Polycarbonate vs Polyfilm Greenhouse Covering
So, you have decided to build a greenhouse, but you are confused about which type of greenhouse covering would be best for you. Let’s take a look at some of the pluses and negatives of polycarbonate sheets and polyfilm coverings. We will discuss R value, installation, cost and lifetime expectancy.
R value is defined as a materials ability to resist heat flow. The higher the R value, the less energy it will take to heat the same size space under equivalent conditions. This may or may not be a concern to you depending on how you are going to use a greenhouse.
The most common single film used for greenhouses is clear 6 mil, 4 year covering. What this means is that the film is 6 mil thick and that the UV protection is warranted for up to 4 years. There are also some other types of film such as overwinter (this film is meant to last one year), white films (lower light transmission) , and reinforced polyfilm (with a rip stop cord).
The R value of single polyfilm is 0.87. It is the least efficient of all of the greenhouse coverings we will discuss.
Installation can be done in several different ways. If you have a wood frame you can use batten tape and staple through the cover and the tape into your frame. If you have a wood frame or a metal frame you can use the base and wire system. The base is screwed to your frame and the cover is secured using a W or wiggle wire. This is a pretty simple installation, but you want to make sure that you do not try to do this on a windy day. You will need several people to do this depending on the size of the greenhouse.
The single polyfilm will be the least expensive of all of the coverings we will discuss.
This film should last you the 4 years that the UV covering is guaranteed for. The only issue with this covering is that you may get a tear. There is repair tape available for this. But, if you are not there and not able to fix a tear in a timely manner you may lose the entire cover. This is why some people will choose to go with the reinforced polyfilm over the clear 6 mil, 4 year. It has a rip stop cord that will keep a tear from spreading. That way you will be able to fix the tear no matter how soon you are able to get to it.
You can do a double polyfilm greenhouse a couple of different ways. You can just buy 2 pieces of the film and attach them together. Or, the easiest way to do this is to buy a tube of the polyfilm. This is just a double layer that is already fastened together. The base and wire system is set up to hold up to 6 layers, so this is not an issue. You will also need a poly inflation fan kit to inflate the 2 layers. I have had some people mention that they think this makes the cover tighter and less susceptible to tears. I have no proof of this, but it does seem a logical conclusion.
The R value of double polyfilm is 1.7. This will save you 40% on the energy costs over a single layer of polyfilm.
The installation on this is done in the same manner as a single layer of polyfilm.
This film will basically be twice the cost of the single polyfilm, as you are basically just using a double layer of the same material. You will have the additional cost of the blower fan. These are typically around $200 or less, depending on the size of the greenhouse and the size of the motor needed.
This film may last a little bit longer than the single layer. As I stated earlier some claim that they inflation keeps the film from tearing as easily. The UV protection is rated for the same amount of time as the single polyfilm.
Polycarbonate is the greenhouse covering of my choice. It fits my needs the best of all of the materials. However, a lot of people don’t like it because of its appearance. It is a rigid plastic configured in much the same way as a piece of a cardboard box. It has a sheet on the outside, a sheet on the inside and a rib that runs through between the 2 sheets. The rib in polycarbonate will be straight vs the wavy rib in a cardboard box. This will distort your view. You will be able to see color, but you will not see form clearly. I use the example that you can see a green leaf and a red flower, but you will not be able to make out if it is a poinsettia, geranium, impatiens, etc. Of course the clear polyfilm is not exactly clear either. It will have a hazy type of appearance to it. If you want a clear glass like view, tempered glass is your only option.
Polycarbonate sheets are available in several different thicknesses. 8mm clear twinwall will have an R value of 1.8. 16mm clear triplewall will have an R value of 2.4. This will be the most efficient of the coverings we are discussing.
Installation of this material requires polycarbonate H profiles to join the sheets side by side and polycarbonate U channels to close off the ends. You will need more framing for this material vs polyfilm greenhouses. Typically polyfilm greenhouses will have 4′ or 6′ wide spaced bays. You can do the same with the polycarbonate, but you will need to run purlins between the rafters spaced according to your snow load needs. There are charts available to determine this. You can typically install this with 2 people as the sheets are not heavy and you install one 4′ wide or 6′ wide sheet at a time. You still do not want to try to install under windy conditions as the sheets can be caught by the wind and sail away.
This will be the most expensive of the coverings discussed in this article. But, it will be the longest lasting. Depending on the manufacturer polycarbonate sheets have between a 10 year and 20 year warranty on the UV. Most sheets also have a 10 year warranty on hail damage, so no need to worry about tears. The practical life expectancy of this material is 20 – 25 years.
When considering which of these materials to use to cover your greenhouse you should look at the initial cost, the cost to install, the cost to heat the greenhouse, and the life expectancy of the material. Once you have made these decisions you should be able to choose the best covering for your greenhouse with ease.