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Design & Construction

Section 1: Important Characteristics of Greenhouse Glazing Materials

The greenhouse glazing generally refers to the translucent (allows light through) greenhouse covering. There are many types of greenhouse glazings and each has specific characteristics. The characteristics of each glazing dictate its best use and its limitations.

Many factors need to be considered when selecting a glazing material. The life of the material, its strength, its weight, initial cost, light transmittance, thermal conductance, maintenance issues and flammability are all very important factors.
Light transmittance
The higher the light transmittance of a glazing, the higher the amount of sunlight that can penetrate the glazing and potentially enter the greenhouse. In northern climates and in the winter, light is often the limiting factor for photosynthesis. Therefore, maximizing the amount of natural sunlight entering the greenhouse is desirable and in most situations, the highest possible light transmittance level is desirable in a greenhouse glazing.  Sometimes, such as in summer or in southern or equatorial locations, the amount of light entering the greenhouse is above optimal levels. In these situations, a shade cloth or shading compound may be used to temporarily reduce the amount of light entering the greenhouse (discussed in more detail under the “Lighting” learning unit). When light levels drop below optimal, the shading material is removed.

Light transmittance of a glazing is not constant. As glazings age, they tend to have a reduction in light transmittance due to scratching from dust and debris and aging or "yellowing" of the glazing material due to U.V. exposure. Additionally, light transmittance values (such as those listed in Table 1) are the percent of light that passes through a clean unobstructed panel of the glazing positioned perpendicular to the light source. The light levels actually entering the greenhouse and reaching the plants will be lower than the listed transmittance of the glazing because of light being blocked by the supporting structure (as well as affects from the STET of the sun to the structure and any debris on the glazing). For example, in one glass-glazed greenhouse tested, the percent of light reaching the plant canopy was only 56% of the outside light level. In a double polyethylene greenhouse tested, the percent of light reaching the plant canopy was only 45% of the outside light level.

Thermal conductance
Thermal conductance refers to heat loss from inside of the greenhouse through the glazing to outside of the greenhouse. In other words, how readily does heat energy move through the glazing.

Thermal conductance may be expressed as Btu loss/ ft2/hr/(oFinside - oFoutside).  However, when evaluating glazings, it is most common to compare the “U” and “R” values of a glazing to determine their heat loss potential. The “U” value is the overall coefficient of heat transfer and includes all elements of construction. It is basically a measure of heat loss from a structure that is glazed with a particular glazing. It is important because different types of glazings are mounted to the greenhouse differently and these mounting materials can also conduct heat. The lower the “U” factor, the lower the rate of heat loss will be from the greenhouse structure. The “R” factor is a measure of the resistance to heat flow. The higher the “R” value the more resistant to heat flow and the more well insulated the glazing.

In most situations with greenhouse glazings, the “U” and “R” values are inversely related. It is desirable in a greenhouse glazing to have as low a “U” factor and as high a “R” factor as possible.

The stronger the greenhouse glazing, the more resistant it is to breakage from debris or weather events such as high winds and hail. Therefore, the higher the strength, the lower the probability of breakage and the resulting costs associated with replacing the glazing. However, often glazings with a high level of strength are not very flexible.

The heavier the glazing material, the higher the dead load on the structure. To account for the increased dead load, a stronger support structure is required. This results in increased costs and may result in a reduction in greenhouse light levels due an increase in obstructions by the supporting structure (i.e. trusses blocking light).

Life span
A short glazing life span means frequent replacement. Therefore, the initial cost of the glazing may be low as compared to other glazings, but after the glazing is replaced several times, it may become less economically attractive than one with a higher initial cost and a longer life span.

Scratch resistance
Dust, soil particles and other debris can scratch the glazing. Scratching reduces the light transmittance of the glazing and can therefore result in reduced light levels inside of the greenhouse. This in turn may require more frequent replacement of the glazing and increased cost.
All aspects of the cost of a glazing need to be considered. These include the initial cost of the glazing material, structural support costs, life span of the glazing and thermal conductance of the glazing. A glazing material that has a high initial cost when compared to other glazing materials may be more economically attractive if it has a long lifespan or has a low thermal conductivity.

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