A hollow square box is made from 1-ft² sheets of a prototype insulating material that is 1 in. thick. Engineers are performing a test to measure the new material’s thermal conductivity. A 100-W electrical heater is placed inside the box. Over time, thermocouples attached to the box show that the interior and exterior surfaces of one face have reached the constant temperatures of 150°F and 90°F. What is the thermal conductivity? Express your result in both the SI and USCS.