So a kind person on the Greenhouse forum directed me to Farmtek to look at their water walls. These are thick black bags that hold hundreds of gallons of water to help with passive solar heat, which sounds like only really works if you indeed have hundreds of gallons of water. I have not yet built my hoophouse, but am planning it. It think it will be 11.5x 12.5, and covered with 6 mil agricultural plastic and possibly some clear bubble wrap. My question for all of you engineers out there is this: would the new xxl ziploc bags (they hold 20 gallons each) filled with water and sitting in some black garbage bags against one wall work as well as the farmtek water bags? The farmtek water bags for 350 gallons (which is about what I estimate I need for my greenhouse) will run me about $500. The xxl ziplocs (for 18 of them) will run me about $40 and some change. I realize that this will be nowhere near as sturdy as the farmtek bags, but I am hoping if I don't jostle them too much or move them too much, they might last me one season at least. Then I can see how well the passive solar heating will work for me. Anyway, does anyone have any thoughts? Thanks! Kim