All greenhouses are solar, but taking efficient advantage of those solar properties can significantly reduce the cost of heating your greenhouse. Capturing solar heat is not the most difficult part, however. Daytime temperatures are typically warm enough, but heat escapes quickly at night and the cooling that results can be significant. Preparing your greenhouse to minimise the loss of heat at night is the key to cheap, effective greenhouse temperature control.
Solar heat can be captured throughout the day by rocks and water-storage tanks strategically placed throughout the greenhouse. Water is the cheapest and most effective solar heat collector. Fill black or dark-coloured 30- or 55-gallon oil drums with water, and place them along the north wall to collect heat throughout the day. Even if you employ additional heating methods, water drums make any type of heating more efficient.
Greenhouses can be situated partially underground where there's a lot of thermal mass available to store heat energy. Excavate a pit in which to situate your greenhouse. Dig down to just below the frost line, where the soil temperature remains near 10 degrees C. all year. This allows the sun's energy that's stored below the frost line to warm the walls of the greenhouse even at night. The south-facing wall can be built into the ground or bermed up to the roof level, and then the roof can slope down from the top of the berm to the ground of the north wall. Every section of the greenhouse that's in the ground should have plastic or clay-sealed walls. Be sure your water table is at least 5 feet below the floor of the greenhouse, or you may have problems with water leakage.
Hold it in
The easiest way to heat a greenhouse is to conserve the heat that's already in it. Insulate at least the wall that receives the prevailing winds in the winter. This could be the north wall or the east and/or west wall. Buy reflective thermal material that's sold for insulating greenhouses, and insulate its foundation by digging down at least 2 feet and placing 1- to 2-inch styrofoam board up against the soil. Double- or triple-glaze the windows, or retrofit single-glazed windows or panels to double- or triple-glazing. Clear bubble insulation can be used for glazing, too. Seal the nooks and crannies on the greenhouse, ensuring that it's airtight everywhere. Use a night curtain of polythene film or vinyl to cover the top and sides to prevent heat loss at night.