Natalie, My husband built my greenhouse a couple of years ago (15 x 30). It is timber frame, and he used polycarbonate for the walls and roof. Along the back wall, which is cinderblock and butts a hillside, we have put 55-gallon barrels all across the back wall, and they are double-stacked. The entire back wall has barrels. I think there are at least 10 on each side, but in the middle is a fish pool. They have lids, of course - since they are doublestacked, there is no way to use them. The barrels are painted black. We have had success with this method. It does add warmth to the greenhouse in the winter and, in fact, it keeps the greenhouse cooler in the summer. If you have more "technical" questions, please feel free to ask me, and I will ask my husband for the answers. I can also give you the names of a couple of books he used.
