The heart of any greenhouse is its glazing or covering. The glazing of a greenhouse will determine how well the greenhouse captures and retains heat and also how well the light needed for plant growth is transmitted. As is usually the case when it comes to greenhouses, there is a wide variety of different types of glazing for the different greenhouses out there. 
The most common glazing that you will find for hobby greenhouses is polycarbonate. Polycarbonate is an extremely durable plastic material that does a great job of insulating the greenhouse and diffusing sunlight. This is especially the case now that most commercial polycarbonate is sold with UV protective coatings to help resist any degradation of the panels. Single layer polycarbonate is seen on less expensive greenhouses and offers a good value for growers in climates with mild winters.
A greenhouse like the Rion Silverline or Enthusiast offers great performance at very economical prices due to their use of clear, single layer polycarbonate. We recommend shade cloths to ensure that harsh sunlight if diffused before it hits your plants to protect against hot spots on clear days
More commonly, you will see that most hobby greenhouse kits use twin wall polycarbonate. The advantages of twin-wall polycarbonate are that you get much better insulation in cold weather and light is diffused in the greenhouse. Different greenhouse kits will use different thicknesses of glazing. Typically, the range is from 4 millimeters up to 10 millimeters. Generally speaking the thicker the glazing, the better the insulation (or R-factor) will be. In our experience, the biggest
improvement comes from stepping up from 4 millimeter to 6 millimeter polycarbonate. Going up to 8 and 10 millimeter thicknesses does give better insulation but the increase isn't as great.
Although twin wall polycarbonate is now the standard in the industry, that doesn't mean that all polycarbonate is the same. Advanced Plastics has developed an innovative polycarbonate solution that they use on their Solexx line of greenhouses Solexx panels offer superior light diffusion and are available in 3.5 mm thickness and a 5 mm thickness for severe winter climates. Although some people aren't fond of the white appearance, there's no arguing with the results!
The latest word on Polycarbonate is triple wall polycarbonate. This provides the ultimate in insulation and because the flute spacing is wider than in twin wall polycarbonate, light transmission is relatively similar. Currently, we only offer Triple wall polycarbonate on our top of the line Grand Hideaway Greenhouses.
A much less expensive alternative is Polyethylene films and laminates. You will see laminated polyethylene on greenhouses like the Weatherguard series or the Flowerhouse line of greenhouses. This style of covering is relatively durable and easily replaced after it wears out. Alternatively, you will find some greenhouses that use a polyethylene film like the Easy2Build greenhouses . The film on these greenhouses does a good job for the price and makes assembly simple. The downside is that it is not as durable as polycarbonate. This is why you usually see large commercial greenhouses use this type of covering. It is cheap for them to cover their vast expanses and the expense of replacement is just built into their cost of doing business. 
Finally, a word on glass glazing. Glass is the traditional glazing for a greenhouse and, for its appearance and longevity, there isn't anything comparable. Typically, you will only see glass on the highest end greenhouses like Victorian Greenhouses and Grand Hideaways. This is partly because of glass's beauty but also because glass is the longest lasting, if not most durable, glazing material there is. The other reason is that greenhouses that utilize glass glazing typically require heavier frames and professional contractors to help assembly.
As you can see, there are quite a range of options out there. Which is best? It depends on your own growing goals and climate. If we had to make a blanket recommendation, it would be hard to go wrong with a greenhouse that has twin wall polycarbonate glazing at lease 6 mm thick. However, for other growers, a laminated polyethylene covering will meet their budgets and needs, while for others only glass will do! If you have any questions, please don't hesitate to contact us and we will be happy to make a recommendation based on your particular needs.