Curtains: Your #1 Fuel Saver

Installing new thermal screen or replacing your outdated fabric can save enough to return your investment in just a few seasons.

With the price of natural gas and other fuels going through the roof, your heat shouldn't be. There's no excuse for not installing thermal screens (also known as heat retention curtains or thermal blankets) in your greenhouses. Research institutions such as Rutgers, Cornell and Penn State have documented energy savings potential of 60% in houses equipped with thermal screens. Other research has shown that screens are as cost effective as any energy conservation method. And cost analyses show that materials and installation expenses for a screen system can be recouped in one to three years.

But what do screen really do? Basically, they save on energy costs in two ways. First, screens cut down the amount of air space in the greenhouse that needs to be heated. Second, thermal screens provide an insulation factor, keeping the cold our and the heat in.

Thermal screens improve both day and night environmental conditions, and they're now available in a number of different designs. To make a proper choice and achieve the desired results, it's crucial to realize that the screen is part of your total greenhouse system.

A little background

Ken Winspear, of the Silsoe Research Institute in the United Kingdom, is credited with developing the first internal greenhouse screen. Ken proved way back in 1948 that greenhouse curtains could be used to manipulate light and heat.

Imperial Chemicals, also from England, is credited with being one of the very first companies to make greenhouse curtains commercially available. At about the same time, a United Sates company called Simtrac began producing greenhouse curtains. Though now out of business, you can still find greenhouses with Simtrac curtain systems.

The Dutch, though often credited with inventing curtains, actually showed tremendous resistance to the use of internal curtains, because they believed the loss of light and consequential loss of productions wouldn't offset the energy savings. But this was when oil was cheap and energy savings weren't much of a concern.

By 1978, energy costs were soaring, and, given that growers spend somewhere between 70 and 80% of their energy dollars in heating their houses, the need for internal screens became obvious.

Screens vs. no screens

If we look at temperatures and energy consumption in two identical greenhouses one with a screen and one without a screen, we find two differences:

First, the screen will allow for the heat to be switched on at a later time, as the temperature drops more slowly under the screen.

Second, when the heat is switched on, less fuel is used for the set-point temperature. Using both effects the total savings of this thermal screen can amount to at least 70%. As an added benefit, you can design a screened house with smaller heating units that are required in an unscreened house.

Along with energy efficiency, research conducted by James Faust, Royal Heins and Paul Kiefer found that thermal screen also increases plant temperature. According to their findings, plants remain warm due to their emission of long-wave radiation. Plants actually exchange energy with the thermal screen. They found that the colder the outside temperature, the more pronounced the benefit of the thermal screen with regards to plant temperature.

Screen materials

Porous cloth materials are often used for shading and are made of white woven materials, but porous cloth is also a common material for internal curtains. The advantages of porous cloth are that it's easy to handle and it allows for water drainage. The major disadvantage of porous cloth is that it's not very efficient in retaining heat. The warm air flows easily through the cloth, giving it a heat transmission of .6 Btu/hr. This compares to having no curtain at all, which equals .8 Btu/hr. heat transmission.

Nonporous materials such as polyethylene, when properly installed, offer a little over .4 Btu/hr. of heat transmission. They're often used as blackout curtain for photoperiod control. But they do trap water and condensation. And they block out all light, which may not be desirable when trying to retain heat during daylight hours. They can also be heavy.

The third material for heat retention is aluminized fabric. The aluminized material is an integration of aluminum and thread. The aluminum reflects harsh sunlight while retaining heat radiation at night. It's lightweight for ease of handling and porous enough to not trap water. Aluminized screens offer roughly .3 Btu/hr. of heat transmission.

The majority of screens now available on the market have aluminum on the upper side, so the radiation of the sun is reflected upwards out of the greenhouse and won't be converted into heat. This lets reflective aluminum screens reduce the temperature during the day. The material also has the advantage of being aesthetically pleasing.

Another advantage of an aluminum upper-side is its low emission value for long-wave heat radiation. This means, when you close the screen during the night, it'll reduce the heat loss, working like a thermos to hold heat in.

Controlling your curtain

While curtains can be opened and closed manually, most growers choose to link them to their environmental control computer to automate them based on time of day, light, heat, humidity or all of the above. But you can also use a simple time clock. Some growers have photo sensors that detect the light level and close or open the screen for shading. An advantage of a fully automatic system is that it can control the speed at which the screen opens to prevent it from opening too quickly and dumping cold air onto your plants.

When you should close or open the screen will depend on outside weather conditions. The screens can be opened slowly, or some growers wait until the sun warms the upper area of the greenhouse. In areas where snow is an issue, growers will leave the screens open to keep from insulating the top of the house from heat.

Keeping it working

Maintenance of the screen fabric is minimal, as it's not exposed to the outside elements. Your biggest risks are gutter drips or water pipe leaks causing algae growth. Be careful trying to clean up such a problem as most chemicals can be harmful to screen materials. Contact your curtain or greenhouse manufacturer for advice.

You do need to pay attention to the drive system that supports and moves the screen. Be sure that limits are set correctly to avoid stressing the screen or drive mechanism. You should check all moving parts for wear and lubricate them annually.

The biggest maintenance issue we see is worn monofilaments on sliding screen systems (where the screen slides on top of monofilament support wires, rather than being suspended below them). This is the biggest reason for screens failing before they should. If you see this happening replace the worn wire immediately to prevent excessive wear to the screen. It's smart to install new support wires whenever you install new material.

The payoff

During the past two decades, technology has revolutionized our industry. With energy costs so much at the forefront of the greenhouse business today, it's good to know there's simple, proven technology that can quickly pay for itself just by the fuel it can save.

John Walters
President North and South American Division of Ludvig Svensson
Charlotte, North Carolina

Article compliments
of Agra Tech, Inc.