A machine shop has a 500-m² metal frame roof that is uninsulated. Determine the payback period of adding insulation (R = 2.0 °C·m²/W). The building is electrically heated. The cost of electricity is $0.07/kWh. The machine shop is located in Paris (Le Bourget, France) and operates 24 hours/day, 7 days/week throughout the heating season. Assume that the installed cost of the insulation is $15/m².