This is a hot topic (mind the pun!) Each material has it’s own advantages and disadvantages.

Glass is the oldest traditional glazing for greenhouses and if the glass doesn’t shatter or smash can last for years however, it doesn’t offer much insulation and tends to let the frosts through in winter. Plants can also burn from the direct sunlight in summer... although you can often get shade covers to go over the greenhouse or alternatively paint the greenhouse white to reduce this effect. Its not the cheapest glazing and replacing panes can be costly.

Polycarbonate (twinwall) glazing provides better insulation and is significantly stronger than glass – which means less breakages. You can get a range of thicknesses but you really need 6mm, 8mm or 10mm to gain any form of insulation benefits. This effectively works like double glazing with an air gap between the two layers, which helps keep the warmth inside the greenhouse, and keep the cold out. It’s easier and safer to work with than glass, and is also safe around children, pets, farm animals. It’s almost unbreakable and easy to replace. The polycarbonate has to be UV treated to last in direct sunlight. Good quality polycarbonate usually has lasts approximately 15-25 years under the New Zealand sun. Low quality, cheaper polycarbonate tends to only last around 3-5 years, so check the manufactures warranty if your not sure.

The single layer, flexible poly/plastic films that are often found on tunnel houses insulate a bit better than glass and are safer to work with. It’s also usually the cheapest option... in terms of $/m2 however, bare in mind this poly/plastic film doesn’t last long compared to glass or polycarbonate glazing and needs replacing every 3-5 years so although these are cheaper, they might cost you more in the long run.