Heat and ventilation

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Given adequate light, one of the most important attributes of a greenhouse is the way it deals with the potentially conflicting needs ot heat retention and ventilation. Greenhouse designers ha ve devoted much thought to the problem of retaining heat. Some years ago double glazing seemed likely to be the answer. In practice, however, the method has serious disadvantages: the two layers of transparent material cut out a great deal of light; the sun's rays take much longer to heat the greenhouse; and the space between the transparent layers attract algae. Moreover, double glazing is considerably more expensive than single glazing.

Whatever the shape or design oi the greenhouse, adequate ventilation must be provided. Greenhouses used to have relatively few ventilators and those that were fitted were often small. As a consequence, on sunny days the greenhouse got too hot for the plants to grow happily, and gardeners were forced to provide shade to prevent damagtr. For the modern designer the problem is to provide adequate ventilation while avoiding draughts. As a rule the ventilator area should be equal to at least one fifth of the total interior floor area. If ventilators need to be covered

Good veniilatirjn is essential 10 the healthy growth of greenhouse plariis

Good veniilatirjn is essential 10 the healthy growth of greenhouse plariis

Forced Ventilation Greenhouse

by lightweight netting to keep out birds, the ventilator area should be increased, for such nets substantially decrease the effect of the vents. For simplicity of manufacture ventilators are usually confined to the roof area; on the more expensive designs, both root and side ventilators may be available, and these are preferable because thc\ allow greater control to he exercised over the growing environment, Doors should not be used as ventilators unless the doorways arc fitted with blinds; otherwise the draughts that inevitably develop are likely to damage the plants near the doors. Incidentally, sliding doors are generally preferable to hinged ones because the opening can be regulated more easily.

Structural i materials_j

Aluminium alloys are now widely used for the framework of greenhouses, and the latest extrusion techniques enable refinements such as drip channels and ventilator hinges to be neatly built in. Alloys of this type need virtually no maintenance, and because of their great strength the glazing bars can be of narrow section, so that they throw little shadow. When new the alloys are usually bright metal, but oxides of aluminium tend to form on the surface over a period of time; although, unlike rust on steel, these oxides do not weaken the alloy, they cause its surface to turn dull. Some manufacturers enamel the alloy to prevent this happening, and it appearance is a major consideration the extra cost of this treatment may seem worthwhile.

Steel is now less widely used for greenhouse structures than hitherto but is nevertheless finding favour with some manufacturers of polythene-clad greenhouses, for it lias great strength and yet can be easily bent into hoops with the simplest of equipment. Steel will rust rapidly in the greenhouse environment unless it is treated in some way. Galvanizing gives long-lasting protection. However, care is necessary in the transport and erection of treated steelwork to ensure that the galvanizing is not removed. Touching up suspect places with a special paint will protect the surface against rusting.

Gardeners who favour Wooden greenhouses are usually quick to point out that heat losses from aluminium alloys are greater than from wood, and that aluminium greenhouses consequently become colder more quickly as temperatures tall. This is so. but 111 my estimation the advantages of the narrow section of the alloy glazing bars outweigh those of the heavier-sectioned timber-framed greenhouse.

Until a few years ago wood was used extensively by greenhouse builders and the many wooden greenhouses Mill in use demonstrate that wood has a long life provided it is properly cared for. It is the necessity for regular painting or other treatment both inside and out that has persuaded many gardeners to turn to the alloy-framed greenhouse, but wood is still preferred by the more traditionally-minded, not least for its appearance.

Softwood, mainly pme, is the favourite wood for glazing bars, while hardwoods are normally used lor the sills. Modern techniques of preserving wood arc now widely employed, but prospective purchasers should ensure that such treatments have been properly carried out, ideally under pressure, to force the preservative into all the timber. Simply brushing on such preservative is virtually useless.

Some manufacturers use a hardwood such as oak throughout, and greenhouses of this type have a long, relatively maintenance-free life. Western red cedar is an attractive looking wood and has been used in the more expensive greenhouse ranges for many years. It is not ,ts strong as some other woods, so the timber sections need to be somewhat thicker, but its attractions are the colour, which looks well in almost every garden setting, and freedom tram the need tor painting, hi fact, no paint applied at ail. just a suitable .preserving oil which needs to be brushed on every two or three years.

Many gardeners of the older generation still prefer wood-framed greenhouses. for they considcrth.it they provide a better growing climate for their plants. There appears to be no scientific evidence for this, but if you are prepared to spend more time on maintaining a wooden-framed greenhouse you will certainly not be at a disadvantage, as far as plant cultivation is concerned, compared to those Working in alloy structures.

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Building Your Own Greenhouse

Building Your Own Greenhouse

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