Greenhouse Glazing Systems
The type of glass used forin Britain is drawn glass weighing 3mm/240 oz per sq ft (was called 24 oz horticultural glass) with a stress factor of up to 470n/m2 (10 lb per sq ft). Continental types of glass are also used and these are slightly heavier. A still heavier type of glass 4mm (32 oz per sq ft), is used for certain large-paned greenhouses, generally of commercial design.
Commercially, and in some amateur greenhouses, the standard size of pane is 60 x 60cm (24 x 24in), while many amateur houses are 50 x 45 cm (20 x 18in), or alternatively Dutch-size glass measuring 14 x 73cm (56 x 28in) and 3mm thick. Venlo sized glass, which is widely used in commercial structures is 142 x 73cm (4ft 6in x 2ft 4in), or 165 x 73cm (5ft 4in x 2ft 4in), and is 4mm thick. There is certainly virtue in having larger pane sizes, as it increases the ratio of translucent to opaque material, which in turn ensures maximum transmission of light, provided the greenhouse is orientated to the best possible advantage. The main drawback in having a large ratio of glass to structural materials, and in particular no base wall is, as stated earlier, the increased heating costs. Replacement costs after breakages are another drawback with large panes.
Alternatives to glass
Various forms of rigid plastics are available and can be used in lieu of glass in conventional greenhouses. Fibreglass sheets can also be used, especially in warm climates. They can readily be used in curved sections, which is a distinct advantage as far as light transmission is concerned. They perform the same function as glass, although it is claimed that in some respects they are more efficient in transmitting light. The most interesting of these are the range of acrylic and polycarbonate twin- or triple-walled sheetings. These are not only easy to cut and fit, but very resistant to breakage. They also give much better (30-40%) heat retention than glass. Newer forms of PVC are also available, such as Biolex 2000.
Several forms of clear polythene are available. The light gauge (38mu) is mainly used for ‘double-glazing’ to reduce heat loss. For plastic greenhouses or polythene tunnels, polythene 150mu (600 gauge) and polythene 180mu (720 gauge) are best used. Anti-fog ‘Thermic’ 720 film is now available as is 4-year AD 200 thermic film (Clovis Lande). Double skinning using a 600g inner and a 720g outer cover or two sheets of 720g — kept apart by a small fan —is now very popular in commercial circles, resulting in very considerable savings in heat with minimal light loss.
PVC film is also available and is clearer than polythene but tends to be more expensive. A range of other plastics is available (eg. woven materials) especially in the U.S.A. Also of interest is the Serac system involving PVF (TEDLAR) outer skin and Melinex 071 inner skin, which provides a combination of excellent light transmission and conservation.
Polythene is efficient in transmitting short-wave solar radiation, the light being all diffused and not directly admitted as in the case of glass. But normal grades of polythene, unlike glass, do not at present trap the long-wave radiation from heated objects within the greenhouse, but new types of polythene are available which are more effective in trapping solar heat and reducing condensation problems. Polythene destruction also occurs constantly, due to ultraviolet rays (which tend to be stronger in areas of unpolluted air), but a large degree of ultra-violet proofing is now incorporated into polythene, which makes it last longer. The build-up of static electricity to which dust clings tends to cloud polythene and detract from its light transmission efficiency and general appearance.
The ‘U’ value or thermal coefficient of polythene is basically higher than that of glass, which results in a great deal of condensation unless the polythene has been treated or designed to disperse moisture.
The problem of how to secure and tension without tearing has been successfully overcome in many of the latest designs: loose flapping polythene quickly deteriorates whereas tightly stretched polythene, even if of thinner grade, remains undamaged. Aluminium channel is ideal for securing and tensioning the plastics.
Glazing methods for glass
The simplest method of glazing is the use of putty and nails, as carried out on wooden glazing bars. Linseed oil putty is bedded in the channel of the glazing bar and the glass laid on this. The panes nearest the eaves in each section are laid first and the glazier then works upwards to the crest of the roof Six brass sprigs are required per sheet of 60 x 60cm (24 x 24in) or 20 x 45cm (20 x 18in), two at the bottom of the lap, two 2.5-5cm (1-2in) from the bottom of the pane, and two in the middle of each side. The next sheet of glass is laid on with a 0.62-1.25cm (¼—1/2in) overlap.
When necessary, glass is cut with a good straight edge and a sharp glass cutter, working on a flat wooden surface such as the top of a table. This system of glazing has been in use for a great many years and is still occasionally used for the conventional wooden house construction. In time putty tends to crack and allow the entry of moisture.
The use of aluminium covered sealing strips has much to commend it on a new structure, to prevent rapid deterioration of the putty. They are often used after the glazing has already deteriorated, as a first-aid measure to avoid re-glazing. A modification of conventional glazing which is quick and effective is to use strips of non-hardening mastic put down the grooves in the glazing bar, tacking the glass with brass nails — not sprigs.
Dry glazing is practised with Dutch light greenhouses, the glass being held in a groove and consequently supported on all four sides. It might seem that this is a highly inefficient method of glazing since the glass rattles and the air space allows loss of heat, besides admitting water. In fact dry glazing is reasonably efficient, especially when the glass is held tightly. Vigorously blown rain can find its way into the structure but many gardeners have, after many years of experience, a high regard for the maintenance-free character of dry glazing and on balance the disadvantages are more than cancelled out by the advantages. Dutch greenhouses, if well constructed, seldom suffer any serious damage from gales or hurricanes.
Dry glazing systems are by no means confined to wooden houses. In recent years they have been adapted in alloy greenhouses in many forms, the glass being retained in an alloy glazing bar extruded to form a groove which retains the glass in a similar manner to the Dutch light system with or without plastic seals. Mastic and clips The more usual way of glass retention with alloy glazing bars is to bed the glass on a strip of mastic and retain it with a stainless steel or alloy clip. ‘Barcaps’ of alloy or PVC can also be fitted over the clips to provide a tight seal and thereby avoid a convenient lodgement area for moss growth. More sophisticated forms of glazing depend on a continuous bar-cap arrangement. Steel houses are also usually glazed on the same system of non-hardening mastic and clips, although one commercial system used non-hardening bitumen and plastic clips. It is obviously more difficult to bar-cap a steel bar than one of extruded alloy.
In very exposed districts something more sophisticated than the simple glazing clip system employed in many amateur structures would be advisable, owing to the degree of vibration of the glass which can be caused by persistent high winds. It needs merely one pane to loosen and slip for the wind to gain entry and rip out glass wholesale. Most glasshouse manufacturers will, I feel, be prepared to provide bar-capping as a not-too-expensive extra where this is not standard.