Solar Power

• Most of the heat that keeps a greenhouse warm comes from the sun, which is why greenhouses are so often made entirely of glass. When siting your greenhouse, be sure it is oriented so that its largest surface area faces due south and it isn't blocked by buildings or overhanging trees. Building the south wall of the greenhouse so it is angled to be perpendicular to the rays of the sun will increase the amount of solar heat that enters. Every bit of heat that you gain from the sun is heat that you don't have to pay for by burning oil or using electricity.
  
  

Insulation

• Insulating the parts of the greenhouse that sunlight isn't entering through can help retain the heat that is gained from the sun. Building a greenhouse so the northern wall is set into a hillside will reduce heat loss from that area. Insulation can also be applied by covering the roof and walls of the greenhouse with thick blankets or insulating covers at night or during very cold, sunless days in the winter. Before spending a lot of money on complicated ways of heating your greenhouse, take steps to make it as efficient as possible so that you can hold on to the heat that you do create.
  
  

Evaporation

• Moisture levels in a greenhouse are as important as temperature. Depending on what type of plants you are growing, you will need to protect them from either drying out or becoming waterlogged by keeping the humidity within acceptable boundaries for those plant species. The easiest and cheapest way of reducing humidity in a greenhouse is to outfit it with hatches at the top of the roof that can be opened to allow hot, wet air to escape. Moisture levels can be increased by putting out large shallow pans of water in the hottest part of the greenhouse and allowing it to evaporate.
  
  

Thermal Mass

• The interior climate of a greenhouse can be regulated by increasing the thermal mass within the structure. Thermal mass absorbs the ambient temperature over time, causing the greenhouse to stay in its current climatic condition for a longer period. A greenhouse with high thermal mass will retain its heat during a cold night. Thermal mass can be achieved by building the greenhouse with a large and thick concrete slab or through alternative means, such as stacks of 55-gallon drums filled with water and painted flat black to absorb the heat of the sun.