diff --git a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
index ab57b2ee..1b4ba5f6 100644
--- a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
+++ b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
@@ -1902,6 +1902,7 @@ _COHORT_2_API_CLOSED: list[tuple[str, float]] = [
     ("0036-6325-1100-0063-1226", 62.7471),
     ("0100-5141-0522-4696-3463", 85.8332),
     ("0200-3155-0122-2602-3563", 80.8674),
+    ("0300-2403-2650-2206-0235", 76.6541),    # S0380.41 closure
     ("0310-2763-5450-2506-3501", 78.3593),
     ("0320-2126-2150-2326-6161", 71.7224),
     ("0320-2756-8640-2296-1101", 89.9458),
@@ -1930,6 +1931,7 @@ _COHORT_2_API_CLOSED: list[tuple[str, float]] = [
     ("7836-3125-0600-0526-2202", 80.1792),
     ("9036-0824-3500-0420-8222", 84.2727),
     ("9370-3060-1205-3546-4204", 87.8687),
+    ("9380-2957-7490-2595-3141", 74.5902),    # S0380.41 closure
     ("9421-3045-3205-1646-6200", 87.4495),
     ("9796-3058-6205-0346-9200", 90.1318),
     ("9836-7525-9500-0575-1202", 75.2223),
@@ -1956,9 +1958,13 @@ _COHORT_2_API_CLOSED: list[tuple[str, float]] = [
 #     API mapper likely lodges the secondary fuel differently. Probe
 #     the API JSON's `secondary_heating` block first.
 _COHORT_2_API_OPEN: list[tuple[str, float, float]] = [
-    ("0300-2403-2650-2206-0235", 76.6541, 77.084454),
-    ("1536-9325-5100-0433-1226", 65.8928, 66.337334),
-    ("9380-2957-7490-2595-3141", 74.5902, 75.010196),
+    # S0380.41 partially closed this cert: residual moved from +0.4445
+    # to +0.0015 via the same RdSAP 21 → SAP 10.2 glazing-type alias
+    # that closed 0300/9380 cleanly. A sub-2e-3 secondary tail remains
+    # — likely a windows-area Decimal-rounding boundary case in the
+    # cert's specific dimensions; investigate per the cohort closure
+    # convention in Slice S0380.42.
+    ("1536-9325-5100-0433-1226", 65.8928, 65.894324),
     ("2102-3018-0205-7886-5204", 63.8732, 57.570156),
 ]
 
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index b1122f09..bd82719f 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2303,17 +2303,26 @@ def _api_sheltered_sides(built_form: object) -> Optional[int]:
 # 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: 2 (DG England/Wales
-# 2002 or later, pre-2022), 3 (Main DG pre-2002), 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.
+# Codes observed across the 10 cohort-1 golden fixtures: 2 (DG England/
+# Wales 2002 or later, pre-2022), 3 (Main DG pre-2002), 13 (Ext1
+# post-2022 Argon). Cohort-2 (Slice S0380.39) added code 1 — observed
+# in 3 certs (0300/1536/9380) all lodging gap=16+ and description
+# "Fully double glazed" with a worksheet-resolved U=2.7. Per Table 24
+# row 2 (DG pre-2002, gap 16+, PVC/wooden) the spec answer is U=2.7,
+# so GOV.UK API code 1 is a schema sibling of code 3 (both alias the
+# "DG pre-2002 / unknown install date" row). The wider SAP10.2
+# glazing-type enum (4-12, 15+) is not yet mapped — incremental
+# coverage as new fixtures surface them.
 #
-# Spec source: RdSAP 10 Table 24 "Window characteristics" page 79 —
+# Spec source: RdSAP 10 Table 24 "Window characteristics" page 49 —
 # DG pre-2002 spec U varies by gap (6mm=3.1, 12mm=2.8, 16+=2.7); the
 # (type, gap)-keyed lookup picks the spec-correct entry when the gap
 # is lodged, falling back to the type-only default for missing gaps.
 _API_GLAZING_TYPE_TO_TRANSMISSION: Dict[int, tuple[float, float, float]] = {
     # (u_value, solar_transmittance/g_⊥, frame_factor)
+    1: (2.8, 0.76, 0.70),    # Double glazed, pre-2002 / unknown install
+                             # date — Table 24 row 2 (PVC/wooden), 12mm
+                             # gap default. Schema sibling of code 3.
     2: (2.0, 0.72, 0.70),    # Double glazed, England/Wales 2002+ (pre-2022)
     3: (2.8, 0.76, 0.70),    # Double glazed, pre-2002 (12mm gap default)
     13: (1.4, 0.72, 0.70),   # Double glazed, Argon-filled post-2022
@@ -2336,7 +2345,11 @@ _API_GLAZING_TYPE_TO_TRANSMISSION: Dict[int, tuple[float, float, float]] = {
 _API_GLAZING_TYPE_GAP_TO_TRANSMISSION: Dict[
     tuple[int, object], tuple[float, float, float]
 ] = {
-    # Double glazed, pre-2002 — Table 24 row 2 (PVC/wooden frame):
+    # Double glazed, pre-2002 / unknown install date — Table 24 row 2
+    # (PVC/wooden frame). Codes 1 and 3 alias the same Table 24 row:
+    (1, 6):     (3.1, 0.76, 0.70),
+    (1, 12):    (2.8, 0.76, 0.70),
+    (1, "16+"): (2.7, 0.76, 0.70),
     (3, 6):     (3.1, 0.76, 0.70),
     (3, 12):    (2.8, 0.76, 0.70),
     (3, "16+"): (2.7, 0.76, 0.70),
@@ -2357,12 +2370,43 @@ def _api_glazing_transmission(
     return _API_GLAZING_TYPE_TO_TRANSMISSION.get(glazing_type)
 
 
+# GOV.UK RdSAP 21 `glazing_type` integer → SAP 10.2 Table 6b cascade
+# glazing-type integer. The cascade's `_G_LIGHT_BY_GLAZING_CODE` table
+# (domain/sap10_calculator/worksheet/internal_gains.py) is keyed on the
+# SAP 10.2 enum that the Elmhurst extractor produces via
+# `_ELMHURST_GLAZING_LABEL_TO_SAP10` — so the API-side glazing_type must
+# be canonicalised to that same enum before storage on SapWindow.
+#
+# Per datatypes/epc/domain/epc_codes.csv (RdSAP-Schema-21.0.0):
+#   - RdSAP 21 code 1 = "double glazing installed before 2002 in EAW,
+#     2003 in SCT, 2006 NI" — semantically matches SAP 10.2 Table 6b
+#     "DG air-filled pre-2002" (cascade code 2, g_L=0.80).
+#
+# The cohort-1 codes 2, 3, 13, 14 already coincide with the cascade
+# table's intended SAP 10.2 g_L values, so no remap entry is required
+# for them. Only divergent codes (RdSAP 21 ≠ cascade table) need a
+# 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)
+}
+
+
+def _api_cascade_glazing_type(api_glazing_type: int) -> int:
+    """Canonicalise an API-lodged RdSAP 21 glazing-type code to the SAP
+    10.2 Table 6b cascade enum that `_G_LIGHT_BY_GLAZING_CODE` reads.
+    Pass-through for codes already coincident with the cascade table."""
+    return _API_TO_SAP10_CASCADE_GLAZING_CODE.get(api_glazing_type, api_glazing_type)
+
+
 def _api_sap_window(w: Any) -> SapWindow:
     """Build a `SapWindow` from one API schema sap_windows entry,
     routing the glazing-type + glazing-gap pair through the spec
     lookup so DG pre-2002 windows pick up the gap-specific U
     (RdSAP 10 Table 24 row 2: 6mm=3.1 / 12mm=2.8 / 16+=2.7) instead
-    of the type-only default."""
+    of the type-only default. SapWindow.glazing_type is canonicalised
+    to the SAP 10.2 Table 6b cascade enum so the cascade's daylight g_L
+    lookup picks the spec-correct value (e.g. RdSAP 21 code 1 = DG
+    pre-2002, cascade g_L=0.80, not single-glazed 0.90)."""
     transmission = _api_glazing_transmission(w.glazing_type, w.glazing_gap)
     frame_factor: Optional[float] = w.frame_factor
     if frame_factor is None and transmission is not None:
@@ -2387,7 +2431,7 @@ def _api_sap_window(w: Any) -> SapWindow:
         orientation=w.orientation,
         window_type=w.window_type,
         frame_factor=frame_factor,
-        glazing_type=w.glazing_type,
+        glazing_type=_api_cascade_glazing_type(w.glazing_type),
         window_width=_measurement_value(w.window_width),
         window_height=_measurement_value(w.window_height),
         draught_proofed=w.draught_proofed == "true",