I appreciate everyone's thoughts on greenhouses. I've been collecting materials for a couple of years now so I really need to decide if I'm going to do this or not. Right now I have raised beds which extend my season by a month or so (2 wks spring, 2 wks fall). Last year I put low tunnels over the raised beds just to see what would happen. We had fresh greens all winter long. The broccoli actually survived long enough to eat. I've declared it a success. I tried some early plantings but it seems germination is a problem in the early spring. So we'll be putting low tunnels on all the beds this fall, just to see how far we can nudge this idea.
I like this temporary set up. I can open the ends on a warm or sunny day to keep the veggies from cooking right there in the ground. As the days become shorter the plants don't grow very much but seem to be "on hold", staying nice and fresh for daily consumption.
Where I live spring is brief and HOT. So fall plantings and my little tunnels are the only way I've found to enjoy broccoli, peas and spinach. I'm still not sure a greenhouse is what I need so we'll see if it ever gets built.