I am interested in building a greenhouse from recyclable materials. It seems like people are giving away windows, etc on Craig's list with great regularity. However, we get hot in the summer, like most places, and I don't want to buy an electric roof opener, fan, ac etc. I would just like to extend the season for annuals and maybe keep some things that more heat than we naturally provide or protect from our coldest days. Do you have any advice for building a temporary greenhouse for the cold half of the year?