Noticed you haven't had any replies, maybe you should post your question on the greenhouse forum. I built a greenhouse which I finally got enclosed early this spring. It is attached to my basement (South end is ground level) I used two small fans (one to blow warm air into greenhouse and the other to draw air back into basement at the other end)
Depending on where ya live in Texas..Since I live near Houston and I don't get many frost nites, I use Xmas lights near the floor of my hothouse..turned on by timer from dusk to dawn...so what zone are you??
It is a 10' x 10' made out of pvc and covered with plastic material. I do not know how many layers the plastic is. How can I find out? My husband made it from stuff he bought at Home Depot. I read somewhere that you can't heat a greenhouse made of plastic. I guess they were saying that the heat would just leak out. I am hoping that is wrong because DH worked SOOOO hard on it.
We have a lean-to greenhouse on the side of the house. After seeing the temp ranges the first year, we took the outlet from the furnace (where it vents to the outside), and piped it into the greenhouse with a 3" black PVC pipe. When the furnace kicks on, it puts warm, moist air into the greenhouse, and it's FREE because that air would just be going outside. I've thought about pushing the clothes-dryer air, too, but it's on the other side of the house and would be a hassle.
Anyway, this works great---when it gets cold outside, it stays about 10 degrees warmer in the greenhouse, which is all I need for seed-starting and frost protection.
pigeon,please tell me you're kidding about the furnace vent!!!! If you work in there for any length of time be aware of the warning signs of carbon monoxide poisoning,headaches,sleepiness,nausea. There's a reason the furnace was vented to the outside.