Don’t Mix PVC Pipe and Polyethylene Greenhouse Film

Emmalea Ernest, Extension Associate-Vegetable Crops;

Here is another article you can file under “Learn from My Mistakes/Mishaps” (previous example here). When my inflated poly greenhouse at the Georgetown research farm went flat about a month ago I thought the blower had gone bad. It turned out that there were actually giant holes in my less-than-one-year-old five-year greenhouse film. The holes corresponded to areas of wear in places where the plastic touched the PVC electrical conduit. There was no sign of wear around the metal support structure. Could the conduit be reacting with the plastic film?

Greenhouse film around the PVC conduit with cloudy discoloration and signs of wear.

One of numerous large holes where the conduit touches the greenhouse film.

Yes, it turns out, that PVC electrical conduit can out-gas chlorine which destroys the UV light stabilizers in the polyethylene greenhouse film and causes the film to degrade. In talking with various greenhouse suppliers and plastic manufacturers I learned that there is a sizable list of materials that can react with greenhouse film and cause wear including any type of PVC pipe or PVC tape, oil based paints or wood preservatives, chlorine based disinfectants, and certain pesticides, especially those containing sulfur or copper.

To prevent premature wear to greenhouse film, manufacturers recommend that surfaces that the film touches should be covered with white acrylic latex paint (but not the mildew resistant kinds that contain fungicides) or non-PVC tape. The film should not be in direct contact with any of the materials listed above that can cause breakdown or wear. In fact failure to follow these recommendations can cause the warranty on the film to be void. There seem to be some differences between manufacturers in tolerance of films to chemical interactions and installation recommendations and requirements, so it is a good idea to check with your greenhouse film supplier and the film manufacturer if you have a concern.

I have been told that some growers wrap PVC pipe that contacts the greenhouse film with old poly film. That would be an inexpensive means of dealing with the problem, but in my greenhouse I was concerned about that creating habitat for pests and diseases. After talking with the plastic manufacturer of the new film I am purchasing, I have decided to try covering the conduit with pipe insulation made out of polyethylene, which is readily available and inexpensive.

Some of the problematic conduit, soon to be covered with pipe insulation.

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