You might try asking your questions at the greenhouse Structure Forum http://forums2.gardenweb.com/forums/strucs/

that is if you don't get enough info. here. They have lots of people with the experience and expertise that you may find valuable. Pat

What plants are you trying to protect?
I have a 9x12 glass greenhouse with some of the side windows missing. It gets pretty hot in summer and is pretty much outdoor temps in winter, although it gets warm during the day with the sun shining through the glass.
I can grow tomatoes there in the winter, even though we get some freezing temps. The glass keeps the frost off the leaves and that is enough for a lot of tropicals/sub-tropicals.
For instance, there is a 15 or 20 yr. old hibiscus planted in one corner of the greenhouse (it was originally a habitat for desert lizards that liked eating hibiscus). It never freezes. I doubt if I could keep coleus alive in there over winter, although I did keep a Tacca alive 1 year, but most all the other potted tropicals that I throw in there for winter survive just fine without additional heat.

If you really want the heat, then I would probably opt for the frame and shade it during summer.

wanda

I also have a 9x12 glass greenhouse. I have given up on summer use for anything but a potting shed. I have a clear plastic covered carport structure, both ends open I use for summer growing. I am thinking of closing the ends with roll up plastic to give enough winter protection to perhaps keep coleus through the winter. I can heat the greenhouse, but cannot afford it. Unheated it will sometimes get down to about 45 degrees part of the night. I am trying to live with that accepting some losses. Al

thanks for the advice. I think reality is setting in. If I want to grow true tropicals it will require a green house with both heating and cooling. I can concede the challange of trying to stretch the zone limits. I'll try to content myself with the limitations of the climate, and arrange the plants for their best advantage. thanks

I am in zone 9 and I grow bananas, guavas, lemons, pummelos, grapefruits, avocadoes, also the cold hardier subtropicals like kumquats, calamondins, mandarins and other citruses. I also have bougainvilleas and madivilla.

There are microclimates around the house which I take advantage of. I have pepper that has survived 5 winters now. there is also a spot where tomatoes won't die even when outside temps go down to 22 deg F.

A cheap burlap from home depot is a very good winter protection that you can place over your plants also good sunshade. In fact there is a mildew resistant sunscreen fabric at Home Depot that are used as Patio Covers and you can build a frame and permanently use that one over your plants. Costs only $29.99 for a 6' x 20' roll and would last 15 years at least. It could serve as frost blanket and summer shade.

You can buy a greenhouse with plastic clear covering over a metal tubing frame for something like 100 bucks these days. It is maybe 6 x 6 feet. Shelves on each side. Portable. One end zips open or closed or you can tie it open just a certain amount.
I've got impatiens and a tomato growing in there now along with some other random stuff. It is fairly handy and as long as it is placed in partial shade, it doesn't get broiling in Summer, but then again it is useful mostly only in Winter & Spring, at least for me anyway.