A polyester polyol composition comprising a phthalic anhydride-initiated polyester polyol, a C4-C6 hydrocarbon blowing agent, and an alkylene oxide modified silicone glycol compatibilizing agent having an HLB of from about 5 to about 8 is disclosed. The blowing agent is soluble in the polyol composition, thereby reducing the risks associated with such blowing agents in processes for making rigid polymer foam articles and providing rigid foams having good dimensional stability and improved insulation properties. An alkylene oxide and isocyanate modified silicone glycol compatibilizing agent is also disclosed.