If you are considering purchasing a greenhouse kit or building your own greenhouse, one of the biggest considerations is what type of greenhouse glazing will be the best for your needs. There are several choices, all with different price ranges and different characteristics. These features will determine the cost, performance and cost to maintain your structure. The most frequently used types of glazing are polyfilm coverings, polycarbonate coverings and glass. Each has its own set of features and benefits
The biggest issue with polyfilm is that some people do not purchase polyfilm with a UV protection. These materials are not designed to be used for long periods of time in the sunlight and most of them get brittle and crack and fail within the first year. There are several different types of greenhouse polyfilm available commercially. The most commonly used polyfilm is a 4 year, 6 mil. This means the UV protection is rated for 4 years and the film is 6 millimeters thick. There is also a reinforced polyfilm available which is laminated material with a cord in between the 2 clear sheets. The cord acts as a rip stop material. If you get a tear in the film it will stop once it hits the cord. This gives you time to do any repairs and avoid future damage. The UV protection on this material is only under warranty for one year, but normally will last much longer. I have had reports back of 12 year old material still in use on this particular film.
The advantages of greenhouse polyfilm are the cost and ease of installation. The disadvantages are that it can tear and that you don’t get as much insulation as with some other materials. A single polyfilm greenhouse is great in areas with warmer climates. In colder climates they are great for crops that do not require a lot of heat. If you are looking to use greenhouse film, but want a higher R value, there is the option of doing a double layer polyfilm covering with an inflation blower.
Multiwall polycarbonate will give you the best insulation factor of the 3 coverings. It is like looking down a cardboard box. It has a sheet on the outside, a sheet on the inside, and a rib running straight between the 2 sheets. This “dead air” space contributes to the higher R values. The cost of the polycarbonate is also between polyfilm and glass. It is a rigid plastic, but it can be bent onsite if you follow the minimum bending radius charts supplied by the manufacturers. Polycarbonate has typically had a 10 year warranty with a 15 plus year life expectancy. But, there is now a Lexan product which has a 20 year warranty with a 25 year life expectancy. The sheets also typically carry a 10 year warranty against hail. These sheets are virtually indestructible. The installation of polycarbonate requires the use of several different profiles, specifically the U and H profile. The H connects the sheets and the U closes the channels at the top and the bottom of the sheets.
The biggest drawback to polycarbonate is the looks. A lot of people do not like the fact that you cannot see through it like you can with glass. Even though the 2 exterior sheets are clear, the rib running between the sheets will distort the view. But, in my opinion, for performance this material can’t be beat.
You should always use tempered glass when building a greenhouse. This is a safety glass that will crinkle when it breaks to help prevent injury to users. This is especially important in overhead glass. You will not get as much insulation from glass as you will from the polycarbonate sheets. Single glass vs 8mm polycarbonate will offer about half as much insulation value. You can just about match the R value of double tempered glass with a 16mm triplewall polycarbonate. But, double tempered glass is a very expensive option and is usually an option used more for living spaces rather than greenhouses. Glass normally has a one year warranty on workmanship, but in most cases glass will last a long time unless it gets broken. If that happens the panel should be replaced immediately. Fortunately, the glass panels do not break very often. Tempered glass (sometimes called toughened glass) is stronger than regular float glass and usually only breaks if it is hit with a sharp pointed object. Glass is the most expensive of the options discussed here, with the double glass being at least twice as much as the single glass.
Here are a couple of quick hints if you are building your own greenhouse frame.
Be sure to use batten tape or a base and wire system to install this. It will hold it much better than just stapling through the film.
The minimum roof pitch is 1 on 12 when using polycarbonate. Be sure to get a specification sheet that states the minimum rafter and purlin spacing for your required wind and snow loads.
If you are building your greenhouse using recycled windows, be sure they have tempered glass. You can spot this by finding an etched label in the corner of the glass. Tempered glass is always marked.
Greenhouses are used in many different manners depending on what you are growing and what the requirements of the plant are. All of these greenhouse glazing materials have their place, just be sure to make the proper choice for your needs.
Tammy Wylie has been selling and installing greenhouses since 1993. She is the owner of Advance Greenhouses AdvanceGreenhouses.com and blogs at GrowYourOwnFoodAnywhere.com where she discusses different growing methods including container, greenhouse and garden growing. She can be contacted at firstname.lastname@example.org.