The next time your greenhouse or hoophouse needs a new covering, consider whether it would be smart to spend more for a special greenhouse film, instead of opting for the least expensive option of clear poly.

An infrared/anti-condensate (IR/AC) film costs more than clear poly, but not a lot more — and should save you a significant amount of money in the long run.

John Bartok, emeritus professor of agricultural engineering at the University of Connecticut, calculated that the payback on the IR/AC film, compared to clear 6-mil poly, is about one week. “With such a short payback, we can conclude that most growers should be using IR/AC film even if the greenhouse is operated only for the spring bedding plant season,” he said in a speech a few years ago.

He calculated that a $15 annual cost of the higher-priced film would generate $792 in fuel savings. “There are not very many places where you can invest $15 and get a $792 return,” he said.

The benefits may be even greater now than when he first did his analysis, with higher heating fuel prices and a smaller price differential between clear and IR/AC poly films.

“Covering a greenhouse with a double layer of poly costs about 30 cents per square foot,” said Zach Carr, greenhouse sales specialist at Growers Supply – FarmTek. “The added expense for IR/AC poly won’t add more than $100 to most structures.”

IR/AC polyethylene film contains two additives; the infrared additive reduces the amount of heat that radiates out of the greenhouse at night, and the anti-condensation additive raises the surface tension, allowing moisture to drain off the poly rather than forming droplets that drip onto plants.

Research showed that the IR additive can save up to 35% of the heat lost on a cold, clear night. Over a heating season the savings can average 15-20 percent.

Mark Miller of G&M Ag Supply said that experts recommend using double inflated poly for the best energy efficiency. The inner layer should be IR/AC poly, and the outer layer should be clear poly, to minimize expense and improve light transmission, which is slightly reduced by IR/AC film.

“It’s also really important to install the IR/AC layer correctly,” Carr said. “Pay close attention to the printing on the film that says This Side Down or This Side Toward Soil.”

The film won’t do any good if it’s installed wrong-side-down.
The anticondensate additive in IR/AC film is equally important to growing success, Carr said, because it prevents water from dripping on plants, which can cause foliar diseases and reduce quality by leaf spotting.

“In early spring especially it’s really cold outside,” he explained. “As soon as the sun comes up,the temperature inside the greenhouse will spike while it’s still very cold outside. That creates a large temperature differential, which creates condensation on the film.”

The AC additive, which is essentially a wetting agent, also improves light transmission by preventing condensation from forming droplets of water. Research shows that leaf temperatures remain higher under IR/AC film, resulting in a slightly earlier crop.

Woven poly film
Solarig is a newer poly film that is winning good reviews from growers in windy areas, according to Miller at G&M Ag Supply.

“It’s a woven, 9-mil thick material with a warranty of six years, and we’re finding it lasts even longer than that,” he said.
“We have been selling a lot of it in western states where they get a lot of wind in the spring. The cost is about double (the price of clear 6-mil poly)but the fact that it holds up well in wind and lasts longer is a big driving force.”
Solarig is more than twice as strong as clear poly.

Miller says that, whatever kind of poly you use, on a single-poly house you should put straps over the top of the greenhouse, between the bows, and pull it snug so the poly doesn’t have a chance to move in the wind. That’s not done with a double inflated poly house, however, because the inflation provides wind protection.

Another factor to consider when choosing a greenhouse covering is the light-diffusing properties of special films. Treatments often help diffuse light, a particular benefit for tall crops such as tomatoes and cut flowers, which can get too hot at the top in a clear-poly greenhouse. Light diffusing properties refract light so it is more evenly distributed throughout the greenhouse.