Determine the SPP for using SIS for walls of a house located in Denver, CO, that has a BLC of 700 Btu/°F·hr. The cost of wall SIS retrofit is estimated to $3.5/ft² of opaque wall area. The opaque walls are initially insulated statically with RSI-3.5 and when retrofitted are operated with two-step control strategy. Assume the house is heated by gas furnace with an efficiency of 85 percent and cooled with an AC with a COP of 4.0. Assume that the electricity price is 0.10/kWh and the natural gas cost is $1.8/CCF.