A rigid polyurethane foam formulation comprising a polyol composition comprising, by weight based on the weight of the polyol composition, more than 70% of at least one polyester polyol having an average hydroxyl number of from 150 to less than 300 mg KOH/g and an average functionality of at least 2; a blowing agent comprising water and an auxiliary blowing agent; a silicone copolymer surfactant; from 1% to 5% by weight based on the weight of the polyol composition, of a cyclic siloxane having a surface tension less than 21 dynes/cm at 25° C., wherein the weight ratio of the cyclic siloxane to the silicone copolymer surfactant is from 0.6 to less than 2.27; a catalyst, and optionally a flame retardant; and a polyisocyanate; such that the isocyanate index is in the range of from 180 to 500; a rigid polyurethane foam formed from the foam formulation; and a method of forming a rigid polyurethane foam.