Rigid Foam Under Roof Deck

Rigid insulation layers exterior to existing roof sheathing must provide sufficient thermal insulation r value to control condensation risk see climate specific guidance.

Rigid foam under roof deck. Attach to deck board. Position the foam board between the rafters and push it upward until it fits snugly against the underside of the roof deck. Nailbase must be vented in high snow load areas. And vented or unvented attics that are insulated above the roof deck with rigid foam.

Rigid foam is produced in various size panels 4 ft x 8 ft 4ft x 4ft etc. The snugness of the foam itself should be sufficient to hold the strips. Install nailbase for cladding. The panels need to fit together tightly to help prevent warm air from leaving the top of the building.

Unvented attics that are insulated on the underside of the roof deck with blown spray foam. Eps is the most vapor permeable of the three types. High efficiency attic insulation helps provide added thermal protection lessens wasted energy and increases enhanced comfort and quiet. 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.

At r 4 it also has the lowest r value per inch.