Which Polycarbonate Sheet is Best?
Polycarbonate sheets –
Which thickness, color, etc should I choose? This is a question that I get asked regularly. It was not so difficult in 1993 when we started dealing with polycarbonate. We had a local business where we sold and installed at that time. We were in the South, so all our projects were done with 8mm clear twinwall. There really were only 2 options readily available at that time – 8mm twinwall and 16mm triplewall. Mostly the 8mm was used in warmer climates and 16mm was used in cooler climates. But all that has changed now.
Always be sure to use polycarbonate sheets that are UV protected one side for any outdoors project such as a greenhouse, patio cover, etc. That is what contributes to the longevity of the sheets.
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What sheets are available?
Today you can get polycarbonate sheets in 6mm (1/4”), 8mm (5/16”), 10mm (3/8”) 16mm (5/8”) and 25mm (1”). Sheets are available in clear, opal, bronze and specialty coatings. These specialty coatings provide properties such as diffused light. I will break it down more sheet by sheet.
What is the difference between the thicknesses?
The different thicknesses will have different properties such as light transmission, etc. But the difference is also in the amount of framing you will need under the sheets. The strength in your structure is really coming from the framing, not so much from the sheets.
You should be able to get a chart like the one below from the dealer you are working with. This will show you the purlin spacing in a rafter and purlin roof design. You will see that as the snow load goes up the frame spacing can be further apart for the thicker sheets.
Also, the thicker the sheet and the more walls – the higher the R value or insulation factor. If you are in an area where heating costs are a concern you will want to get the highest R value for your budget. That is unless you do not plan on heating your greenhouse in the winter. Then the R value is not that much of a consideration.
What is the difference between the colors?
Clear sheets will have the most light transmission and are the choice for a greenhouse unless you are using a specialty sheet such as Polymatte, Polycoolite, Lexan Softlite or Polisoft. These preceding 4 named sheets all supply 100% light diffusion with a light transmission comparable to clear. With 100% light diffusion the light is spread so that it reaches all parts of the plant equally, even under the plant canopy. This leads to higher production and a healthier plant.
Opal and bronze sheets do not have enough light transmission for a greenhouse unless you are growing plants with low light requirements. They are better off being used for patio or pergola covers. Remember, if you are covering a pergola the sheets need at least a 1 on 12 roof pitch. You cannot lay them flat.
Which thickness should I use?
Now you not only need to consider the thickness you will also have the option of how many walls. A twinwall sheet has two sheets on the outside with a rib running straight between them. A triplewall sheet will have an interior wall running parallel to the two exterior sheets. Also, when you get into polycarbonate sheets with 5 or more walls you may sometimes have an X shaped configuration.
- 6mm twinwall – This is an excellent choice for a patio cover, etc as you are not so worried about a higher R value on this project. It is also an excellent choice for a greenhouse in a warmer climate. Although I prefer the 8mm twinwall as it typically has an anticondensate coating on it. This is a coating that allows condensation to sheet off rather than come off in droplets. Healthier plants are the result of this. But, if the budget does not permit the 8mm, the 6mm is your next best bet.
- 8mm twinwall or triplewall – This is my choice for greenhouse glazing in a warm climate. It has the anticondensate coating as previously mentioned and all the profiles are readily available for it. The 8mm triplewall will have a higher R value than the 8mm twinwall. It will be a comparable value to a 10mm twinwall sheet.
- 10mm twinwall or triplewall – 10mm sheets do not require framing to be spaced as close as the 8mm sheets. So, if you are in a moderate climate with a small amount of snow this would be an option to consider. The triplewall sheets will have a higher R value than the twinwall sheets.
- 16mm triplewall or 5 wall – These are both excellent options for greenhouses for cold weather climates with a heavier snow load. Remember to build your frame with appropriate spacing as per the chart above. The 5 wall will have a higher R value than the triplewall.
- 25mm 3 wall – If you want to go even higher with the R value, this would be your choice. Please keep in mind though that typically you will have to use the aluminum profiles with this sheet. Polycarbonate profiles for 25mm are not readily available. The only real difference in the profiles is the look and the fact that the pricing will be higher for the aluminum. Just keep that in mind when you are looking at your budget.
Polycarbonate can be difficult to work with the first time you are using it. I think that most people tend to over think it some. There are a few basic installation rules that you must follow, but it is not all that complicated. There are a lot of options today and a lot of decisions to be made, more than when we started using it. Hopefully this breakdown will simplify this process for you. The biggest mistake is getting overwhelmed before you start and not getting your project done. So just take it step by step and enjoy your greenhouse when it is completed!