Rigid Foam Under Roof Deck

High efficiency attic insulation helps provide added thermal protection lessens wasted energy and increases enhanced comfort and quiet.

Rigid foam under roof deck.

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. Unvented attics that are insulated on the underside of the roof deck with blown spray foam. 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.

Rigid insulation layers exterior to existing roof sheathing must provide sufficient thermal insulation r value to control condensation risk see climate specific guidance. 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. Eps is the most vapor permeable of the three types.

After the roof deck is prepared the rigid foam boards need to be cut to size to fit snugly together on your roof. The panels need to fit together tightly to help prevent warm air from leaving the top of the building. Nailbase must be vented in high snow load areas. 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.

Attach to deck board.