diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 97f8b218b..41066c19d 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -6026,7 +6026,7 @@ def _map_sap_window(window: Window) -> SapWindow:
         glazing_gap=window.glazing_gap,
         orientation=window.orientation,
         window_type=window.window_type,
-        glazing_type=window.glazing_type,
+        glazing_type=_pashub_glazing_type_int(window.glazing_type),
         window_width=window.width_m,
         window_height=window.height_m,
         draught_proofed=window.draught_proofed,
@@ -7261,6 +7261,37 @@ def _pashub_wall_insulation_type_int(label: Optional[str]) -> Union[int, str]:
     return _PASHUB_WALL_INSULATION_TYPE_TO_SAP10[key]
 
 
+# PasHub surveyed window `glazing_type` label → SAP10 cascade glazing code that
+# the calculator reads for both the window U (`_GLAZING_CODE_TO_UWINDOW` /
+# u_window, RdSAP 10 Table 24) and the solar g-value
+# (`_G_PERPENDICULAR_BY_GLAZING_TYPE`, Table 6b). The install-year band is
+# load-bearing on U (double pre-2002 = 2.8, 2002-2021 = 2.0, post-2022 = 1.4);
+# an unknown install date defaults to the conservative pre-2002 code (per the
+# `_API_GLAZING_TYPE_TO_TRANSMISSION` "pre-2002 / unknown install date" pairing).
+_PASHUB_GLAZING_TYPE_TO_SAP10: Dict[str, int] = {
+    "Double glazing installed before 2002": 3,  # double pre-2002 (U 2.8)
+    "Double glazing installed between 2002 - 2021": 2,  # double 2002-2022 (U 2.0)
+    "Double glazing, Unknown install date": 3,  # unknown → pre-2002 default (U 2.8)
+    "Double glazing installed during or after 2022": 13,  # double 2022+ (U 1.4)
+}
+
+
+def _pashub_glazing_type_int(label: Optional[str]) -> Union[int, str]:
+    """Resolve a PasHub surveyed window `glazing_type` label to its SAP10 cascade
+    glazing code at the mapper boundary. A blank label is "no lodging" and passes
+    through as the empty string (the field is `Union[int, str]`; the calculator's
+    isinstance-int guards then fall to their double-glazed defaults); a non-empty
+    label the lookup does not cover strict-raises `UnmappedPasHubLabel` so the
+    coverage gap is fixed here rather than silently pinning every window to the
+    double-pre-2002 U=2.8 default downstream (#1562)."""
+    if label is None or not label.strip():
+        return ""
+    key = label.strip()
+    if key not in _PASHUB_GLAZING_TYPE_TO_SAP10:
+        raise UnmappedPasHubLabel("glazing type", label)
+    return _PASHUB_GLAZING_TYPE_TO_SAP10[key]
+
+
 def _resolve_elmhurst_underfloor_subtype(
     main_floor: ElmhurstFloorDetails,
     main_age_band: str,
diff --git a/datatypes/epc/domain/tests/test_from_site_notes.py b/datatypes/epc/domain/tests/test_from_site_notes.py
index 6a1b7e19c..c6647ca82 100644
--- a/datatypes/epc/domain/tests/test_from_site_notes.py
+++ b/datatypes/epc/domain/tests/test_from_site_notes.py
@@ -151,6 +151,43 @@ class TestWallInsulationTypeCoding:
             EpcPropertyDataMapper.from_site_notes(survey)
 
 
+class TestGlazingTypeCoding:
+    """`from_site_notes` must int-code the PAS Hub window `glazing_type` label to
+    the SAP10 cascade glazing code the calculator reads (`_GLAZING_CODE_TO_UWINDOW`
+    for the window U, `_G_PERPENDICULAR_BY_GLAZING_TYPE` for solar g), not copy the
+    raw label. A raw label reads as `None`, so every window falls to the double-
+    pre-2002 U=2.8 default — over-counting heat loss on the ~852/1669 windows the
+    survey dates to 2002-2021 (which are U=2.0) (Guinness cohort, issue #1562).
+    """
+
+    @pytest.mark.parametrize(
+        "label, expected_code",
+        [
+            ("Double glazing installed before 2002", 3),  # double pre-2002, U 2.8
+            ("Double glazing installed between 2002 - 2021", 2),  # double 2002-2022, U 2.0
+            ("Double glazing, Unknown install date", 3),  # unknown → pre-2002 default, U 2.8
+            ("Double glazing installed during or after 2022", 13),  # double 2022+, U 1.4
+            ("", ""),  # no lodging — passes through (calc nulls it)
+        ],
+    )
+    def test_label_maps_to_sap_code(
+        self, label: str, expected_code: "int | str"
+    ) -> None:
+        data = load("pashub_rdsap_site_notes_example1.json")
+        for window in data["windows"]:
+            window["glazing_type"] = label
+        survey = from_dict(PasHubRdSapSiteNotes, data)
+        result = EpcPropertyDataMapper.from_site_notes(survey)
+        assert result.sap_windows[0].glazing_type == expected_code
+
+    def test_unknown_label_strict_raises(self) -> None:
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["windows"][0]["glazing_type"] = "Quadruple glazing, vacuum"
+        survey = from_dict(PasHubRdSapSiteNotes, data)
+        with pytest.raises(UnmappedPasHubLabel):
+            EpcPropertyDataMapper.from_site_notes(survey)
+
+
 class TestFromSiteNotesExample1:
     """
     Fixture: pashub_rdsap_site_notes_example1.json
@@ -325,9 +362,9 @@ class TestFromSiteNotesExample1:
         assert result.sap_windows[0].orientation == "South East"
 
     def test_window_glazing_type(self, result: EpcPropertyData) -> None:
-        assert (
-            result.sap_windows[0].glazing_type == "Double glazing, Unknown install date"
-        )
+        # windows[].glazing_type: "Double glazing, Unknown install date"
+        # → SAP10 cascade code 3 (double pre-2002 default, U 2.8)
+        assert result.sap_windows[0].glazing_type == 3
 
     # --- building parts ---
 
@@ -529,7 +566,7 @@ class TestFromSiteNotesExample1:
                     glazing_gap="16 mm or more",
                     orientation="South East",
                     window_type="Window",
-                    glazing_type="Double glazing, Unknown install date",
+                    glazing_type=3,  # "Double glazing, Unknown install date" → cascade code 3
                     window_width=1.0,
                     window_height=1.36,
                     draught_proofed=True,
@@ -542,7 +579,7 @@ class TestFromSiteNotesExample1:
                     glazing_gap="16 mm or more",
                     orientation="South East",
                     window_type="Window",
-                    glazing_type="Double glazing, Unknown install date",
+                    glazing_type=3,  # "Double glazing, Unknown install date" → cascade code 3
                     window_width=0.96,
                     window_height=1.33,
                     draught_proofed=True,
@@ -555,7 +592,7 @@ class TestFromSiteNotesExample1:
                     glazing_gap="16 mm or more",
                     orientation="North West",
                     window_type="Window",
-                    glazing_type="Double glazing, Unknown install date",
+                    glazing_type=3,  # "Double glazing, Unknown install date" → cascade code 3
                     window_width=0.96,
                     window_height=1.04,
                     draught_proofed=True,
@@ -568,7 +605,7 @@ class TestFromSiteNotesExample1:
                     glazing_gap="16 mm or more",
                     orientation="North West",
                     window_type="Window",
-                    glazing_type="Double glazing, Unknown install date",
+                    glazing_type=3,  # "Double glazing, Unknown install date" → cascade code 3
                     window_width=0.97,
                     window_height=1.02,
                     draught_proofed=True,