Many N. Texas homes like mine have "attic" type 2nd floors. Commonly known as a "1 1/2 story," The 2nd level is fully finished out with 3 bedrooms and 1 bath. But the tax rolls define it as a 1 story house.
The 2nd story has a 7.5' ceiling. The 2nd level roof is flat and has a tiny attic space. Since the entire house is cooled by a single 5-ton air conditioner, the 2nd floor gets uncomfortably warmer in summer, and the AC runs less effeciently.
2 summers ago, I purchased 18 sheets of R Matte Plus 3 aluminum-sided foam sheathing from Home Depot, for about $8 a sheet. I placed them shiny side up on the flat portion of the roof. The panels are weighed down with strategically placed bricks. In the fall, I stack the panels up in the center of the roof, cover them in a tarp, and weigh the stack down with the bricks. To deploy or stack the sheets takes only about an hour.