Glass Type and Pane Size
Almost all glass-glazed garden greenhouses for the amateur are made from glass standard to your country and this differs slightly from country to country.

There is no choice and really no reason to select a different type of glass unless you plan to have a very large greenhouse for commercial purposes.

There is variety and a true choice in the size of the glass panels. The larger the size of the panes the better the light transmission which is a benefit.

The disadvantage of larger panes is the heat loss is increased and therefore heating costs are higher. One further disadvantage of large panes of glass is that the cost of replacing a broken pane will be higher. None of these factors are critical, so we wouldn't really recommend making the pane size a key factor in your choice of garden greenhouse.

Other considerations with glass are that it is a good transmitter of light (typically 94% of light goes through glass) which is the good news. The bad news is that it is a bad transmitter of heat which means the inside of a glass greenhouse can get very hot very quickly with disastrous results for some plants. Glass is also very heavy compared to the materials below which means the frame has to be stronger (and therefore more expensive) and packing costs are higher.

Plastics, Polycarbonates and Other Glazing
There is now a very wide choice of alternatives to glass as glazing material for a garden  greenhouse. Many of these are beyond the scope of this article, but two of the more popular alternatives are noted below:

Fibreglass / Polycarbonate Sheet The most common of these are acrylic and polycarbonate double (sometimes triple) glazed sheets with excellent heat retaining properties. These sheets perform at least as well as glass for light transmission, although they do deteriorate with age. This material is far less liable to breakage and very easy to form into curved sections. These really are an excellent alternative to glass, especially if children are liable to be in the same area as your greenhouse. Twin wall or double glazed polycarbonate has a light transmission of around 85%, single wall polycarbonate has a light transmission even better than glass, around 96%. 

Fibreglass has much the same properties as polycarbonate, but they turn yellow much quicker and are therefore more costly in the long run. 

Polythene Surprisingly, polythene is very good at transmitting light because it diffuses it, spreading it more evenly round the greenhouse. Previous problems with polythene tearing appear to have been solved with new methods of securing it to the frame. The main problem with polythene is that it is always being damaged by the suns rays. This slowly reduces it's light transmission qualities and causes the material age. 

One other key disadvantage with polythene is that condensation gathers on the inside of the greenhouse which attracts dirt and makes it more likely to encourage plant diseases. Some types of polythene greenhouse are treated to help the moisture disperse but this does not seem to be 100% satisfactory. 

Methods of Securing Glass
If you buy a second hand glass greenhouse the glass may well be secured by seating it on a bed of putty and held in place by nails. This method is not commonly used nowadays because the putty deteriorates and cracks. Treat a greenhouse of this type with caution.

The most commonly used glazing method is simply holding the glass in place on a plastic strip and securing it there with metal clips at various intervals along the frame. See the top picture on the left (click picture to enlarge). This works fine in most cases and is long-lasting. 

In areas exposed to high winds however, the clips may not be strong enough and it is necessary to secure the glass in place with with a continuous strip of metal - commonly referred to as bar-capping or edge ridging. See the bottom picture on the right (click picture to enlarge).

Remember, in high winds the loss of one single pane of a greenhouse will let the wind in which will destroy all the panes of glass in a few seconds.

The Future
Greenhouse glazing methods have changed very slowly over the years. It seems that the polycarbonate sheets are the way forward. Triple glazed polycarbonate retains the heat extremely well and the light transmission is excellent. Polycarbonate also has the advantage that it is stronger than glass or plastic, making it less vulnerable to damage.