diff --git a/backend/onboarders/epc_descriptions.py b/backend/onboarders/epc_descriptions.py
index e9b770fd..a92565d1 100644
--- a/backend/onboarders/epc_descriptions.py
+++ b/backend/onboarders/epc_descriptions.py
@@ -396,7 +396,32 @@ def flat_insulated_efficiency_age_band(age_band: EpcConstructionAgeBand) -> EpcE
 
 
 def flat_insulated_efficiency_thickness(insulation_thickness: int | None) -> EpcEfficiency:
-    pass
+    """
+    12mm -> Very Poor
+    25mm - 50mm -> Poor
+    75mm - 125mm -> Pitched, insulated, average
+    150mm - 250mm -> good
+    270mm+ -> very good
+    :param insulation_thickness: Insulation thickness in mm
+    :return: EpcEfficiency
+    """
+
+    if insulation_thickness is None:
+        raise ValueError("Insulation thickness is required for flat insulated efficiency calculation")
+
+    if insulation_thickness >= 270:
+        return EpcEfficiency.VERY_GOOD
+
+    if 150 <= insulation_thickness <= 250:
+        return EpcEfficiency.GOOD
+
+    if 75 <= insulation_thickness <= 125:
+        return EpcEfficiency.AVERAGE
+
+    if 25 <= insulation_thickness <= 50:
+        return EpcEfficiency.POOR
+
+    return EpcEfficiency.VERY_POOR
 
 
 def loft_insulated_efficiency(age_band: EpcConstructionAgeBand) -> EpcEfficiency: