diff --git a/datatypes/epc/domain/tests/test_from_site_notes.py b/datatypes/epc/domain/tests/test_from_site_notes.py
index cc532b6e7..af90f142f 100644
--- a/datatypes/epc/domain/tests/test_from_site_notes.py
+++ b/datatypes/epc/domain/tests/test_from_site_notes.py
@@ -74,6 +74,75 @@ class TestRoofConstructionTypeCoding:
         assert extension_part.roof_construction_type == "Flat"
 
 
+class TestRoofInsulationNoneCoding:
+    """A surveyed roof-space "Insulation At: None" is the assessor explicitly
+    recording ZERO loft insulation — `from_site_notes` must lodge an explicit
+    `roof_insulation_thickness=0`, not leave it `None` ("unknown"). With `None`
+    the calculator's `u_roof` falls through to the age-band "assumed insulated"
+    default (e.g. 0.16 W/m²K at band J/K) instead of the Table 16 uninsulated
+    row (2.30) — a ~14x understatement of roof heat loss (issue #1590 bug 2;
+    fixtures 507665533138 / 507670893756 over-rate by ~7 SAP each).
+    """
+
+    def test_insulation_at_none_lodges_explicit_zero_thickness(self) -> None:
+        # Arrange — the surveyor recorded "Insulation At: None" (no thickness
+        # line follows; there is no insulation to measure).
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["roof_space"]["main_building"]["insulation_at"] = "None"
+        data["roof_space"]["main_building"]["insulation_thickness_mm"] = None
+        survey = from_dict(PasHubRdSapSiteNotes, data)
+
+        # Act
+        result = EpcPropertyDataMapper.from_site_notes(survey)
+
+        # Assert — explicit zero, so u_roof takes the uninsulated branch.
+        assert result.sap_building_parts[0].roof_insulation_thickness == 0
+
+    def test_surveyed_thickness_still_passes_through(self) -> None:
+        # Arrange — the fixture's as-lodged "Joists" + 100mm.
+        survey = from_dict(
+            PasHubRdSapSiteNotes, load("pashub_rdsap_site_notes_example1.json")
+        )
+
+        # Act
+        result = EpcPropertyDataMapper.from_site_notes(survey)
+
+        # Assert
+        assert result.sap_building_parts[0].roof_insulation_thickness == 100
+
+    def test_blank_insulation_at_stays_unknown(self) -> None:
+        # Arrange — a blank lodging is "not surveyed", NOT "no insulation";
+        # it must stay None so the age-band default still applies.
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["roof_space"]["main_building"]["insulation_at"] = ""
+        data["roof_space"]["main_building"]["insulation_thickness_mm"] = None
+        survey = from_dict(PasHubRdSapSiteNotes, data)
+
+        # Act
+        result = EpcPropertyDataMapper.from_site_notes(survey)
+
+        # Assert
+        assert result.sap_building_parts[0].roof_insulation_thickness is None
+
+    def test_extension_insulation_at_none_lodges_explicit_zero(self) -> None:
+        # Arrange — same behaviour on the extension building part.
+        data = load("pashub_rdsap_site_notes_example2.json")
+        data["roof_space"]["extensions"][0]["insulation_at"] = "None"
+        data["roof_space"]["extensions"][0]["insulation_thickness_mm"] = None
+        survey = from_dict(PasHubRdSapSiteNotes, data)
+
+        # Act
+        result = EpcPropertyDataMapper.from_site_notes(survey)
+
+        # Assert
+        extension_part = next(
+            bp
+            for bp in result.sap_building_parts
+            if bp.identifier != BuildingPartIdentifier.MAIN
+        )
+        assert extension_part.roof_insulation_thickness == 0
+
+
 class TestCountryCode:
     """`from_site_notes` must set `country_code` so the cascade's England /
     Wales / Scotland / NI U-value tables (`u_floor`, `u_basement_floor`,