Deciding on a covering system for a new greenhouse can be difficult due to the many options now available. The choice used to be glass, rigid poly or double co-poly film. Today, growers can incorporate energy conservation and shading into the mix. Energy conservation must be considered in many northern states, whereas shading is more important in southern locations. Three main factors affect the choice: light, energy conservation and shading. Here are a few suggestions that may help you make a decision.
Winter light is still the limiting factor for plant growth in most areas of the U.S., even in the Southwest, where the average daily light integral (DLI) is only 20 to 25 mol/sq. m/day (endowment.org/dlimaps). In most current greenhouses, only 50 percent to 60 percent of outside sunlight gets through to plant level.
Research continues to find better ways to capture and make use of natural light. Glass and plastic with nonreflective surfaces and better diffusion having up to 95-percent transmittance are becoming available. Using larger panes and smaller composite material, structural members can also increase the amount of light reaching the plant level by up to 10 percent.
Although great strides have been made in improving the efficiency of artificial illumination, the increasing cost of electricity is offsetting some of the benefits. For example, Connecticut power suppliers were given the go-ahead to increase electricity costs last year, and the new policy went into effect on Jan. 1.
Growers in northern climates are usually more concerned with heat loss during the winter. Energy requirements in cold weather are frequently 15 percent to 30 percent of production costs for many crops. To offset this, energy screen systems can be installed to reduce heat loss by 50 percent or more. New screen materials plus the use of multiple screens can also offset the loss of heat, compared to having a single-layer glazing.
Research has shown that using a translucent screen left closed both day and night on cold, cloudy days can both provide good light levels and save energy. The greater hours of use can more than offset the use of a screen having a higher U-value (meaning lower insulating qualities) but greater shading.
In warmer climates, high summer temperature is of greater concern than energy savings. Typical material shading choices are in the 40-percent to 60-percent range, but materials with a shade factor as high as 86 percent are available. By using multiple screens, the benefits of both shade and energy can be achieved.
Put all this together to choose a covering system:
For the glazing, select a lightweight material such as a corrugated polycarbonate or modified acrylic with a 90-percent light transmission. This allows a lighter greenhouse frame than if glass was chosen. A lighter frame means more light to the plants.
Add a shade screen that diffuses the light and provides the level of shading that the crop requires.
Install a transparent energy screen with 40-percent to 50-percent energy savings, translucent strips and a closed structure that can be left extended during the day without much light reduction.
If daylight exclusion is required, install a blackout screen material instead of the energy-saving material. Most blackout screens have a greater energy-saving rating. Select a material with one reflective aluminum surface. Face it up to reflect summer heat; face it down to reflect winter heat or supplemental artificial lighting.
The above suggestions will help you select a good covering system that will have at least a 10-year economic life. With the rapid development of new materials and production concepts, such as photoselective glazings and photovoltaic electric power generation panels, any structure with a longer life may be obsolete in a few years.