Update app/materials_library.py

app/materials_library.py

@@ -76,10 +76,10 @@ class Material:
             return "High"
 
     def get_u_value(self) -> float:
-        # Calculate U-value: U = 1 / (R_si + (λ / d) + R_se) (W/m²·K)
-        if self.default_thickness > 0:
-            r_value = R_SI + (self.conductivity / self.default_thickness) + R_SE
-            return 1.0 / r_value if r_value > 0 else 0.0
+        # Calculate U-value: U = 1 / (R_si + (d / λ) + R_se) (W/m²·K)
+        if self.default_thickness > 0 and self.conductivity > 0:
+            r_value = R_SI + (self.default_thickness / self.conductivity) + R_SE
+            return 1.0 / r_value
         return 0.0
 
 class GlazingMaterial: