In doing energy audits on several hundred greenhouses, it has become apparent that there is considerable heat savings and a good payback to insulating the wall areas of a greenhouse that are not important for sunlight. Payback is usually less than 2 years. Compared to the insulated wall in your home, a double wall glazed greenhouse wall loses more than 20 times as much heat. Heat loss is measured by "R-value", the resistance to heat flow. The generally accepted value of R for a standard 6" thick house wall with fiberglass insulation is R = 30. The standard greenhouse wall with either double wall polycarbonate or air inflated poly is about R = 1.5.
The savings that can be obtained varies with the length of the heating season, the growing temperature and the interior layout of the greenhouse. In a typical 30' x 100' freestanding greenhouse, the amount of wall area to three feet height is 15.6% of the total surface area. Insulating this area can reduce fuel use significantly and lower the heating bill. If additional area such as headhouse or storage walls can be insulated the savings increases.
What type of insulation can be used?
Insulation is available in many forms and materials. The most common materials utilize the principle that air is a good insulator. Air, trapped in foam bubbles or the tiny spaces in fiberglass insulation creates many barriers to heat movement. Some materials also utilize gas other than air that has a higher insulating value.
The air film at a wall surface also provides insulation. The effectiveness of the air film depends on the surface conditions and the air velocity next to the surface. The air layer on the glazing inside the greenhouse is almost four times more effective than the air layer on the outside in a 15 mph wind.
As much as 35% of the heat loss from a greenhouse can be attributed to radiation. During the day, energy received from the sun is converted to heat within the greenhouse. About 50% of this heat is converted to latent heat to evaporate water from plant surfaces. The remaining 50% is added to the plants, structure and soil. The use of glass or polycarbonate that naturally traps radiation or polyethylene with an infrared inhibitor will reduce this loss.
Aluminum has low emissivity (high reflectivity) that reflects heat. It is frequently added to insulation to increase the R-value. It works best when the surface is bright and free of dust and dirt and there is an air space in front of it. Adding insulation to sidewall and endwall areas can provide good energy savings. What are some of the options?
Low cost materials are aluminum foil or aluminized polyester plastic. I found a 40ºF decrease in greenhouse knee wall temperature by adding a sheet of aluminum building paper behind the heat pipes. Aluminum foil is available with a kraft paper or poly backing or polyester film with a reinforcing scrim to give greater strength. Cost is about 10¢/sq ft Installation can be attached to a kneewall, clipped to the frame or woven between the greenhouse frame and the plastic. If you have rollup sides, the material can be installed for the winter and then removed in the spring when ventilation is needed. The material should last several years.
A better choice is to install a double-bubble insulation with foil faces. This provides additional insulation due to the air space in the bubbles. It is available in rolls of 500 or 1000 sq ft in several widths. Cost is about 40 - 50¢/sq ft. This material is available from home centers.
For a more permanent solution and to get a greater R-value, rigid board insulation is commonly used. The material should be a closed cell material such as polystyrene or polyurethane. These materials are available in several thicknesses from ½" to 2" with R-values from 5 to 6/inch. The addition of a foil face on one or both sides can increase the R-value some. The insulation board can be attached to the sidewall with large head nails, tec screws or clips. I have seen where the aluminum foil breaks down after several years but the insulation is still good. This material is readily available in home centers and lumber yards. Cost is about 50 - 70¢/sq ft for the 1" thick material. Based on a double poly or polycarbonate wall and fuel oil at $4/ gallon, payback is about 6 months. With natural gas at $1.50/therm, payback is less than a year.
A permanent material commonly used for walls in gutter-connected greenhouses is Thermax Heavy Duty Insulation by Dow Chemical Company. It consists of a glass fiber reinforced isocyanurate foam core with 4 mil aluminum on one side and 1.25 aluminum on the other. Available in 4' x 8' sheets it can be purchased with a joint closure system for easy installation. R-value is 7.2 for 1" thickness. Cost is about $1/sq ft. This material will give good service for many years.
Adding insulation to bench height on the walls reduces fuel consumption by keeping the heat in and helps to offset increasing fuel prices. Cost recovery for insulation is one of the shortest of the energy conservation measures.
John W. Bartok, Jr., Extension Professor Emeritus & Agricultural Engineer
Department of Natural Resources and the Environment, University of Connecticut, Storrs CT - 2015