I've been getting mixed views on how to identify fins in different situations. From my class I know that fins have a lot of convection around them, and the fin material in itself has high thermal conductivity, so there is negligible temperature gradient along its breadth. However today we were analyzing a long structure, like a steel joist in a roof. It had insulation lining its bottom surface (towards the interior of house), but the TA said that its still a fin, though one surface is effectively adiabatic due to the insulation lining. Could someone please tell me a full-proof way to identify a fin?