diff --git a/backend/documents_parser/tests/test_pashub_sap_accuracy.py b/backend/documents_parser/tests/test_pashub_sap_accuracy.py
index d660d8747..34141c47b 100644
--- a/backend/documents_parser/tests/test_pashub_sap_accuracy.py
+++ b/backend/documents_parser/tests/test_pashub_sap_accuracy.py
@@ -81,7 +81,10 @@ _MANIFEST_PATH = _FIXTURES_DIR / "manifest.json"
 # gap) — see docs/HANDOVER_PASHUB_838_PROBLEM_PROPERTIES.md. Re-baselined to the
 # full-coverage aggregate; ratchet back DOWN as those two are resolved.
 _MIN_WITHIN_HALF: float = 0.12
-_MAX_SAP_MAE: float = 2.71
+# Ratcheted 2.71 -> 2.64 by #1590 bug 2 (roof "Insulation At: None" -> explicit
+# zero thickness): observed MAE 2.632 — the two ~7-SAP roof over-raters
+# (507665533138 / 507670893756) correcting is exactly 14/205 ≈ 0.07 of MAE.
+_MAX_SAP_MAE: float = 2.64
 
 _KNOWN_GAP_REASON = (
     "pashub `from_site_notes` mapper does not int-code a main-heating "
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 8ce2fc426..1c38228a1 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -5961,6 +5961,14 @@ def _map_roof(
 ) -> tuple[Optional[str], Optional[Union[str, int]]]:
     if roof is None:
         return None, None
+    # "Insulation At: None" is the assessor explicitly recording ZERO loft
+    # insulation (no thickness line follows — there is nothing to measure).
+    # Lodge an explicit 0 so `u_roof` takes the Table 16 uninsulated row
+    # (2.30 W/m²K) instead of falling through, on thickness=None, to the
+    # age-band "assumed insulated" default (issue #1590 bug 2). A blank
+    # `insulation_at` stays None — "not surveyed", not "no insulation".
+    if roof.insulation_at == "None":
+        return roof.insulation_at, 0
     thickness: Optional[Union[str, int]] = (
         roof.insulation_thickness_mm
         if roof.insulation_thickness_mm is not None