Types of Greenhouse
Types of Greenhouse
We turn now from the actual structural materials and their inherent strengths and design points to consider the basic forms of greenhouse. Before discussing them in detail there are a number of criteria for selection to be taken into account. These may be summarized as follows:
1. Maximum light transmission compatible with heat loss, bearing in mind the main purpose. Whether double-glazed or not. eg. polycarbonate.
2. Freedom from maintenance, taking into account not only the structural materials, but also the glazing system.
3. Easy access to, and within, the greenhouse. Doors should be wide enough to permit entry of a barrow.
4. Reasonably easy and simple erection.
5. Cost. A relative matter. There is an excellent choice of structures to suit everyone’s pocket.
6. Adequate ventilation, a really important issue which will be referred to in detail later.
7. Adaptability for automation, ie for ventilation, watering, heating, thermal screens etc.
This is the conventional shape with straight sides and a sloping roof, fairly standard heights for amateur houses being about 2.1-2.25m (7— 7-1/2ft) to the ridge and 1.65m (5ft 6in) to eaves. Most alloy greenhouses are glass-to-ground type. The length and breadth of the greenhouse varies from approximately 2.4m (8ft) long by 1.8m (6ft) wide, up to 3.6 x 2.4m (12 x 8ft), which is an ideal width module as it allows two good sized widths of border or benches. There is a very wide range of sizes available and most are readily extendable in length.
A most interesting type of greenhouse for the amateur is the single-span Dutch light greenhouse constructed of interlocking Dutch light frames. The interlocking principle involving male and female lights fitting snugly into each other gives rigidity and avoids drip. It also lends itself admirably to being mobilized by the fitting of pulley wheels to the bottom of the lights and running them on angle iron rails. If the ends of the greenhouse are hinged so that they can be raised clear of a crop the whole house can be moved en bloc and a system ofpractised.
Mobile greenhouses need not, of course, be of Dutch light design, indeed many of those in Britain, Holland and Germany are not. Plastic structures with their cheaper capital costs, coupled with newer cropping techniques, have tended to oust mobile greenhouses to a great extent.
There is a vast range of sizes and types, and in this highly competitive business much experimentation. It is interesting to note that the massive ultrawide-span houses erected in Britain in the sixties have given way recently under pressure of Continental competition to less costly narrower span structures and layouts designed to achieve maximum light transmission. In Europe, however, the popular Venlo type (improved Dutch light) 3.2m (10ft 6in) houses are being superseded in many cases by a width module of 6.4m (21ft), again with economy of production in mind although double and triple supported or ‘floating’ gutter venlos are still very popular.
Mansard and curvilinear greenhouses
Mansard or curvilinear design simply sets out to provide a range of angles so that at any sun angle on the vertical plane there is a surface area of the glasshouse near the ‘normal’ angle (90°) for maximum solar radiation distribution if orientated east-west. This can give rise to problems in summer, necessitating shading, since variable-pitch roofs give more headroom for less air volume, and the increased solar radiation heats the air more than would be the case with a conventional greenhouse.
Mansard and curvilinear greenhouses have not found a great deal of favour in commercial circles, being more costly to build than span-roofed houses and not so amenable to multi-span erection. For private gardens or parks’ nurseries, however, the extra cost may be justified, especially when there is a lot of light-demanding winter work.
Greenhouses of this kind are available as glass-to-ground structures, or on a base block or wood or brick based wall. The Mansard roof is also suitable for the lean-to greenhouse.
The lean-to greenhouse was much favoured by earlier gardeners on account of the heat- reflecting and heat-storing qualities of the wall on which it was constructed. In recent years, lean-to greenhouses have fallen out of favour, largely because they pose tremendous ventilation problems in very hot weather, sited as they are invariably on a south-facing wall. But in the last year or so the high costs of fuel and the universal installation of central heating in the home have revived interest in the lean-to or conservatory, since heating can possibly be linked to the domestic system and run very economically.
Lean-to greenhouses, or conservatories, can take three forms:
1. The three-quarter span, where it is preferable to extend the ridge of the greenhouse above the top of the wall and gain the full benefit of two-sided ridge ventilation.
2. The single pitch lean-to glass-to-ground, or on a base wall.
3. The curvilinear and Mansard-shaped lean-to, variable pitch.
There has been sufficient discussion on light transmission for the finer points of configuration to be appreciated. The variable pitch roofed structure is obviously best for good light transmission throughout the year, although excessively hot in summer. But both the three-quarter span and single-pitch types, if facing towards the sun, suffer from excessive summer heating. Shading can be carried out, both outside and inside, or a fan ventilation system installed to moderate temperatures.
Circular and geodesic greenhouses
Not a great deal can be said about circular greenhouses other than to extol their obvious advantages for light and solar heat transmission. They are more expensive to manufacture than conventional shaped houses offering comparable growing space. Nevertheless, the circular bench arrangement necessary with a circular greenhouse allows a high percentage of bench for pot plants or propagation under conditions of excellent light transmission and easy management. Effective ventilation can be a problem; fan ventilation or air conditioning is more or less essential if temperatures are to be kept to an acceptable level in very hot weather.
Plastic greenhouses made their enthusiastic debut in kit form in the early 1950s, and followed the basic design of a rigid tubular steel or wood structure of conventional shape over which the plastic was stretched. At that time, however, plastic deteriorated very quickly due to the dual effects of ultraviolet rays and cold. It was particularly vulnerable when in contact with structural members due to heat build-up. Modern experience has shown, however, that provided a good quality polythene or PVC is used and treated as being reasonably expendable (three years on average), such greenhouses can be cheap and efficient for crop production. A basic fault is the condensation of moisture which occurs, necessitating very efficient ventilation if it is to be prevented. Double skin polythene helps in this respect, especially if the skins are kept apart with air, using a small fan. This arrangement has excellent heat conservation properties but results in the loss of a small percentage of light.
Various types of polythene greenhouses can be bought ready to erect and the more successful designs tension the polythene effectively. Recent recommendations indicate the need to orientate plastic greenhouses so that the prevailing wind can assist ventilation and the need to keep the polythene tightly tensioned. Aluminium “Grip Strip” is available to help in this direction. Disease incidence should be no greater than in a glass greenhouse if correct ventilation is achieved. A recent criticism of plastic greenhouses is that carbon dioxide starvation occurs due to lack of air change when ventilation is poor. Water requirements in a properly ventilated plastic structure will be basically the same as those in an ordinary well ventilated greenhouse.
While the majority of polythene greenhouses are of simple curved roof design, they can also be obtained in more conventional multi-span form. In do-it-yourself designs, where wood is used for the structural members, it is essential to use a wrap-round system of securing the polythene so that it does not tear at local stress points which occur when nails are used (or use “Grip Strip”). Much can be done by using polythene roofs and netting using “Grip Strip” to join them.
While a modicum of ventilation can be obtained in the polythene greenhouse by leaving doors or flaps open, orientating the house to catch the prevailing wind, or using vents, really effective ventilation can be ensured by the installation of a suitably sized fan, and here of course is where the cost of the total structure starts to rise. Alternatively a strip of Nicofence, Lobrene or similar ‘net’-like material helps greatly with ventilation.
One of the most interesting types of polythene greenhouse is the ‘bubble’. Here the polythene is kept inflated by a fan, ventilation being effected by an ancillary extractor fan, which brings a counterbalanced ventilator into use in an inlet.