Conserving Energy in Greenhouses

Double poly - Double polyethylene (poly) coverings reduce heating costs about 50% compared to single poly coverings. Most greenhouses in Virginia that are used for winter production are inflated double poly houses. Different polyethylene films vary from 35% to 60% heat loss. Ask your supplier about the film's thermal value. Selecting films that reduce water condensation will enhance light transmission and improve heat retention. Maintaining proper inflation between double poly layers is critical to maximizing the insulation value of the covering.

Winterize openings - A tight greenhouse with few air leaks around vents, fans or doors will cost less to heat. Repair any holes in the plastic, glass or doors. Keep doors closed and caulk or weatherstrip door frames and other openings.

Maximize the Insulation
Endwalls - Insulate the endwalls of the greenhouse, especially the north endwall. In most parts of Virginia, the north endwall provides very little light for crop production. This wall can actually be constructed of a solid material like wood. Plywood (1/2-inch thick) will lose about the same amount of heat as a double poly wall. At least, insulate this wall for winter production. Reflective (foil backed) insulation boards provide better insulation than other rigid foam boards. Place them with the reflective side facing into the greenhouse. If possible, add windbreaks outside the greenhouse along the north wall. These may be conifers planted for screening or a temporary fence material to divert the wind over the greenhouse. The south endwall can be insulated with an extra layer of plastic.

Foundations on new construction - On new construction, foundation heat loss can be reduced by half through the installation of 1 to 2 inches of polyurethane or polystyrene insulation. This insulation should be installed 1.5 to 2 feet deep around the foundation wall with care given not to leave gaps or openings. This is especially important when installing any type of floor heating system.

Existing foundation and side walls - If the foundation of the greenhouse was not insulated during construction, make sure that all gaps or holes below the foundation board are filled or repaired. If the greenhouse has a concrete kneewall, insulating the inside of it with insulation board can significantly reduce heat loss. Reflective insulation boards can be added to the inside of any flat greenhouse wall but should not extend above the crop or bench height. Leave a small airspace between the insulation and the sidewall to prevent freezing of the greenhouse wall. Be sure that the reflective surfaces are not in contact with perimeter heating pipes. Sidewall insulation can reduce annual heating costs 5% to 10%.

Fans and vents - To reduce other air leaks, insulate secondary fans and vents to reduce heat loss through unused areas during the winter. Do not cover all of the vents; remember that winter ventilation is required for humidity control and to restore the oxygen/carbon dioxide balance in the greenhouse. Keep these vents in good working condition so that they close tightly when not in use.