The greenhouse effect is what we use to heat up the house and to cool it down. Basically we put a huge south facing facade (in the northern hemisphere) and we let the sun shine inside. The 4 layers of acrylic sheet instead of glass and the thick insulation, will capture the heat within. Our biggest concern in the beginning of the project was overheating the house in winter. Even in the coldest climates. Read about the chimney effect and batteries and shutters. We use this effect for cooling. Check out this article: pre heated, cooled air for free.