Rigid Foam Under Roof Deck

And vented or unvented attics that are insulated above the roof deck with rigid foam.

Rigid foam under roof deck.

Install roofing underlayment flashing and cladding as per best practices. The purpose of the requirement that rigid foam installed above roof sheathing meet certain minimum r values is to ensure that the roof sheathing stays warm enough during the winter to avoid moisture accumulation and possible sheathing rot. Install nailbase for cladding. Rigid insulation layers exterior to existing roof sheathing must provide sufficient thermal insulation r value to control condensation risk see climate specific guidance.

After the roof deck is prepared the rigid foam boards need to be cut to size to fit snugly together on your roof. Rigid foam is produced in various size panels 4 ft x 8 ft 4ft x 4ft etc. Eps is the most vapor permeable of the three types. Nailbase must be vented in high snow load areas.

The snugness of the foam itself should be sufficient to hold the strips. The panels need to fit together tightly to help prevent warm air from leaving the top of the building. Position the foam board between the rafters and push it upward until it fits snugly against the underside of the roof deck. At r 4 it also has the lowest r value per inch.

Attach to deck board.