Greenhouse Galzing
Properties of Greenhouse Glazing
Most glazing materials are manufactured to meet specific industry standards for strength and light transmission according to ASTM (Association of Standards and Test Methods) guidelines (Table 2.1). The NGMA Glazing Standards lists various ASTM test procedures that apply to typical greenhouse glazing materials. There are tests recognized by the NGMA, including those for measuring light transmission, tensile strength, flexural strength, impact strength, and more. Most glazing materials are manufactured to meet industry standards for light transmission according to the Association of Standards and Test Methods (ASTM) guidelines. Some of the properties to consider in the selection of the glazing material include photosynthetically active radiation (PAR), heat loss, thermal transmission, and life expectancy. There, too, are other factors that need to be considered in the selection of the glazing material, such as maintenance and installation costs, budget, tax laws, and zoning regulations.
Photosynthetically Active Radiation (PAR) Transmission
Glazing materials should have a high transmittance for photosynthetically active radiation (PAR). Photosynthetically active radiation is the amount of light available for photosynthesis and plant growth in the wavelength range of 400 to 700 nanometers, which approximates the same range as visible light.
Direct and Diffused Radiation
Based on physical properties, surface orientation (angle of incidence), and the types of glazing material, portions of the incoming light are either transmitted, reflected, and/or absorbed. On a cloudless day, most sunlight travels in a straight path through the Earth’s atmosphere. Under these conditions, the incoming light is termed direct radiation. On a cloudy day, the sunlight is diffused by the many water vapor particles in the moisture-laden air. This light is called diffuse radiation.
Heat Loss
The amount of heat loss from the greenhouse glazing material should be considered, especially in colder climates. The energy efficiency of specific glazing materials is rated by the manufacturers with either U or R values. Heat loss reported as the U-value is the measure of the amount of heat that the material will transmit through a material, such as a greenhouse glazing, and is measured in British thermal units per square foot per degree Fahrenheit per hour (Btu/ft2/°F/hr).
Thermal Transmission
Thermal transmission is the ability of IR radiation (heat) to travel through the 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.
Life Expectancy
The life expectancy of the glazing material is another important consideration. According to the NGMA, the usable life of glazing materials is defined as the number of months or years that a material will function effectively in a commercial growing application.
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