Greenhouse Glazing Materials
Many materials other than glass have been used to create a clear covering for greenhouses. Indeed, the Romans some two thousand years ago use thin sheets of mica in frames to cover plants.
Sheets of polycarbonate, 3mm/1/8in thick are amazingly strong – thick sheets are said to be bulletproof! – and about one third of the weight of glass. But although tough and initially allowing about 85% of light to pass through, it deteriorates after about fifteen years and eventually its structure breaks down.
Acrylic is slightly cheaper but unfortunately also has the ageing problem of polycarbonate.
Hooped tunnels are covered with sheets of clear or opaque plastic, but they do not have the good warmth retention properties of glass and after a few years discolour and perish due to the action of ultraviolet light. In commercial horticulture, these structures are very popular and create inexpensive protection for plants.
The Greenhouse Effect
Radiation from the sun contains infrared radiation as well as visible and ultra-violet radiation. When this radiation reaches a greenhouse, the glass reflects long wavelength infrared radiation, but allows short-wavelength infrared radiation as well as visible light to pass through. These are then absorbed by the soil and plants, thereby raising the temperature. Additionally, infrared radiation emitted by the soil and plants is of a longer wavelength and does not pass back out through the glass. Therefore it becomes trapped inside the greenhouse, thereby causing the temperature to rise.
If this continued, the temperature within a greenhouse would become unbearable. Therefore, provision must be made to open up ventilators to make the temperature suitable for the plants. Low external temperatures, and wind and rain are also responsible for decreasing temperatures within the greenhouse and make heating the structure very expensive.
Greenhouse Glass
Although initially expensive during the early 1800’s, glass has proved to be the best covering for greenhouses, sunrooms and conservatories. The greatest impetus to its use in Britain was the abolition of the glass tax in the mid 1800’s and the construction of the glass-clad Exhibition Hall for the Great Exhibitions of 1851.
Early greenhouses had panes of glass 30 cm/12ins or less wide, but when glass-making techniques improved these became 45cm/1 1/2ft wide and are now frequently 60cm/2ft across. The use of wider panes was made possible by the introduction of extruded aluminium glazing bars, which are both stronger and lighter than wooden types.
Glass used in greenhouses must be free from bubbles, with a standard weight of 7.32kg/sq cm (24oz/sq ft). The total weight of glass in a greenhouse is considerable, giving the framework rigidity as well as weight to resist winds and storms.
When clean, only 85-80% of the available light passes through, but if dirty this decreases dramatically. In summer, this reduction is not a problem, as the glass will probably be covered in a shading material, but in winter and spring all available light is needed.
