If you just want to keep plants from freezing in winter, you might be able to wrap your frame with a couple layers of heavy-duty clear polyvinyl and hang lights, some infrared bulbs (for heat) or run an extension cord to a heater and get by. A greenhouse for both winter and summer is *much* more complicated than that and would require water, electricity, heating *and* cooling. You definitely don't need a greenhouse for spring/summer veggie gardening. If you intended to grow orchids or hostas, you would.

Here's one guy's take on building a "winter greenhouse". It's a bit more complicated than you'd think.

Here is a link that might be useful: building a winter greenhouse

I am also planning my greenhouse but since you have your frame up, you are farther along than I am.
What I have noticed is that cooling is a big concern here in the valley. I plan on building mine out of block,concrete, glass and steel. In my plans, I want to have the north side and west sides fully enclosed except for some shaded ventilation windows or openings of some sort.

The winter before last, I made a temporary greenhouse out of thick opaque plastic sheeting. I noticed that it seemed to actually be colder in there than outside unless I put the heat lamps in there. During the day everything roasted but then at night they froze (unless heated). So, I had to open the vent areas early in the day and shut everything up later in the day. Then in the early evening I would turn on the lamps. It worked and my pineapples, tomatoes, adeniums (yes, even these) as well as several other frost sensitive plants all lived.

I have noticed at some of the nurseries like Baker and Tropica Mango, they have plastic type enclosed greenhouses. The one at Tropica Mango is similar to what you are describing. The man there is very friendly and helpful. I'm sure he wouldn't mind if you wanted to check his out.

Keep us updated on your progress and please post pictures when you are done.

Except for the opening statement about locating a greenhouse where it "gets maximum sunlight", this is a really informative article about all the factors involved in planning and building one.

Here is a link that might be useful: building a greenhouse

I had a 16x24 "hoop house" in Tennessee - and true enough, the toughest part about a greenhouse is cooling, not heating - and it was much cooler there. Unless you have shade cloth or a really big tree with leaves in the summer, you'll fry everything.

The best feature of my house was the roll up sides - very simple to make and made a huge diff with the heat exchange. But once the air outside warmed up, ventilation fans were a necessity.

HTH!