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.

Unvented attics that are insulated on the underside of the roof deck with blown spray foam. Install roofing underlayment flashing and cladding as per best practices. At r 4 it also has the lowest r value per inch. Rigid insulation layers exterior to existing roof sheathing must provide sufficient thermal insulation r value to control condensation risk see climate specific guidance.

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. After the roof deck is prepared the rigid foam boards need to be cut to size to fit snugly together on your roof. Position the foam board between the rafters and push it upward until it fits snugly against the underside of the roof deck. All three types of rigid foam insulation expanded polystyrene eps extruded polystyrene xps and polyisocyanurate are suitable for use on the exterior of walls and roofs though they do not perform equally.

High efficiency attic insulation helps provide added thermal protection lessens wasted energy and increases enhanced comfort and quiet. The panels need to fit together tightly to help prevent warm air from leaving the top of the building. The snugness of the foam itself should be sufficient to hold the strips. 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.