How to Install Tapered Rigid Insulation

Use either extruded polystyrene, called XPS, or polyisocyanurate, called polyiso. Both some in tapered styles designed for roof applications. Locate the roof drains and design a pattern to direct water to the drains. Make two slopes from a middle peak or a pyramid with four slopes from a center or any combination depending on where down drains are situated. Consult a manufacturer's design manual for guidance.

Build "crickets," humps above the basic insulation level, if necessary to guide water to drains. You can form crickets by cutting triangles of foam board and placing them in diamond or half-diamond shapes to direct water toward drains. Make crickets for roofs that slope in more than one direction, toward four-corner drains, for instance.

Fasten foam board panels mechanically, with either screws and a screw gun or hammer and nails, as recommended by the insulation manufacturer. Space fasteners per panel according to the manufacturer and installation, but usually at least every 2 feet. Use an adhesive if that is approved by the manufacturer and local building codes.

Install panels to slope toward a drain, combining panels for longer runs. Manufacturers provide 4-foot long panels designed to be put together for a continuous slope. Make two or more layers if a steeper slope is needed. Basic tapered insulation slopes from high side to low at about 1/4-inch per foot. Stack panels to increase that ratio to 1/2-inch per foot or more. Use regular flat foam panels under top sections to elevate them to the proper height.