A 55 gal. drum full of
water?
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.
