I am planning a greenhouse and was trying to come up with an economical way of heating and cooling it. I was thinking of laying a vapor barrier then foam insulation then a pex heating loop covered by 6 to 8 inches of sand covered by a few inches of pee gravel. Heat source would be solar with gas back up. Do you think this would work or would it be so inefficient as to be a waste of time. Also thinking about a simple cooling system that would be a fan with cooling pads that I will pump pond water over with a gravity return to the pond. Not sure how much I will increase my evaporation losses to the pond, Iâm concerned it may be considerable. May have to go to an inclosed exchanger. Does anyone know of a formula to figure how much surface area per volume of air, water temp or do I just guess?