The covering material used on a greenhouse influences the productivity and performance of a structure. Covering materials impact on the level and quality of light available to the crop. Diffused light is better than direct light. Fluorescent and pigmented films can increase the proportion of good red light. Dust, attracted to plastic films, will reduce the transmission of radiation. Water droplets on the inside of coverings have been shown to reduce light transmission by 8% and will also block thermal radiation.
Greenhouse coverings all reduce light to some extent. As coverings become dirty and as they get older, less light enters the greenhouse. Condensation (water drops) on the covering material also reduce light. Light coloured materials in the greenhouse, such as white weed matting, increase the light available to the crop.
Key characteristics that should be considered in selecting a covering material are the cost, its durability (how long it lasts), its weight and ease of repair or replacement, how much light is transmitted through the material and how much energy moves through the material.
Diffusing materials are designed to scatter incoming light and result in better light conditions for crops – for example, a cloudy white plastic film diffuses light better than a clear plastic film.
Glass has long been the traditional covering. Its favourable properties include:
Essentially there are three materials in this category - polycarbonate, acrylic (polymethyl methacrylate) and fibreglass. Plastic sheeting is not used extensively in Australia but its use is increasing. Sheeting products are more durable than plastic films and have fairly good heat retention, good initial transmission in the PAR range and low UV light transmission.
Films are the most common and lowest cost type of covering material. The types of film available are polythene (polyethylene), EVA (ethyl vinyl acetate) and PVC (poly vinyl chloride). With the constant improvements in plastics, these covering materials offer a lot of flexibility and performance options. Coverings can have a variety of additives which are used to give plastic films useful properties. For example, films may be used to exclude ultra violet (UV) light for chemical free pest control or reflect long wave infra red (IR) radiation to improve heat retention at night. As a result, some plastic covering materials are coloured or tinted.
Additives to the plastic determine its;
The process of making multilayer films enables thin layers of materials with different properties to be joined to make superior composite films.
Properties such as durability, creep (deformation over time) and long wave radiation absorption can be improved.
A poorly maintained covering material can lose a lot of energy and significantly increase production costs.
Glass coverings should be kept clean and broken panes replaced. Plastic coverings need to be replaced routinely. The performance of plastic coverings declines over time. Old coverings reduce light transmission which can restrict yield. The useful life of plastic films depends on the specifications of the plastic purchased. All plastic covering materials need to be replaced before they visibly start to break down; discolouration, for instance, is an early indication of wearing.