While cavity insulation helps to create an insulated envelope around the house, installing rigid foam insulation on the roof before putting on shingles significantly improves the house's energy efficiency, and if done correctly, eliminates any leaky spots in the process. Foam sheathing can as much as double the R-value of the house according to the Green Building Advisor website, as it helps to create a continuous barrier on the roof, where most of a home's energy escapes.
Things You'll Need
- Tape measure
- Utility knife
- Roofing nails
- Spray foam insulation
- Carpenter pencil
- Safety equipment
- Roofing tar
Tie your safety line off in a manner that will keep you from falling. Attach the safety line to your safety harness. Roll out the felt onto the roof's plywood, starting at the bottom of the roof and working up. Overlay a foot of the felt over the previous layer, as this helps to shed water. Secure in place with roofing nails at regular intervals.
Install flashing in all gulleys and places required on the roof. The last flashing to install is the flashing that covers the end of the foam, before installing shingles.
Draw a center line down the center of the roof. Work from this center line outward for the first course. Other courses are laid to stagger potential leak lines.
Lay the first foam panel onto the roof working from the center line out. Nail into place according to the engineering specifications and nailing pattern. Butt the sheets tightly next to one another to minimize the gap between the rigid foam panels. Don't insert the nails past the rigid foam face or make an indentation into the foam; keep nails flush with the foam surface.
Stagger the next course so that the abutment lines don't line up with the coarse below.
Apply tape and caulk to all seams and edges. The idea is to ensure a leak proof roof. Dab a bit of caulk onto each nail. It's an extra step, but well worth it in the end. Alternatively instead of caulk, use a foam sealer, especially along the edges to create a tight seal.
Install end flashing over the rigid foam. Secure with roofing nails. Seal all nail holes with a dab of caulk. This helps prevent water from getting under areas, it sheds water away and aids in preventing insects and birds from getting under the foam and shingles.
Cut pieces to fit, if needed, when reaching a course end or to fit an odd angle.
Add shingles working from the bottom up, over the foam. Remember to install each coarse of shingles over the previous one laid according to the markings on the shingle. Nail each tab. If installed correctly, the shingle from above covers the nail and tab of the shingle below. Don't put any nails into exposed shingles, only install the nail on the tab and then cover with the next course.
Tips & Warnings
- Always use a safety harness and a safety line when working up high or on a roof. Safety is the first priority.
- Any hole into the rigid foam, flashing or shingles is a potential leak. Caulk any extra nail holes and ensure all edges are properly flashed. Remember the idea is a sealed roof. Apply roofing tar to edges around attic roof vents, drain and breather pipes. If you can slip something under a shingle (except at the bottom edge), then it potentially might leak. After install, asphalt shingles take a while to set in. The asphalt becomes soft and spreads a bit and then hardens to provide a solid roof.
- Energy Savers.gov: Foam Board Insulation
- Green Building Advisor: How to Install Rigid Foam Insulation Outside a House
- "Fine HomeBuilding"; Rigid Foam on the Roof and Walls; Martin Holladay
- Scotts Contracting: Insulating Roofs, Walls and Floors
- Energy Savers.gov: Polyisocyanurate Insulation Materials
- Photo Credit Comstock/Comstock/Getty Images