_GLAZING_TYPE_TO_TRANSMISSION: Dict[int, tuple[float, float, float]] = {
+    # (u_value, solar_transmittance/g_⊥, frame_factor)
+    3: (2.8, 0.76, 0.70),    # Double glazed, pre-2002
+    13: (1.4, 0.72, 0.70),   # Double glazed, Argon-filled post-2022
+}
+
+
 def _api_build_sap_floor_dimensions(
     fds: List[Any],
     floor_heat_loss: Optional[int],
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 59676ff0..2c462ccd 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
@@ -95,8 +95,8 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
         cert_number="0300-2747-7640-2526-2135",
         actual_sap=78,
         expected_sap_resid=+2,
-        expected_pe_resid_kwh_per_m2=-0.9139,
-        expected_co2_resid_tonnes_per_yr=-0.9974,
+        expected_pe_resid_kwh_per_m2=-0.2955,
+        expected_co2_resid_tonnes_per_yr=-0.9443,
         notes=(
             "Large semi-detached, TFA 526, age D, gas boiler PCDB-listed "
             "(no Table 4b code). Cert lodges open_flues_count=1 + "
@@ -119,8 +119,8 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
         cert_number="6035-7729-2309-0879-2296",
         actual_sap=70,
         expected_sap_resid=-5,
-        expected_pe_resid_kwh_per_m2=+37.7305,
-        expected_co2_resid_tonnes_per_yr=+0.8510,
+        expected_pe_resid_kwh_per_m2=+39.1452,
+        expected_co2_resid_tonnes_per_yr=+0.8845,
         notes=(
             "Mid-terrace, TFA 128, age A, gas combi Table 4b code 104. "
             "Slice 59 per-bp window apportionment tightens all 3 "
@@ -132,9 +132,9 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
     _GoldenExpectation(
         cert_number="7536-3827-0600-0600-0276",
         actual_sap=68,
-        expected_sap_resid=+2,
-        expected_pe_resid_kwh_per_m2=-11.7633,
-        expected_co2_resid_tonnes_per_yr=-0.3124,
+        expected_sap_resid=+1,
+        expected_pe_resid_kwh_per_m2=-8.8124,
+        expected_co2_resid_tonnes_per_yr=-0.2337,
         notes=(
             "Detached + 2 extensions, TFA 152. Multi-age bps (Main=D, "
             "Ext1=L, Ext2=F). Slice 59 (per-bp window apportionment) and "
@@ -147,8 +147,8 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
         cert_number="8135-1728-8500-0511-3296",
         actual_sap=72,
         expected_sap_resid=+1,
-        expected_pe_resid_kwh_per_m2=-13.0069,
-        expected_co2_resid_tonnes_per_yr=-0.2200,
+        expected_pe_resid_kwh_per_m2=-10.0737,
+        expected_co2_resid_tonnes_per_yr=-0.1645,
         notes=(
             "Semi-detached, TFA 102, age C, gas PCDB-listed. Cert lodges "
             "blocked_chimneys_count=1. Slice 59 per-bp window apportionment "
@@ -160,8 +160,8 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
         cert_number="2130-1033-4050-5007-8395",
         actual_sap=82,
         expected_sap_resid=+2,
-        expected_pe_resid_kwh_per_m2=-44.8941,
-        expected_co2_resid_tonnes_per_yr=+0.2250,
+        expected_pe_resid_kwh_per_m2=-43.5103,
+        expected_co2_resid_tonnes_per_yr=+0.2414,
         notes=(
             "End-terrace + 1 extension, TFA 64, gas combi PCDB index 17505, "
             "postcode DE22 (PCDB Table 172 match), PV: 2× 2.04 kWp arrays "
@@ -179,9 +179,9 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
     _GoldenExpectation(
         cert_number="0390-2254-6420-2126-5561",
         actual_sap=65,
-        expected_sap_resid=+1,
-        expected_pe_resid_kwh_per_m2=-3.5091,
-        expected_co2_resid_tonnes_per_yr=-0.0136,
+        expected_sap_resid=+0,
+        expected_pe_resid_kwh_per_m2=-1.9117,
+        expected_co2_resid_tonnes_per_yr=+0.0102,
         notes=(
             "End-terrace + 1 extension, TFA 80, gas combi PCDB index 18119, "
             "no PV, no secondary, postcode LN12 (PCDB Table 172 match). "