Glass is the ideal material for covering a greenhouse, but when you’re shopping for greenhouse glass, you have several types to choose between. Some glasses are better for walls and others for roofs, based on how they’re designed and the stressors they can handle. Some greenhouse glasses can pose a serious cut risk if they break, so you need to balance your cost-savings with safety as you pick the glass for your greenhouse. These are the main types of greenhouse glass available and in common use:
Annealed glass – Annealed glass, the plain glass we’re all familiar with, is heat treated and allowed to cool in a controlled way so that the internal stresses relax slowly. Unfortunately, this means the glass is very weak and prone to cracking or breaking when the temperature on either side of the glass changes rapidly. Annealed glass under pressure breaks into large, jagged shards, creating a hazard for anyone nearby. It’s the cheapest glass option and would work well for wall panels in a greenhouse where snow loads and high winds aren’t a concern.
Tempered glass – Tempered glass is a heat-treated glass—the stresses in the glass are induced in a very specific way to increase the surface stress in relation to the internal stress. Tempered glass is very clear, but up to six times stronger than annealed glass and breaks into tiny, nearly square fragments that pose little risk to people. It’s a good choice for greenhouse roofs, walls and doors due to its ability to handle fluctuating temperatures, but shouldn’t be used on roofs where snow loads or high winds are a concern.
Laminated glass – Laminated glass is made of two or more sheets of glass bonded together using a polyvinyl butyral (PVB) layer—it’s the same glass your car’s windshield is made from. It’s extremely strong, like tempered glass, but unlike tempered glass, laminated glass doesn’t break. It may shatter, but the PVB layer holds the pieces together so they don’t pose a risk to anyone nearby.
This is a very good thing, especially when the glass is being used for a roof application where shattering may result in the rest of the panel or other panels falling to the ground. In addition, laminated glass can block up to 99 percent of incoming UV radiation, making your greenhouse safer for seedlings. Laminated glass is an excellent choice for greenhouse roofs—many people choose to use it for walls, as well, and the only major drawback to this is the additional cost over other glasses.
A word about double-pane glass
Although it would seem that double-pane glass is the only way to go with a greenhouse, what works for homes isn’t always best for the hot, humid conditions inside a greenhouse. The seals inside a typical double-pane window are typically guaranteed for five to 10 years, but that guarantee is void if you install them in a greenhouse or indoor pool area. These areas accelerate the destruction of the seal, leading to premature failure and fogging. When it comes down to it, you’re much better off insulating your glass greenhouse with bubble wrap than spending the money on most double-pane products. At the end of the day, you can always take the bubble wrap down, but a busted seal in a double-pane unit means the whole unit must be replaced.
Deciding between glass and plastic? Read Glass greenhouse vs. plastic greenhouse—pros and cons>>[box]by Kristi Waterworth[/box]