Theoretical investigation on thermal insulation performance of closed cavity containing a hollow cylinder

► Heat transfer through a square cavity enclosing a hollow-cylinder is theoretically investigated. ► Better insulating properties are attainable when placing the hollow-cylinder close to the middle of the isothermal boundaries. ► Better insulating properties are attainable when increasing the hollow-cylinder diameter. ► The maximum reported enhancement in the insulating property is 1.96 times that of the pure square cavity. ► A simple least expensive building envelope based on present work is proposed.