A water-heater is covered up with insulation boards over a total surface area of 30 ft2. The

Question:

A water-heater is covered up with insulation boards over a total surface area of 30 ft2. The inside board surface is at 175 F and the outside surface is at 70 F and the board material has a conductivity of 0.05 Btu/h ft F. How thick a board should it be to limit the heat transfer loss to 720 Btu/h?