So I got one of those 6x8 polycarbonate Harbor Freight Greenhouses for Christmas....I still have not put it up yet but will do so in about another week. My Question is what would be the best way to heat it?....by best I mean cheap and effective. I live in Pennsylvania,zone 6b . I would like to start seeds in this greenhouse and grow a few containerized tomatoes.
A thermal mass in the form of a lot of hot water can provide significant BTUs in your greenhouse. There are two ways to go about it: (1) the nerd way, and try to calculate everything, the heat loss of all the surface area of the greenhouse, and try to replace that with buckets of hot water, candles, electric blankets, etc., or (2) the empirical way, get a thermometer that registers the nightly low and compare that to the low outside the greenhouse and see how much of a difference you have to make up (using hot water, candles, hair dryers, etc.).
My greenhouse is 2-ply 6 mil polyethylene on 2x3 studs, 8'x20', and 3 5-gal. buckets of hot water keep me above freezing down to about 20F. Down into the teens, I light some candles and keep them going all night as well. Of course here in GA, our soil temperature rarely drops below 50F, so there is always ground heat coming up, and the greenhouse keeps plants warm mainly by preventing radiation out into the clear night sky.
Diego Footer on Permaculture Based Homesteads - from the Eat Your Dirt Summit