 This is a problem where we have three layers for a wall. And those three layers are made up of stone, polyurethane board, and plywood. The first one is stone, and its thickness is 8 inches. And each inch of stone wall would provide an R value of 0.8. Therefore, all these eight inches would provide together 0.64. And the second layer is made up of three inches of polyurethane. And each inch provides an R value of 6.25. Therefore, together all three inches would provide an R value of 18.75. The third layer is, again, three-quarters inch plywood. And it provides an R value of 0.94. So all these three together would have an R value of 20.33. Or the composite R value is 20.33 degrees Fahrenheit foot square hour over BTU.