Greenhouse plastic film is easily the least expensive method of covering a greenhouse.
But greenhouse poly film, if used correctly, is also quite effective.
In addition to cost savings, using polyethylene film to cover your greenhouse will provide other benefits.
But there are also a few disadvantages to using this most popular form of greenhouse covering.
If you’re considering covering your greenhouse with plastic poly film, you’ll need to be selective when making your purchase.
Because there are many variations of plastic greenhouse film available, all of which significantly impact both the effectiveness and durability of the plastic film.
These variations primarily involve the thickness of the greenhouse film, and also the resistance of the film to degradation by sunlight.
Whether you’re building a large commercial greenhouse or a little backyard hobby greenhouse, you’ll need to understand these differences to be able to select the best greenhouse plastic film for your needs.
Greenhouse poly plastic is available in a range of thickness, generally ranging from 3 mils up to 8 mils (a mil is 1 thousandth of an inch).
The thickness of the plastic mostly impacts the durability and expected useable life of the plastic. Also, the thicker the plastic, the less light is transmitted through it. But the thickness of the plastic has little effect upon the amount of heat that can be lost through the plastic.
6 and 8 mil plastic are the most commonly used thickness of greenhouse plastic.
Aside from the occasional tornado, hurricane, or hailstorm, more damage is done to greenhouse plastic by sunlight than any force of nature. It’s the ultraviolet component of sunlight that does the damage, resulting in yellowing and weakening of the plastic.
Fortunately, you can buy greenhouse plastic that has built-in resistance to the damaging effects of UV radiation.
If you buy a plastic that does not have UV protection, you can expect a very short service life from that plastic – very possibly no more than a year. Most greenhouse plastics that ARE treated with UV protection, on the other hand, have a service life rating of from 3 to 5 years.
Though less critical than thickness and UV resistance, there are other features available in greenhouse film that increase the effectiveness of the film, but also raise the cost:
Most manufacturers offer an optional feature called an anti-condensate coating.
This coating is applied to only one side of the plastic, and that side is to be used as the inside surface of the plastic. If you were using a double-layer of plastic, you’d use a sheet with the anti-condensate only on the inner layer; it’s not needed on the outer layer.
The purpose of the anti-condensate is to prevent droplets of water forming on the inner surface of the plastic.
A greenhouse filled with plants tends to be a highly humid environment. And with cold outside temperatures and warm, moist air inside of the greenhouse, there’s a tendency for condensation to form on the inner surface of the plastic covering. Just like on a glass of iced tea on a humid summer’s day.
If there’s enough condensation, the droplets coalesce on the surface of the plastic and eventually become heavy enough to fall off and rain down on the plants below.
That’s bad, because wet plants in the humid environment of a greenhouse is a prescription for disease problems. And if you’re working in the greenhouse in these conditions, you’ll find that it’s none too pleasant to be rained upon while you work.
Condensation on the inner surface of the greenhouse plastic film also creates a secondary problem by reducing the amount of sunlight that enters the house.
The anti-condensate coating is applied at the time the plastic is manufactured, and is designed to cause the water to sheet off of the plastic rather than forming droplets. It’s also possible to purchase the anti-condensate formula separately, and spray it onto the plastic yourself.
(I should add that in my years of greenhouse growing, I haven’t found the anti-condensate coating to be terribly effective. Though we’ve selected that option each time we’ve purchased new plastic for one of our greenhouses, the actual reduction in condensation has been minimal, at best, and not very long-lasting. Just my personal experience; others may have found the coating to be more beneficial.)
Greenhouse plastics are now available that can block infrared radiation (IR).
This feature can be used to great advantage in a greenhouse, because the infrared portion of sunlight is what produces the heat. But the light that plants need for photosynthesis is a different portion, or wavelength, of sunlight.
So by cutting down on the amount of IR entering the greenhouse, there’s less tendency for the greenhouse to be too hot during summer. But conversely, the amount of heat that can leave the greenhouse is also reduced. So there’s less heat loss in winter, making the greenhouse more energy efficient.
Greenhouse plastic film can be used as either a single-layer covering, or as a double-layer covering with an air space between the 2 layers.
Since air is an excellent insulator, a double-layer greenhouse will retain heat far more efficiently than a single-layer greenhouse. The trade-off is that a single-layer house allows more sunlight to reach the plants within.
In a double-layer house, a blower runs continuously to maintain an air layer of a few inches between the inner and outer covering.
The blower is normally set-up to draw air from outside the greenhouse, rather than drawing humid air from within the greenhouse. This reduces the risk of moisture condensing between the 2 layers of plastic and causing a reduction in the amount of sunlight entering the greenhouse.
The combination of relatively low cost, effective and efficient operation, and ease of installation have made greenhouse plastic film the most popular type of greenhouse covering.
If you choose this form of covering for your greenhouse, wind will be your enemy when the time comes to apply the plastic. And if that enemy shows up in strength, you will not be able to defeat it (unless you’re only covering a very small greenhouse).
So no matter how long you have to wait for the weather to cooperate, make sure you pick a calm day for the job.
A very calm day.
Otherwise, I predict your nerves will be anything but calm by the end of that day! (Yep – been there, done that. But that’s a story for another page.)