Q. Am I Allowed to Apply Spray Foam to the Underside of the Roof Without Ventilation?
A. Yes. Section R806.4 of the 2006 International Residential Code specifically permits unvented attics with certain provisions; namely, that an air-impermeable insulation is applied to the underside of the roof with an appropriate r-value necessary to maintain the monthly
average temperature of the surface of the foam above 45°F in order to prevent a condensing surface. The condensing surface is on the face of the foam and not the roof sheathing because the foam bonds to the sheathing creating a single cohesive, air-impermeable assembly. (Please note, fiberglass and cellulose do not qualify as air-impermeable. This also means there are certain limitations when combining fiberglass over the foam in an unvented roof). The code defines an air-impermeable insulation as having an air-permeance less than 0.02L/s-m2 (both open and closed-cell foam qualify).