When you are building a solar greenhouse, one of the important considerations is insulation. Many people have questions about greenhouse insulation. What to use, how much, and where? There are also special considerations when integrating insulation into a greenhouse, that are different from other buildings.
Ideally, especially in colder climates, only the south wall of a solar greenhouse should be glazed. The other three walls and the ceiling should be insulated. The type of insulation and the amount can be the same as for regular buildings. Check with your local building code for R-values for attic and wall insulation. If the north wall of the greenhouse is cement, which is good for thermal storage, insulate it with rigid insulation panels on the outside of the wall. If you have a perimeter foundation, insulate it with rigid panels, on the outside as well. The foundation insulation should go below the frost line, even if the footing does not. Extending the insulation a few inches below the frost line keeps the earth beneath the greenhouse significantly warmer.
One important difference between greenhouses and other buildings is the amount of humidity inside. For this reason it is very important to have a good vapor barrier between the insulation and the inside of the greenhouse. Otherwise you will have problems with mildew and rot. It is also important to insulate the glazing at night. Solar greenhouses should have double glazing, as well as insulation to prevent heat loss.
You can use insulating curtains or shutters. The inside of the shutters or curtains should be reflective to increase their heat retaining capacity. The rigid foam boards with foil on one side make great shutters. For a curtain, aluminized mylar attached to thinsulate works very well. Make sure your curtains are very durable, as they will get a lot of use, especially if they are on the outside of the greenhouse. Also make sure you have a tight seal between the greenhouse and the shutters or curtains. A lot of cold air can escape through a small gap.
Getting the design of your solar greenhouse is not too difficult, and planning it well can give you a greenhouse that does not need supplemental heat through even the coldest Winters.
