Congratulations! You have decided to buy or build your own greenhouse. Now you are trying to figure out which is best – a polycarbonate greenhouse or a glass greenhouse? For this discussion I am going to assume that the framing is the same material for each. I will only be discussing the glazing materials and any benefits or potential issues that may arise out of them. First let’s discuss the polycarbonate greenhouses.
Benefits of Polycarbonate Greenhouses
Polycarbonate sheets that are used for greenhouses have typically been a multiwall material. But, this is not true anymore. There are some greenhouses that have single clear sheets of polycarbonate for the glazing. Multiwall polycarbonate – This includes, but is not limited to, 6mm twinwall, 8mm twinwall or triplewall, 10mm twinwall or triplewall and 16mm triplewall or 5 wall. These are sheets that are much like looking down the end of a cardboard box. There will be a sheet on the inside, a sheet on the outside and a rib that runs between the sheets. In corrugated cardboard the rib is wavy. In polycarbonate panels it is straight between the exterior sheets. The numbers give you the distance between the 2 exterior sheets in millimeters. For clarification the 6mm is about 1/4″ thick, 8mm is approx 5/16″, 10mm is 3/8″ and the 16mm is 5/8″ thick. Simple enough. The twinwall, triplewall and 5 wall refer to the number of walls. The twinwall will only have the 2 exterior walls. The triplewall will have a third sheet halfway between and running parallel to the exterior sheets. The 5 wall can have several configurations. One is to have three interior walls and the 2 exterior walls. There is also some 5 wall available with one interior wall and a wall that forms and X between the exterior sheets. Bottom line is that other than the thickness, these sheets will all pretty much look the same while looking at them from the outside. The thicker the sheets and the more walls, the better the insulation. Depending on the manufacturer, multiwall sheets will have a 10 to 20 year warranty that the light transmission will not vary within a certain amount. This is a fancy way of saying that it won’t get yellow or brittle. The life expectancy on this material is anywhere from 15 – 25 years. Most manufacturers also have a 10 year warranty against hail damage. Typically this will be a less expensive option than the single tempered glass. So, the pluses to the twinwall and the triplewall, etc is that they are virtually unbreakable, will last a long time and they give you a better insulation than a single glass. The negative (and the only one I can think of) is the view. If you want to see clearly out through these sheets, this is not the material for you. Even though the exterior sheets are crystal clear, you will have a distorted view. The rib will break up your field of vision. You will be able to see color, but not form. I have told people for a long time that you will be able to see a red flower on a plant with green leaves, but you will not be able to determine what type of plant it is because of the ribs. On the other hand, there is now a clear single layer polycarbonate sheet that is used in greenhouses. This is a UV protected material as well. You do not get as good of insulation with it as you do with the multiwall material. Also, the warranty is typically only 5 years for light transmission and they do not typically carry a hail damage warranty. That does not mean that these sheets would not hold up in a hail storm. It is still the same tough, durable material as the multiwall sheets. On the plus side, you can see clearly through this, just like a window. And, the pricing will be in between the twinwall material and single tempered glass. The greenhouses available with this glazing will sometimes have a twinwall roof. A good example of the clear polycarbonate sides (and sometimes roof) are the Palram greenhouses
Benefits of Glass Greenhouses
Single tempered glass – The single tempered glass will not have as good an insulation factor as the twinwall polycarbonate sheets. But, you will see the timeless beauty of a victorian conservatory when using this glazing. You should always be sure to use tempered glass in a greenhouse, especially in the roof. If you use regular float glass it could break in sharp pieces and injure someone. Tempered glass is a safety glass. It will crinkle into small pieces if and when it breaks. The downfall of the tempered glass is that it will break if it is hit correctly. A lot of people choose to do a kneewall at the bottom to keep the glass from the ground. This may help with flying objects such as stones when mowing. If you use glass in the roof of the greenhouse you may get hot areas. A good way to do away with this problem is to use a multiwall polycarbonate in the roof. This is an excellent choice and gives you the best of both worlds. Glass greenhouses typically weigh more than polycarbonate greenhouses, so you need to keep this in mind when building your foundation. Glass greenhouses have bigger issues with the freeze / thaw cycles that many of us experience. You do not want the foundation to “heave” and break your glass pieces. You should always build a foundation according to your local building standards when using glass for the glazing. Polycarbonate is a little bit more forgiving. A small polycarbonate greenhouse may be placed on a base rather than a foundation. A base would be something like timbers that are placed on the ground and anchored into the ground. A foundation would be where you dig a footer and build as you would any other type of building in your area. You can get away with a base on a smaller glass greenhouse (up to 8′ x 12′), but I would definitely recommend a foundation for any size over 12 x 16 whether it is glass, polycarbonate or polyfilm. Of course this is going to be ultimately determined by your local building code department. You can up your insulation factor by using double tempered glass. You can also use add ons like Low E coatings to increase the efficiency of the glass. Some people find that the initial cost for the double tempered glass will outweigh the increase in insulation. It all depends on if you are truly using this as a greenhouse or if you are looking for more of a living area conservatory type setup. Bottom line is that polycarbonate and glass both have their benefits and their detractions. You should weigh these carefully when deciding which will be the perfect backyard greenhouse..