An insulated roofline like this house has does several really frugal things. By spraying insulation between the rafters rather than insulating with batts between the ceiling joists, you not only get better insulation, but the entire attic stays close to room temperature. So pipes won’t freeze, and ducts aren’t subjected to temperature extremes, making them more efficient. Anything stored in the attic is protected as well. And you can use a simple board flooring on top of the ceiling joists, exposing them to view and gaining several inches of ceiling space as shown in this image.