The three main types of glazing for greenhouses are horticultural glass, toughened glass or polycarbonate/perspex.
- Horticultural Glass. 3mm thick, comes in 610x610mm squares. It is brittle and shards when broken producing sharp, knife like, edges. usually overlaid when used, causing unsightly and disease retaining algae and dirt to build up at the point of overlay. It cannot withstand movement of the greenhouse frame and will crack if the frame is moved off the square. It has been the standard greenhouse glass for many years, but is rapidly being replaced by toughened glass.
- Toughened Glass. 4mm thick glass with a toughened film added which ensures that, should it break, it will shatter like a car windscreen. This produces many small pieces of glass, but no dangerous knife like edges. It comes in large sheets. The combination of increased size and toughness makes it robust, giving the greenhouse frame more rigidity, strength and weight. it's a safer glass, especially with children about. We always recommend toughened glass.
- Polycarbonate or perspex. Both made from plastics, these glazing options are quite flexible. Polycarbonate is usually a "sandwich" which traps air in the centre improving heat retention in colder months. It is opaque: you cannot see directly through it. Dirt and algae build up in the air spaces of the sandwich. Perspex is clear and used in greenhouses that have curved areas which require glazing. The key weakness with both forms of plastic is their flexibility.This leads to them popping out of their fittings in high winds. the moment this happens the wind blows many of the other sheets of polycarbonate or perspex out as well.
Also consider how windy the site is, whether there are children in close proximity, how sturdy you wish the greenhouse to be and what level of maintenance you are prepared to accept (horticultural glass generally needs more cleaning and replacement than toughened glass).