We lose heat from the greenhouse through your glazing (glass or plastic) at night. When you have two or more layers of glazing you gain insulation from having dead air space between the layers. Some glazing has as many as five layers thick as in the case of this polycarbonate pictured to the right. Sometimes you can forgo night insulation because you have a glazing with multiple layers. By the way, the more layers of glazing you have, the cooler your summer daytime temperatures will be. But, there is a trade off in light transmission. I wouldn’t go with more than a three layer polycarbonate if you live in an area that doesn’t have many sunny days. I live in a sunny location and do fine with a five layer polycarbonate.
A cheap way to add another layer of glazing is to add a bubblewrap material to your glazing.
You can also add insulation by using a insulation barrier. Commercial growers have long used aluminized curtains for both holding in the heat and to provide some shading when needed. Styrofoam beads have been blown in between glazing layers to provide night insulation but have suffered from static electricity problems, making the beads adhere to the glazing. This was first experimented with at Kansas State University by Architecture professor Gary Coats back in the 1980s.
More recently a number of people have been experimenting with the use of soap bubbles to insulate between greenhouse glazings (see video below).
- Aluminized curtains: http://www.greenhousecanada.com/content/view/2791/38/
- Styrofoam beads: http://research.archives.gov/description/555305
- Soap Bubbles: http://www.youtube.com/watch?v=3D516P0UYik