diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 1f55d4e3..72e10f44 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2988,6 +2988,16 @@ def _api_glazing_transmission(
 # remap — incremental coverage as new fixtures surface them.
 _API_TO_SAP10_CASCADE_GLAZING_CODE: Dict[int, int] = {
     1: 2,  # RdSAP 21 DG pre-2002 → cascade DG (g_L=0.80, not single 0.90)
+    # RdSAP-21 codes 4 and 5 DIVERGE from the cascade enum in the 1-6 range
+    # (cascade 4 = double low-E soft-coat, 5 = secondary). Without these the
+    # g-tables read the wrong slot: a single-glazed window (RdSAP-21 5) took
+    # the cascade-5 secondary g (g⊥ 0.76 / g_L 0.80) for solar + daylight
+    # gains instead of single (0.85 / 0.90), and a secondary-glazed window
+    # (RdSAP-21 4) took cascade-4 double-low-E (0.63). Correctness fix
+    # (gains are second-order — negligible SAP impact; the dominant single-
+    # glazing error was the U-value, closed in the Table 24 transmission map).
+    4: 5,  # RdSAP 21 secondary glazing → cascade secondary slot (0.76/0.80)
+    5: 1,  # RdSAP 21 single glazing  → cascade single slot   (0.85/0.90)
 }
 
 
diff --git a/datatypes/epc/domain/tests/test_from_rdsap_schema.py b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
index 8cadcc6b..b882a4c0 100644
--- a/datatypes/epc/domain/tests/test_from_rdsap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
@@ -1188,3 +1188,54 @@ class TestApiGlazingTransmissionTable24:
 
         # Assert
         assert result == (2.0, 0.72, 0.70)
+
+
+class TestApiCascadeGlazingCodeDivergentRemap:
+    """`_api_cascade_glazing_type` must translate the RdSAP-21 glazing codes
+    that DIVERGE from the SAP 10.2 Table 6b cascade enum the g-value tables
+    (`_G_PERPENDICULAR_BY_GLAZING_TYPE` / `_G_LIGHT_BY_GLAZING_CODE`) are
+    keyed on. Only code 1 was ever remapped; codes 4 and 5 sit in the 1-6
+    range where the two enums disagree — RdSAP-21 4=secondary / 5=single,
+    cascade 4=double-low-E / 5=secondary — so an API single-glazed window
+    (5) read the cascade-5 (secondary) g slot for solar + daylight gains."""
+
+    def test_single_glazing_code_5_remaps_to_cascade_single_slot_1(self) -> None:
+        # Arrange — RdSAP-21 code 5 = single glazing; cascade single slot
+        # is 1 (g⊥ 0.85, g_L 0.90), not cascade 5 (secondary, 0.76/0.80).
+        from datatypes.epc.domain.mapper import _api_cascade_glazing_type  # pyright: ignore[reportPrivateUsage]
+
+        # Act
+        result = _api_cascade_glazing_type(5)
+
+        # Assert
+        assert result == 1
+
+    def test_secondary_glazing_code_4_remaps_to_cascade_secondary_slot_5(self) -> None:
+        # Arrange — RdSAP-21 code 4 = secondary glazing; cascade secondary
+        # slot is 5 (g⊥ 0.76, g_L 0.80), not cascade 4 (double low-E 0.63).
+        from datatypes.epc.domain.mapper import _api_cascade_glazing_type  # pyright: ignore[reportPrivateUsage]
+
+        # Act
+        result = _api_cascade_glazing_type(4)
+
+        # Assert
+        assert result == 5
+
+    def test_double_pre_2002_code_1_remap_unchanged(self) -> None:
+        # Arrange — regression guard: the existing code-1 remap (→2) stands.
+        from datatypes.epc.domain.mapper import _api_cascade_glazing_type  # pyright: ignore[reportPrivateUsage]
+
+        # Act
+        result = _api_cascade_glazing_type(1)
+
+        # Assert
+        assert result == 2
+
+    def test_rdsap21_native_codes_pass_through(self) -> None:
+        # Arrange — codes 9-15 already coincide with the g-table's RdSAP-21
+        # extension slots, so they must pass through untranslated.
+        from datatypes.epc.domain.mapper import _api_cascade_glazing_type  # pyright: ignore[reportPrivateUsage]
+
+        # Act / Assert
+        assert _api_cascade_glazing_type(14) == 14
+        assert _api_cascade_glazing_type(9) == 9