Greenhouses: poly vs glass

With the massive trend in ‘‘grow your own’’, many more gardeners are finding that if you have some permanently covered space, you can raise plants in an environment where they can establish to a stage where they are less vulnerable to the vagaries of weather or peckish pests. At this time any greenhouse builder must consider which materials to use for constructing their greenhouse.

Any traditionalist at heart if challenged would defend glass as the best material for greenhouses, however,  many gardeners in Europe (especially Germany) are now using polycarbonate rather than glass.

Over the last few years the drawbacks of polycarbonate have been removed, as polycarbonate had a reputation of clouding over time, the yellowness index as it is called, but these problems have been overcome with the development of new films. Its qualities in many respects are now superior to glass. In addition to longevity, the safety factor of polycarbonate over glass – especially when younger children or the elderly are involved – is a big plus.



The insulation properties are another big concern. Does polycarbonate sheet retain the heat radiated from the ground at night? This is crucial in any greenhouse. Thermal insulation of a material is the “R” value. Horticultural glass is usually 3mm thick and a single layer has an R value of 0.93 while clear, single-layer polycarbonate retains slightly less heat with a value of 0.83. If, however, opted for the 4mm twin-wall polycarbonate, as is most popular in Germany apparently, the “R” value rises considerably to 1.42.

The other worry, for a gardener, is the amount of light that gets through to the plants. Single-skin polycarbonate has around 94-96 per cent light transmission compared to 3mm glass having a value of 97-98 per cent; 4mm polycarbonate has a value of 80-84 per cent. Although the transmission value for the latter looks significantly lower than glass there is the benefit of the light that is scattered as it passes through the second wall. The rays become more diffuse, meaning it penetrates into areas that light coming through glass does not reach, this can be quite an advantage.

The final advantage for any nudist gardeners among you is that polycarbonate filters out nearly all harmful UV rays, so you need have no fear of sunburn.

More on greenhouses here, here & here

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