Ever wonder what actually keeps a greenhouse warm, bright, and bursting with life—even in the dead of winter? That magic layer is called glazing.
The glazing you choose for your greenhouse can make or break how well your plants thrive. In this guide, we’ll explain what glazing is, how it affects light and temperature, and which materials perform best for different goals.
Glazing Defined
According to the Oxford dictionary, glazing is ‘the action of installing windows.” When it comes to greenhouses, manufacturers tend to refer to the glazing as the outer covering of a greenhouse.
Glazing materials allow the sun to work for you by letting in light and holding in heat, allowing you to control the climate.
Greenhouses however, typically have three different types of glazing. Some greenhouses use polycarbonate, some use polyethylene, and some use glass. It is also common to see a combination of glazing materials.
Polyethylene and polycarbonate are often used together for larger high-tunnel greenhouses, while a glass and polycarbonate combination will mostly be used for home or hobby greenhouses.
Glazing Properties
Different materials are good for different things. Here we will go through the common types of greenhouse glazing and their properties.
Glass
The prettiest of all glazing materials is glass. Glass is great for adding strength to your greenhouse structure because it is solid and holds its shape. Clear glass also has up to 90% light transmission, making the space very bright.
Glass has its issues when it comes to insulation. Single pane glass has a very low R-value which means it does not insulate well. The sunlight coming through can heat up the space hotter than it is outside in the summer, and the heat will easily escape in the winter.
This issue can be solved by getting double pane glass, with insulating treatments. That, however, gets more expensive very fast, creating a good use case for combining glass with polycarbonate.
Polycarbonate
Polycarbonate is a hard plastic similar to plexiglass with properties that are beneficial for greenhouse use. It comes in different thicknesses being corrugated single wall, 6mm or 8mm twinwall, or up to 16mm five wall.
Polycarbonate can bend with curved roof lines and is very strong. It is virtually unbreakable and lasts up to 20 years if applied correctly and insulates better than glass.
The R-value of twinwall polycarbonate is about one point higher than glass (1.6 compared to 0.9), while also being cheaper. This makes it a better option especially for budget projects. If you are looking for strength and more insulation, 16mm polycarbonate has an R-value around 2.5 to 3.0.
Polycarbonate has about 80% light transmission, which is less than glass. This can protect plants from overheating and extreme exposure to the sun. The design of polycarbonate diffuses sunlight coming through, evenly spreading it throughout the greenhouse.
Polyethylene
Polyethylene is the thin plastic film that you will see on high tunnels or farm greenhouses. It is basically just a high strength plastic wrap.
It has the lowest R-value of the three glazing types (0.85), but the standard for most high tunnels is to use two layers on the roof and fill it with air to create an insulating bubble (R-value: 1.25).
Polyethylene comes in different thicknesses and with different light diffusion capabilities. Light diffusion helps with evenly lighting the greenhouse while still having around 85% or more of sunlight coming through.
The main downside of polyethylene is that it does not last long. The typical lifespan is around two to five years. UV treated polyethylene can last longer, sometimes up to eight years, but often other factors like weather can shorten the lifespan.
Choosing the right glazing
When making the choice for glazing material, we recommend you think about the goal of your greenhouse. As outlined above, different types of glazing offer different benefits.
If you’re wanting to extend your growing season, or maybe start your garden a month early, a high tunnel like greenhouse with polyethylene will be great for that.
If your goal is to produce vegetables or plants year round, a higher insulating material like polycarbonate is best. Or even a mix of polycarbonate and glass will work for this.
If you want a beautiful sunroom that fits your home’s aesthetic while doubling as a greenhouse, glass is the best option with its strength, durability and beauty.
And if you want a completely unique DIY greenhouse, you can recycle windows that have been replaced on old homes. If you do this please send us pictures of your DIY greenhouse!
If you are looking for more information about what greenhouse might be right for you, check out our homeowners guide here.
Re-covering a greenhouse
As mentioned before, polyethylene and polycarbonate glazing will need to be replaced.
Polycarbonate, if installed correctly, will last up to 20 years and will usually start to yellow. If installed incorrectly, or with the UV protective coating facing inward, it will yellow much faster and start to break.
There are several polycarbonate manufacturers throughout the U.S., but we like to work with Gallina USA to supply replacement panels for our customers. They can supply the size needed for any greenhouse.
Some people have asked us if they can replace polyethylene with polycarbonate. This can be done, but depending on the size of the greenhouse, it can be very expensive.
Some manufacturers don’t have polycarbonate greenhouse options. This means that the greenhouse would need a custom design, and potentially custom materials, to make it work.
In most cases it is cost effective to replace your greenhouse glazing with the same material as was originally installed. But if you would like to explore options, we would be happy to consult with you.
Conclusion
Glazing is the window to the effectiveness of your greenhouse, so it is important to get the material that works best for your needs.
Glass, polycarbonate and polyethylene are not the only options for glazing, but are the most common.
At Monarch Greenhouse Solutions, we work with our manufacturers and suppliers to source the best materials for greenhouse owners. With years of experience in greenhouses and with glazing materials, we have figured out some of the best practices for installation and replacement of different types of glazing.
If that is something you need, please give us a call or fill out our contact form today!