Choosing Your Greenhouse Construction
Design, Materials and Construction
Does shape matter?
A little research around the world, both into the past and the present, would no doubt bring to lightof every conceivable size and shape.
Such diversity provokes the question: does shape or design really matter anyway? For most greenhouse gardening purposes what matters most is stability coupled with light transmission. Only where the absolute maximum light is sought for certain commercial crops, especially during the duller winter months, and when earliness of cropping or the quality of bloom is crucial, does glasshouse design become a critical matter.
Light is of two basic forms: the total illumination of the whole sky from all directions and the direct light transmitted by the sun as solar rays. The translucent material of a greenhouse will transmit about 90% of the total illumination of the whole sky through the roof and sides of the greenhouse with almost equal efficiency. Direct radiation from the sun is only transmitted at maximum efficiency when the sun’s angle is at 90° (normal) to the glass. Each degree away from normal results in a greater amount of deflection. A little thought along these lines shows the extreme difficulty of designing and erecting a greenhouse so that it will enjoy maximum light transmission at all times. The situation is, however, not quite so bad as it looks, since during the summer months the total radiation from the sun is usually far in excess of the requirements of many plants;
Important note: Remarks generally apply to the northern hemisphere. The converse is true for the southern hemisphere.
It is only during the winter months that maximum light transmission is vital.
As the sun is never in the same position for long, any conventionally shaped greenhouse receives the maximum amount of light only for the very short period when the sun’s rays penetrate the glass at 90°. Curved greenhouses were quite popular in earlier days, especially the all-important centre planthouse of the greenhouse range in the private estate garden; these offered a varied surface, part of which would always be at around 90° to the sun’s rays. Modern types of round or geodesic greenhouses have been designed expressly to allow maximum light transmission over a period.
Much research has recently been concerned, however, with the conventionally shaped greenhouse. Assuming a roof pitch of approximately 30°, it can be seen that with a winter sun angle of approximately 15°, the angle of incidence at best is 180°— 45° = 135°. The vertical side of the greenhouse is better placed to transmit light with an angle of presentation of 75°. The higher the vertical side wall, the greater the amount of light transmitted, which in a single span house of normal width will reach the rear of the house easily. If the side wall is slightly angled off the vertical it is possible for the low-angled winter sun to have a 90° angle of incidence. Recent experiments have aimed at making the roof pitch steeper on the side facing south, to transmit instead of deflecting the low-angled winter sun. A roof pitch of 60°, for example, gives an angle of incidence of 105° which is much more efficient in light transmission than the 135° achieved with the normal roof pitch.
Obviously there is more than winter to consider, and the normal roof pitch of 25 — 35° (Dutch light houses are often 15-25°) is much more efficient for light transmission in the spring and summer months than the 60° pitch, although as light transmission is so often in excess of the plants’ needs, the sleep roof pitch is trot necessarily detrimental.
It becomes apparent that if a conventionally shaped greenhouse is to derive maximum winter light it must be sited with the ridge running east-west; to have it running north-south would present only the south-facing gable end favourably for winter light, the sides and roof deriving very little owing to the very acute angle of incidence presented on two planes. The glazing bars or astragals also create a lattice blind effect.
1. The ideal greenhouse for light transmission would be completely rounded, but these have some drawbacks in respect of size, cropping, etc.
2. The ideal greenhouse of otherwise conventional shape would have its roof pitched more steeply on the south-facing side. A greenhouse of such a design is available, but can be somewhat impractical unless as a conservatory.
3. For maximum light transmission in winter the conventional greenhouse should be orientated east-west to present its long axis to the south.
4. Where summer cropping only is concerned, north-south orientation is acceptable and indeed allows for a more equable distribution of light on both sides of the greenhouse and perhaps better ventilation.