An isocyanate-based rigid foam comprising the reaction product of an organic and/or modified organic polyisocyanate, a resin blend and, optionally, a relatively low molecular weight chain extender or crosslinker in the presence of a catalyst, and, optionally, further auxiliaries and/or additives, wherein the resin blend comprises a phthalic anhydride-initiated polyester polyol, a blowing agent comprising a C4-C6 hydrocarbon, and a fatty acid or fatty alcohol ethoxylate compatibilizing agent. 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.