DETAILS FOR A THICK ROOFRigid-foam insulation installed above the roof assembly can create an energy-smart roof. With three layers of 11⁄2-in. polyisocyanurate insulation above the sheathing and approximately 91⁄4 in. of cellulose insulation in the rafter cavities, the roof shown here has an R-value of approximately 63. The three layers of foam help to seal air leaks, but to make this assembly perform well, you need to incorporate some unconventional details.Click To Enlarge