You can save a considerable amount of energy in the greenhouse with a few simple steps.
1) Insulate the greenhouse foundation (See Foundation FAQ).
2) Insulate the north wall and north roof. Little light comes through the north wall of a greenhouse in the northern hemisphere, so insulation of this wall helps with energy losses without sacrificing any sunlight. One approach is to install foil-backed foamboard insulation, with the foil side facing into the greenhouse.
3) Seal off vents, weatherstrip doors, and caulk or otherwise close off other areas of air infiltration.
4) Double up on glazing. A double or triple layer of glazing can be added with an additional layer of polyethylene or with the addition of "bubble wrap."
5) Add thermal mass to the greenhouse (See FAQ on thermal mass).
Information provided by: Stressbaby