The present invention provides a rigid polyurethane foam formed by the reaction product, at an isocyanate index of about 160 or less, of a polyisocyanate, a polyol component containing an aromatic polyester polyol, and a sucrose-initiated polyether polyol and a flame retardant component made from a chlorine-containing combustion modifier and a bromine-containing combustion modifier in the presence of a hydrocarbon or hydrofluorocarbon blowing agent. The inventive polyurethane foam contains, based on the weight of the foam, at least about 3 wt. % chlorine, from about 1 wt. % to about 2 wt. % bromine and less than about 1 wt. % phosphorus, and exhibits good properties, good processing and good adhesion to metal whilst satisfying the ASTM E-84 Class I burn requirement.