A solar greenhouse is one in which the sun provides most, if not all, of the energy necessary to keep the inside temperature above freezing. A solar greenhouse captures energy of the sun during the day using water and gives it back at night. The amount of water to use will depend on the size of the greenhouse and the glazing.
Build a solid north wall. Use siding to close the outside and use bat insulation to fill gaps. Cover the inside with Oriented Strand Board (OSB), a type of sheet wood, or cheap plywood and paint it white to get sunlight to bounce back in the greenhouse to create natural heat.
Insulate the floor of your greenhouse. Dig a trench around the perimeter of your greenhouse, 3 to 4 feet deep and insert foam board insulation. Alternatively you can build a raised floor and use foam board insulation between the joists.
Use double wall glazing (glazing with two plastic walls separated by an insulating pocket of air) to cover the three remaining walls and the roof. Follow the directions that come with your glazing, as each manufacturer makes its own type of attachment. Use as thick double wall insulation as your budget allows. Choosing a thick glazing will improve efficiency.
Go around the envelope with a smoke generator (such as a smoke pen) to check for air leaks, as drafts or leaks will decrease the efficiency. Plug leaks with a silicone-based filler.
Calculate the square footage of your greenhouse's southern wall. Multiply the length by the width to get the square footage.
Calculate the number of drums you need for water. The rule of thumb is two gallons per square feet of the southern wall. A 10-foot by 8-foot wall is 80 square feet, which would need 160 gallons or three drums.
Buy or re-use drums that you own. Clean them and fill with water. If the drum is not black, use spray paint at least half of it black. The black side needs to face the south wall.