Polycarbonate and acrylic panels are popular alternatives to glass for covering greenhouses, but be sure to purchase specially designed panels for greenhouse use. Polyethylene plastics are energy-savers because of their double-layer of insulation, which reduces heat loss 20 to 30 percent. Greenhouse-grade film plastics, if applied properly to the frame, last 18 to 30 months in Colorado.
It is difficult to have two separate areas for warm and cool temperature requirements for specific crop needs in a small greenhouse. But all plants grow better when the night-time temperature is 10 degrees Fahrenheit lower than the day-time temperature.
A good heating system is also important in Colorado’s climate. As a general rule, you will need 200 BTUs per hour per square foot of ground area to keep your standard-design greenhouse at 50 degrees inside when outside temperatures are 10 degrees below zero.
Greenhouses collect solar heat, and on sunny, winter days when the temperature outside is 20 degrees, the temperature inside the greenhouse can reach 65 degrees and above.
When designing your greenhouse, include vents to allow air to be mixed and exchanged. You can effectively cool greenhouses smaller than 200 square feet of ground area with an evaporative cooler. Larger greenhouses are easier to cool with a fan-and-pad system. Special non-loading fans and cooling-pad systems are available through greenhouse supply companies.
Greenhouse heating and cooling systems should be controlled with thermostats placed at bench level in the center of the greenhouse, preferably in a protected area. You also might want to invest in a battery-operated high and low temperature alarm system.