Glazing
Traditionally,
greenhouses have been made from glass, which maximizes light transmission
and reduces maintenance compared to plastic coverings. Glass requires
only regular cleaning and sealing of the edges whereas plastic film must
be replaced every two to three years. Glass is expensive, heavy, hard
to work with, and relatively inefficient in terms of energy conservation,
however. Glass must be tempered before it can be used as larger panels
(up to 1.8 m by 3.6 m) which reduce structural shading and breakage from
hail [16]. This further increases
the cost.
Rigid clear plastics used in greenhouse construction, include fiberglass reinforced polyester, polycarbonate, acrylic (polymethymethacrylate) and polyvinyl chloride panels [17]. Although some of these are energy efficient and have good light transmission, and last at least 10 years, they are more expensive than polyethylene films [18]. Acrylic and fiberglass panels are also potential fire hazards. Thus, it is more common for new greenhouses to be covered with a double layer of polyethylene film than with glass or rigid plastic panels. Compared to glass, plastic panels are generally stronger and so require fewer support frames. Thus they shade the house less. However, insulated double-walled plastic panel reduce the rate of snowmelt compared to glass or plastic film, so more snow accumulates, which reduces light and can potentially collapse the greenhouse. With plastic films, which are also energy efficient, the double layers collapse together when snow accumulates, increasing the rate of melt. There are a number of different types of greenhouse plastic films available [19], including low-density polyethylene, polyvinylchloride and ethylene vinyl acetate copolymer, but polyethylene is the most common film is the US. It is now possible to co-extrude plastics or to extrude multiple layers of plastics, which offers the opportunity to create an infrared barrier, reduce condensation, and reduce disease or insect pressure or regulate plant growth through wavelength selective plastics. Wavelength-selective plastics have not been widely tested, however.
Plastic coverings must be replaced every 2-3 years to maintain high levels of light transmission although the useful life and strength of the film can be extended by the addition of 1-5% vinyl acetate [20]. Temperature extremes and air pollutants further reduce life expectancy of the plastic. Air pollutants can be either external or internal, such as pesticide sprays. Degradation from high temperature is accelerated where the plastic contacts the greenhouse structure and if high temperatures persist.
Compared to glass or rigid plastic panels, polyethylene film greenhouses have low air infiltration from outside because of the continuous plastic film covering. However, this also contributes to higher humidity levels, and flattened arch-shaped roofs contribute to dripping of condensate, subjects discussed further in the environmental control section below. Plastic film greenhouses also require fan ventilation for cooling rather than ridge vent openings for natural ventilation. However, recently designs have become available for plastic film greenhouses with roof openings [21].
Frame Types and Greenhouse Orientation
Greenhouse frames are generally made of aluminum or galvanized steel. The shape varies (Fig. 6), depending on the expected snow load, whether a number of houses are to be joined, whether the covering is to be glass or plastic, and the height of the crop to be grown. The straight sidewall greenhouse with arch roof is probably the most common shape because it can be covered with double-layer plastic and connected to other houses at the gutters to expand the growing areas. The height to the gutter is as high as 5 m in some European-style greenhouses. More commonly in greenhouses in the southeast it is 2.5-3.5 m. High sidewalls increase the greenhouse area relative to the growing space, thereby increasing heating costs, but also make the environment more uniform and stable. This is particularly important where the ridge vents open to provide passive cooling. Hoop styles greenhouse are inexpensive to build, and cheap to heat, but space near the sidewalls cannot be used efficiently with a tall-growing crop such as tomatoes. Gothic arch frame structures, which have a peak at the top, but curving sides, can provide adequate sidewall height without loss of strength and can be free-standing or part of a range of multi-span, gutter-connected units.
A free-standing, single-bay greenhouse generally provides more light than a gutter-connected, multibay greenhouse because there is less overhead structure [22]. However, in all types of greenhouses, the most important feature is to have as little of the overhead area as possible obstructed by the framing, shades, lights, fans, or other greenhouse devices. Another important consideration is greenhouse orientation. Tall crops, such as tomatoes, grown in gutter-connected, multibay greenhouses should be oriented with the gutters (or ridges) north-south to improve light uniformity to all locations within the growing area [23]. Shadows in the house will move east to west during the day, and no single location will be shaded all day.