A polyisocyanurate foam insulation product is produced from an isocyanate component and a polyol-containing component including a blowing agent. The polyol-containing component comprises one or more polyols, a fire retardant, and a hydroxyl-containing low-molecular-weight additive effective to increase the R-value per inch of the insulation product as measured at 40° F., as compared with an otherwise-identical formulation lacking the hydroxyl-containing low-molecular-weight additive. The polyisocyanurate foam insulation product may have an R-value per inch of at least 6.0 when measured at 40° F. and may have an R-value per inch of at least 5.5 when measured at 25° F. Suitable hydroxyl-containing low-molecular-weight additives may include triethanolamine, trimethylolpropane, N-methyldiethanolamine, 1,4 butanediol, or another suitable multi-functional alcohol or combination of multi-functional alcohols.