I just wanted to jump in here to say that I've found my cold frames are better for rooting cuttings than for seedlings. Because my husband did such a good job of making them insulated, they are a little too air tight for seedlings. The lack of air flow doesn't seem to pose a problem for the cuttings. I have had winters where I only watered the cuttings once and every single one took root. I think they get a lot of their water from condensation on the double glazed window which forms the lid.
I do have two small pre-fab greenhouses that are mounted on a south facing stone wall. I use them to start seeds in the spring. A lot of these seedlings are traded with friends and neighbors for other varieties. I do find that I need to start peppers, melons and eggplant in the greenhouse or they won't get enough of a head start. Our summers are usually three months of drought with very high temperatures during the day and cool temperatures at night. During the summer months my little greenhouses are far too hot for plants, but they do an excellent job drying fruit. Over wither, they house young or sensitive plants. I don't monkey around with worrying about trying to keep the inside temp above freezing. I sort of use my mini greenhouses like sun traps rather than real greenhouses. We had an awful cold snap this year which had lows around -15 C. Because I don't keep anything that isn't frost hardy up to -8 C, I didn't have to worry, which was nice.