The ideal glazing will transmit light well, yet not transmit thermal infrared, the wavelengths at which the solar energy absorbed in the greenhouse is reemitted. The ideal glazing will also have a high R-value, will be inexpensive, and will last a long time. The most common types of glazing are glass, polycarbonate, and polyethylene films.
Glass, the original glazing, is the longest lasting of the glazings (in the absence of large hail), particularly if the glass is tempered. It is expensive relative to other glazings. It transmits light very well, yet it is opaque to thermal IR. Glass does not diffuse light very well and may result in shadowing and plant burn. It is more difficult to work with and in fact tempered glass cannot be cut at all. Single pane glass has an R-value of about 0.8.
Polycarbonate (PC) generally comes in twinwall or even triplewall sheets in thicknesses from 4 to 16mm. It is easy to cut. Twinwall PC has an R-value of about 1.4, and triplewall PC has an R-value of 2.5. PC has a high degree of light transmittance and low thermal IR transmittance. Light is diffused with PC, lowering the risk of foliage burn. PC lasts 10-20 years.
Polyethylene (PE) films are relatively inexpensive and are the easiest with which to work. Single layer of PE film has an R-value of about 0.8 but and inflated, double layer PE glazing has an R-value of about 1.4. Light is transmitted and diffused well, but plain PE film is not opaque to thermal infrared wavelengths. Therefore radiation energy losses at night can be greater with PE films than with other glazings. PE with infrared additives are available, and these films reduce heat loss by about 20% compared to plain PE films. The lifespan of PE films is generally felt to be about 3-4 years.
Other less commonly used glazings include acrylic, fiberglass, and PVC.
Information provided by: Stressbaby
Entered by orchiddude
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