Materials of Greenhouse construction FAQs

Most of the greenhouses on sale locally have metal frames. Are they better than timber-framed models?

Although metal is a better heat conductor than timber, the area of frame in proportion to glass is so small that this can be discounted. The metal is stronger than timber, so the frames of metal houses have much thinner members than those of timber—and thus more of the sun’s radiation can enter, giving extra free heat. Most greenhouses for the amateur are now made of aluminium alloy, which is virtually maintenance-free, will not corrode, and needs no painting or other treatment (although it can be given a decorative finish if desired). Timber, in contrast, may be subject to rot, insect attack, warping, and other problems, although a good-quality cedar greenhouse regularly treated with preservative can last a lifetime. In general, alloy-framed greenhouses are cheaper than cedar ones and are probably a better buy for most amateurs, although it can be argued that cedar-framed structures look better in older cottage gardens.

Which is better material for glazing—glass or plastic?

Plastic is not a substitute for glass: it does not trap the sun’s warmth so effectively, it will not retain artificially created warmth so efficiently, and it will not weather so well or remain unaffected by time. In short, it is not recommended for permanent or long-tern) glazing. On the other hand it is useful for light-weight portable structures, especially where crops are grown in the ground soil and can be rotated. On sites subject to trouble from vandals, many plastics are suitable due to their unbreakability.

Which is the more useful—a base-wall structure or glass-to-ground?

A glass-to-ground house is the most versatile and is the better choice if size has to be limited; a base wall may be preferable if the greenhouse is heated and is sited in a cold area, if it is sited alongside a pathway where low glass could get broken, or if it is used to store materials under staging which might be an eyesore if visible from outside. A structure with a base wall, often called a ‘plant house’, is usually fitted with staging—wide shelves at waist height. Some kinds of conservatory may be better with a base wall if overlooked by neighbours—the wall gives a certain amount of privacy. Base walls may be timber boarded as well as of brick or concrete block.

Would it be worthwhile to design and build my own greenhouse?

It might well be considerably more expensive for the ordinary handyman to build his own greenhouse. It is not possible to compete with the bulk-buying of raw materials and mass-production methods of the leading greenhouse manufacturers. The time spent on the work would also have to be taken into account: a prefabricated structure can be erected very quickly. In general, building your own deserves serious consideration only if you need a specially designed structure or one of an out-of-the-ordinary shape or size.

Fittings

How many ventilators do I need, and what associated fittings must I buy?

Good ventilation is essential. The typical small greenhouse should have at least two vents—one in the roof and one in the side. More will be an advantage, since you do not have to open them all at the same time, and they enable you to allow for wind direction so as to avoid draughts and sudden gusts of air blowing through the greenhouse and causing damage. Few other fittings are essential, but a maximum-and-minimum thermometer is very important: it allows you to see how the temperature is behaving, so that remedial action can be taken to avoid rapidly fluctuating temperatures.

What is the best design for staging?

For the home greenhouse some form of slatted-top or mesh-topped staging is the most convenient. In winter this allows free circulation of air, which is greatly beneficial, as well as more light to the area below, which may be useful. In summer it can be covered with plastic sheeting and strewn with a moisture-retaining material such as grit or vermiculite (there are several proprietary aggregates that are kept moist to maintain humidity).

Are automatic aids worth the expense?

A thermostat is an essential money-saver for all heating systems. For those who have to leave the greenhouse to fend for itself for long periods, automated watering of some kind is a great boon, and a form of capillary watering is usually the most adaptable. Automatic ventilation arms are self-operating, very useful and reasonably priced. For larger greenhouses, electric-fan ventilation may be more practical.

What exactly is capillary watering?

This depends on the natural tendency of liquids to rise against gravity when absorbed by fine particles of fibres—as happens in the operation of a wick in a paraffin lamp. Traditionally, a layer of wet sand was spread onto the staging, and pots with unobstructed drain holes were firmly pressed down upon it so that the moisture in the sand could rise up into the compost in the pots. As the plants depleted the moisture in the compost, more water was automatically taken up by ‘wick action’ from the sand. Modern refinements include automatic moistening of the sand. More recently a synthetic fibre matting has replaced the sand; it is more convenient, lighter, and easier to keep clean.

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