What can I use to keep greenhouse warm passively
I know people put 55 gallon barrels inside their greenhouse to keep it warmer at night because the barrels of water absorb heat during the day and release them at night. My little greenhouse is small and can only hold two 5-gallon containers. What material will work better than water in absorbing heat during the day so that I can maximize the heating capacity of the 5-gallon containers. Does anyone know if tar absorb more heat than water? With the tar containers, I will not have to paint the outside. :) Thank you for any suggestions.