Did you use any weather stripping? I find the Harbor Freight GH to be leaky. I caulked the windows in as well, but thought that was supposed to help hold them in rather than keep cold out. I am putting in door seal on the doors tomorrow. The blog recommends using a rope-like foam insulation on the crown. I plan to also put some above the windows. I also use heaters. See the Harbor Freight Greenhouse thread in the Greenhouse Forum.