Greenhouse Structures and Design
Selecting Greenhouse Covering Materials
There are many greenhouse configurations, both in design and dimensions. No two greenhouses are exactly alike, if one considers the overall size, dimensions of length, width and height, number of bays, orientation, etc. The covering material or glazing is one most important of the component systems for the greenhouse. Even when a greenhouse covering material offers strength, consistency, durability, manufacturing quality control, and cost, other factors should be considered. These include response to environmental conditions such as light diffusion, transmission of phosynthetic light (PAR), the R-value (insulation measurement) of the covering, condensation control, and how these interact with glazing/superstructure systems. There are plenty of options available when selecting greenhouse-covering materials, and they vary among glass, rigid plastics, and plastic films. No covering material is ideal. The following sections focuses on a number of selection criteria and the specific attributes of the various covering materials.
Properties of Greenhouse Coverings
There are a number of properties (See Table 1.1) that may be found associated with greenhouse glazing materials; it is important to have an idea of what they mean, as this will help to determine the quality and performance advantages from one material to the other, and will “level” the playing field between proprietary materials. Some of the more common properties are discussed below.
The U-value is the measure of the amount of heat that the material will transmit through a material, such as greenhouse glazing and is measured in British thermal units per square foot per degree Fahrenheit per hour (Btu/ft2-°F-hr).
Thermal transmission is the ability of IR radiation (heat) to travel through material and is particularly an issue in greenhouses on cloudless winter nights. Shortwave radiation from the sun passes through the glazing and heats the greenhouse plants, soil, and structures. As the plants, soil, and greenhouse structures become warmer than the surrounding temperature, they emit heat in the form of IR radiation.
Photosynthetically Active Radiation
Photosynthetically active radiation (PAR) is the measure of the amount of sunlight needed for photosynthesis and is regarded by many horticulturalists as being critical for proper plant growth and development. Optimal PAR wavelength range is between 400 to 700 nanometers. Typical PAR values glazing materials are:
Plastic Material Additives
Some polyethylene film as well as rigid FRP, acrylic, and polycarbonate panels are available with can be formulated to control or utilize more effectively the heat and light energy from the sun, and also heat energy radiated from the soil. These forms of energy are part of the electromagnetic spectrum and differ only in their wavelength. By using different polymers and additives, it is possible for films to transmit, absorb or reflect different wavelengths preferentially. It is possible, therefore, to create a wide variety of microclimates by using different types of film, either as covers over crops or as a mulch on the soil surface.
Water vapor condenses on the cold inner cover surface forming small droplets of liquid water. Condensation is particularly a problem in the morning when, due to a cold greenhouse covering and humid air inside, a thick layer of droplets often forms. This has negative consequences on light transmission.
Most polymers are poor electricity conductors, particularly prone to the accumulation of static electricity when two surfaces are rubbed against each other or when there is friction caused by the wind.
Light Diffusion Materials
In areas with clear skies and high solar radiation, direct radiation can cause leaf burning in greenhouse crops on warm days. New plastic films have been developed to increase the percentage of diffuse radiation in the greenhouse.
NIR-blocking Plastic Materials
Only about half of the energy that enters a greenhouse as sun radiation is in the wavelength range useful for photosynthesis (PAR: photosynthetically active radiation). Nearly all the remaining energy fraction is in the near infrared range (NIR). During the day, objects such as plants, the greenhouse frame and covering, and soil warm by absorbing light, particularly in the infrared (IR) range. When it becomes colder outside, such as at night, the warm objects re-radiate thermal energy to cold objects outside, such as the ground and the sky.
UV-blocking materials absorb or reflect specific wavelengths of light; these films can manipulate plant growth (enhance yield or quality) or suppress disease or insect pest growth or behavior. For example, UV-blocking materials have the potential to suppress grey mold of tomato and cucumber, and downy mildew of cucumber, as well as whiteflies, aphids, thrips, and leafminers. Red films such as Dupont IR and Smartlite Red film reduce PAR light and create a shading effect.
Click on the following topics for more information on greenhouse structures and design.