Polyurethane foams having a NFPA 101 Class B rating (ASTM E-84) which pass the FM 4450 Calorimeter Test are produced by reacting: (a) an organic polyisocyanate, (b) at least one polyether polyol or polyester polyol with a nominal hydroxyl functionality of at least 2.0, (c) a blowing agent composition and (d) at least one halogen-free flame retardant. The blowing agent composition includes: (1) no more than 10% by weight, based on total weight of the foam-forming composition, of one or more hydrocarbons having an LEL less than 2% by volume in air, and/or (2) a hydrocarbon having an LEL greater than 2% by volume in air, and (3) up to 1% by weight, based on total weight of foam-forming composition, of water.