Rigid foam board is a standard insulation for exterior walls and roof decks. It most often is extruded polystyrene -- XPS -- trademarked by Dow Chemical as Styrofoam and known in the construction industry as blueboard. It has high heat resistance or R-value and high compressive strength, meaning it does not dent easily. Polyisocyanurate -- polyiso -- has a higher R value and is thinner than comparable XPS, but it is much harder to install because its facing cannot be penetrated or cracked. Either can be used under corrugated or standing seam metal roofing.
Things You'll Need
- Moisture barrier
- Rigid foam board
- Big headed galvanized nails
- 2-inch galvanized steel screws with plastic washer heads or other fasteners
- Roof and edge caps recommended by manufacturer
Use XPS foam board for most roof applications, under either corrugated or standing seam metal. Install a moisture barrier or house wrap on the roof first. Lay XPS panels horizontally, so those seams do not align with the seams of the metal roofing. Nail XPS to roof decking with big-headed galvanized nails and a hammer. Drive nail heads flush with the XPS surface, but try not to dent it.
Install either corrugated or standing-seam metal starting at the bottom of one roof side. Fasten panels with galvanized steel screws with plastic washer caps or other fasteners recommended by the manufacturer; some standing-seam panels are installed with clips rather than screws. Use 2-inch screws with plastic washer heads in corrugated to penetrate typical 1 1/2-inch foam board. Drive screws with a screw gun until the caps are tight with the metal but do not compress it.
Overlap corrugated panels at vertical joints and secure with screws through both panels. Fasten standing seam panels by sliding a lip on one panel into a slot on the adjoining one. Cover the roof with metal panels. Finish a peak or other roof top with special roof or edge caps which overlap the joint. Use recommended edge finishing on the sides and eave of the roof; this will vary with the roofing type and manufacturer.
Consider as an alternative special metal roofing that combines the metal exterior with polyiso insulation under it. Use this style, produced by several manufacturers, to combine the high R-value of polyiso with ease of installation. Fasten these panels according to manufacturer's directions, typically with some type of clips secured to the decking and over the metal facing.