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.

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. Attach to deck board. Install nailbase for cladding. Eps is the most vapor permeable of the three types.

Unvented attics that are insulated on the underside of the roof deck with blown spray foam. 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. Install roofing underlayment flashing and cladding as per best practices.

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. At r 4 it also has the lowest r value per inch. Nailbase must be vented in high snow load areas. And vented or unvented attics that are insulated above the roof deck with rigid foam.

The panels need to fit together tightly to help prevent warm air from leaving the top of the building.