1. The problem statement, all variables and given/known data So you have a fridge, the walls of the fridge are made of aluminum, plastic and insulation. Which is the most relevent and why for the fridge. 2. Relevant equations Thermal conductivity of aluminum=250 Plastic=.03 insulation=0.035 - 0.16 3. The attempt at a solution I thought by just common sense to be insluation because insulation is mainly used to keep heat either in or out. But i don't know how to describe it in a scientific way? help.