Blends comprising an aromatic polyester polyol and a 1 to 10 wt. % of a fatty acid derivative are disclosed. The fatty acid derivative is a C8 to C18 fatty acid ester or a C8 to C18 fatty acid amide. Also disclosed are rigid PU or PU-PIR foams that comprise a reaction product of water, a catalyst, a foam-stabilizing surfactant, a polyisocyanate, a blowing agent, and the polyester polyol/fatty acid derivative blends. Surprisingly, low-temperature R-values of rigid foams based on pentane blowing agents can be improved significantly by using blends of aromatic polyester polyols and a minor proportion of readily available fatty acid derivatives. In some aspects, the difference between initial R-values of the foam measured at 75° F. and 40° F. is at least 5% greater than that of a similar foam prepared in the absence of the fatty acid derivative.