Posted: 01/03/2013 at 08:01
The key to keeping anything warm is insulation.
An eskimo igloo is heated purely by the body heat of its occupants. The eskimo don't have heaters of any kind. It can be -40C degrees outside, but it's as warm as +10C inside.
A human being gives out about 100Watts of heat - only as much as a small lightbulb. But a couple of people is enough to keep an igloo warm. An individual human being in an efficient sleeping bag can easily remain warm in sub-zero conditions.
So in terms of keeping a greenhouse warm, it's essential to elimate any draughts, and maybe use more than one layer of bubblewrap. Though additional layers of bubble wrap will reduce the light, which is important for seedlings. And it's easier to keep a small volume warm than a large volume.