diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index b725555a8..3260dd181 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -5879,6 +5879,10 @@ _CYLINDER_INSULATION_TYPE_FACTORY: Final[int] = 1
 # which SAP 10.2 Table 2 Note 1 gives a SEPARATE (higher) loss factor
 # L = 0.005 + 1.76 / (t + 12.8) vs the factory L = 0.005 + 0.55 / (t+4).
 _CYLINDER_INSULATION_TYPE_LOOSE_JACKET: Final[int] = 2
+# RdSAP 10 field 7-11 code 0 = NO insulation. SAP 10.2 Table 2 has no
+# separate uninsulated row — the loose-jacket formula at t = 0 IS the
+# uninsulated factor (L = 0.005 + 1.76 / 12.8 = 0.1425 kWh/L/day).
+_CYLINDER_INSULATION_TYPE_NONE: Final[int] = 0
 
 
 def _cylinder_storage_loss_insulation_label(
@@ -7106,9 +7110,18 @@ def _cylinder_storage_loss_override(
     insulation_label = _cylinder_storage_loss_insulation_label(
         sh.cylinder_insulation_type
     )
+    thickness_mm: Optional[float] = (
+        float(sh.cylinder_insulation_thickness_mm)
+        if sh.cylinder_insulation_thickness_mm is not None
+        else None
+    )
+    # SAP 10.2 Table 2: an uninsulated cylinder (lodged insulation type 0)
+    # takes the loose-jacket formula at t = 0 — the WORST loss SAP knows,
+    # never the zero-loss combi default (which over-rates the dwelling).
+    if _int_or_none(sh.cylinder_insulation_type) == _CYLINDER_INSULATION_TYPE_NONE:
+        insulation_label, thickness_mm = "loose_jacket", 0.0
     if insulation_label is None:
         return None
-    thickness_mm = sh.cylinder_insulation_thickness_mm
     if thickness_mm is None:
         return None
     storage_56m = cylinder_storage_loss_monthly_kwh(