The present disclosure provides blowing agents for polyurethane and polyisocyanurate foams, comprising Z-1-chloro-2,3,3,3-tetrafluoropentene (HCFO-1224yd(Z)) and cyclopentane in amounts of 65 wt. % to 99 wt. % and 1 wt. % to 35 wt. %, respectively. The present disclosure further provides foams formed from polyurethane or polyisocyanurate foams and blowing agents comprising Z-1-chloro-2,3,3,3-tetrafluoropentene (HCFO-1224yd(Z)) and cyclopentane having low initial lambda values.