One thing a greenhouse buyer should never skim over is the choice of frame material. Whilst glass and other cladding materials can usually be replaced, as can door and window fittings, any problems that develop in the frame may be difficult to put right later.
Your choice will be between PVC, aluminium, galvanised steel and wood. Although each choice has some advantages, the decision is easier to make if you consider the materials’ weaknesses.
Steel & Aluminium
Steel and aluminium – being metals – have poor insulating qualities. If your greenhouse will ever be used for plants that need protection from late spring or early autumn frost, or full over-wintering, a metal frame will not be ideal from the heating point of view. Conversely, if plants need protection from over-heating at the height of summer, they will cook that bit quicker if you have a highly heat-conducting frame.
Another potential problem with metals is condensation. Moist air will condense more readily on cold metal and the ensuing water could damage other components or contents of the greenhouse. A third drawback is that metal expands and contracts according to temperature; as a result, extreme temperatures may loosen joints and rivets or crack other components, such as panes of glass.
Whilst PVC has better weather resistance, temperature stability and insulating qualities than metal, it has several drawbacks of its own. Firstly, although PVC is a relatively durable and stable form of plastic, it can still perish over the course of time – and this is accelerated when it is subjected to weather extremes. Secondly, it is not the easiest material to alter or repair if any part is broken. Adhesives have limited reliability – and even less when exposed to weathering.
Its light weight and flexibility are often listed as advantages; however, in time, movement can cause windows and doors to stop opening or closing, as many of us with them in our home know only too well. If your greenhouse is ever exposed to extra strong winds, it might prove to be a little lighter than is desirable.
One of the drawbacks of wooden constructions in the British climate has been its tendency to rot unless regularly weatherproofed. Damaged wood then readily admits pests, such as mice and rats. Most woods also expand and contract – when your new timbers dry out a little, gaps may open up that weaken the structure and are wide enough to admit insect pests.
Different woods have different qualities, however. One exceptional kind now readily available in the UK is the western red cedar, often simply called red cedar. This wood is durable but 30 per cent lighter than many timbers of comparable strength. The grain is straight and almost knot-free, making it easy to work with, alter or repair. It has excellent water and rot resistance; in fact, it is a traditional choice for kayaks and canoes. Its pleasant, slightly aromatic fragrance is naturally fungicidal and somewhat repellent to insects and rodents.
Unlike metals or PVC, red cedar is a renewable resource. It is a tall, fast-growing tree and for every tree felled, at least one is planted to replace it. As a natural and easily-worked product, comparatively little energy is expended in making it into your greenhouse frame.