I missed this thread, sorry.
If you have electric power you have endless possibilities. I agree with SH about a wood stove over a fireplace. I built a fireplace because I had no money to purchase a wood stove...and had 7 hours to get it up and running before a cold front came in. The cinderblocks (filled with cement) are actually very good at storing thermal mass much like a woodstove. Long after my fires are out the cinderblocks are still hot/warm to the touch.
I'd bulk up that PVC hoop house with long horizontal bracing using 2x2's and further brace up the ends.
6ml plastic is almost zero R-factor. Honestly I think it will be very difficult to keep it warm in there at night. Last night I had temps in the teens and lit a fire at 7 pm in my GH. I checked temps at 11 pm and inside it was 40 (not bad). I lit another fire anyways and checked in on it again at 2:30 am. It was 3o degrees and IMO thats about as low as you want it without damaging your plants. I lit another fire and checked things again at 5 AM and again it was 30 degrees inside. I lit it once more. Three separate fires, getting up in the middle of the night.. that's a lot of work...and not real good sleep. I can not imagine trying to keep temps up in Jan or Feb and doing that every single night.
If you go with pots you could use some type of electric grids (like electric blankets) and wrap the pots up. If you plant directly in the ground you will have to do some more work to keep the soil warm. Do a google for constructing a "cold sink." Basically this a ground area that is dug deeper than your garden area which acts as a cold sink for cold to gather thus keeping the heat above. There are also a few ways to heat the soil by burying flexible drainpipe underground and inducing heat into the hosing.
For winter the challenge is to keep it warm at nights. It will be a breeze to keep temps up in the daytime. Yesterday in my GH (in the day) it was 30 degrees outside and 75 degrees inside with one vent cracked open. I think the solution for a winter greenhouse is to build it out of wood (more beefy) and use some type of insulating covering like Solexx (double layers). Cover the north facing side with insulation and sheathing...as there is no need to use plastic where the sun don't shine. Maybe evn paint the interior north wall black as well (or another dark color..I like forest green). I tend to lean towards some type of electric grids and keep the plants in pots instead of in the soil. (less work) And if need be, the potted plants can easily be hustled indoors should the weather become super-nasty or if encountered by a growers worst nightmare, the dreaded power outage. As far as extra lighting goes? You can use T-5's for a cheap supplements of lighting. HIDs will cost more but those will also be a nice heat source as opposed to T-5s. If you can afford it I'd lean towards HPS or MP just because of the heat factor. They eat up more electrical juice though (than T-5s). Another option is heating the GH with those oil/electric heaters. Plain electric heaters (like baseboard heaters) are the worst type of heat in regards to cost-to-run factors.
It surely is a challenge. The thing to consider is how much money is this going to cost you to grow your own. Look at you initial start up fees (equipment and stuff) and then factor in your other costs like electric bills, cost of wood, etc...and don't forget to include your actual labor (personal time) that will be needed to keep everything sweet and productive. There sometimes comes a time where all your work and investments just won't make it worth your while. Therefore..try to keep it simple as possible..which often seems to be the most inexpensive way. It often comes down to creativity and trial and error.
Heres something you may want to think about: You could also tap into your homes heating system (ie gas furnance) and tee off a heat duct and pipe it directly into your GH. But again you'd have to monitor inside GH temps and see if they would hold you above 32 degrees.
SH made a good suggestion. An attached grow room off your house would be nice. And you use heat from your main house into the grow room. IMO this would be the cats ass. (I wonder how that expression came to be...cats ass.)
I COULD attempt a winter grow in my current set-up but the amount of labor involved steers me away from taking on the challenge. You would have to be married to your grow, never miss one night and constantly be a slave to your commitment. A damn rough row to hoe but OTOH not impossible. either. Lets us know what ya do and good luck.
