Decide on the purpose of your greenhouse. While the National Sustainable Agriculture Information Service points out examples of solar greenhouses used to both grow food and provide home heating, they note that because the two purposes have somewhat conflicting requirements, most people build a greenhouse to satisfy one or the other.
Choose a site. If you are in the Northern Hemisphere, the solar-collecting slope should face south. Sunlight should not be impeded by trees, buildings or other obstructions.
Design the dimensions. The University of Missouri Bradford Research and Extension Center recommends using a 2-to-1-to-1 ratio of length, width and height. Calculate the slope of the roof by adding 10 to the latitude of your location. For example, if your home is located at the 35 degree latitude, the roof slope should be 45 degrees.
Measure and outline the foundation of the greenhouse with stakes and string. Double-check that measurements that should match up do. For example, both short sides of a rectangular greenhouse should be the same length. Dig a trench 6 inches wide to accommodate the foundation beams. Use a level to be sure that the foundation is flat.
Construct the frame using 2-by-6-inch boards and add exterior walls.
Select and install a glazing material on the sun-facing slope. Several types of glazing materials are available for greenhouses. The National Sustainable Agriculture Information Service recommends plastic materials that repel ultraviolet rays, absorb infrared and include anti-condensation drip surfaces. Make sure that the glazing material fits snugly and seal up all cracks.
Insulate any part of the greenhouse not being used for solar collection or heat storage. Apply weather stripping to doors and vents. Line other surfaces with an insulating material, such as fiberglass batting.
Measure the height and width of the glazing materials and multiply them to obtain the area. Multiply the area times five to obtain the number of gallons of water needed for heat storage.
Obtain enough 55-gallon drums to meet heat storage needs. Paint them with non-reflective black paint to aid in heat absorption, stack them against the north wall (the south wall in the Southern Hemisphere) and fill them with water. Do not allow them to touch the wall, as this will cause heat losses during cold weather.
Paint all surfaces not involved in heat absorption with a reflective paint that will direct sunlight toward plants or heat-absorbing barrels.
Install ventilation to keep the greenhouse cool during the summer. While areas that receive frequent high winds may be able to rely on them for ventilation, according to the National Sustainable Agriculture Information Service, the Bradford Research and Extension Center's solar greenhouse required a fan to move air on still days.