How to Keep a Greenhouse Above Freezing

All greenhouses require some form of heat during the winter.

Greenhouses should be orientated to face south for maximum solar energy gain. The sunlight, solar gain, can then be collected and stored passively during the day. The heat collected by the solar gain is then released during the colder evening hours to keep the interior of the greenhouse above freezing. Retaining heat inside the greenhouse structure may not be economically feasible as this entails adding a second layer of glazing to the exterior. The best method is to add mass to the interior, that will collect the solar heat and then emit that heat when needed.

Things You'll Need

• Tape measure
• 55-gallon metal barrels
• Black paint
• Paintbrush
• Pliers
• Garden hose

Instructions

1. Calculate the amount of water storage required for the greenhouse by measuring the overall square footage of the glazing. Use the tape measure and find the length and width of the plastic or glass that is covering the greenhouse.

2. Multiply the width and length together to find the square footage. Each square foot of glazing requires 4 gallons of water for heat storage during the evening hours. In other words, a single 55-gallon metal barrel will store enough heat gain for 13.75 square feet of greenhouse glazing.

3. Move the empty 55-gallon drums to an outdoor location. Paint the barrels with an even coating of black paint with the paintbrush. The black paint will aid in the absorption value of the metal barrels. Allow the paint to dry for at least 24 hours.

4. Line the painted 55-gallon metal barrels against the rear wall of the greenhouse. Stand the barrels on end so the large bung holes, filling holes, are facing upward. Remove the bung hole plugs with the pliers. Be sure the barrels are sitting level and in the exact location where you want them. Once the barrels are full of water, they are hard to move due to the excessive weight.

5. Fill each barrel with water from the garden hose. Replace the top bung hole cover after the barrel is full. The water-filled barrels, painted black, will absorb the solar heat gain during the day.

Tips & Warnings

• Keep all items from the front of the barrels so direct sunlight will physically hit the painted surface.

References

• "Passive Solar Energy"; Bruce Anderson & Malcolm Wells; 1981
• "The Passive Solar Energy Book"; Edward Mazria; 1979

Resources

• University of Missouri: Model Green