February 2002

Greenhouse Curtain Systems By National Greenhouse Manufacturer's Association

What are greenhouse curtains? Greenhouse curtain systems are called shades, screens and evenblankets. They consist of moveable panels of fabric or plastic film used tocover and uncover a greenhouse. Curtains may cover an area as small as a singlebench or as large as an acre. Small systems are often moved by hand, whilelarge systems commonly use a motor drive. Curtains are used for heat retention,shade and day length control.

How do the curtains provide heat retention? Any interior curtain system can be used for heatretention at night when the heating demand is greatest. Blackout systems canserve this purpose, even when day-length control is not a consideration. Theamount of heat retained and fuel saved varies according to the type of materialin the curtain. Curtain systems can save energy in three ways: they trap aninsulating layer of air, reduce the volume that must be heated, and when theycontain aluminum strips reflect heat back into the house. A curtain system usedfor heat retention traps cold air between the fabric and the roof. This coldair falls into the space below when the curtain reopens in the morning. Toavoid stressing the crop, it is important to uncover the curtain gradually to allowthis cold air to mix with the warm air below. Alternatively, if the crop cantolerate the shade, the curtain can be left uncovered until sunlight warms theair below the system.

How can I attach the curtain to my greenhouse? The fabric panels in a curtain system can be drivengutter-to-gutter across the width of the greenhouse or truss-to-truss down itslength. In a gutter-to-gutter system, each panel of curtain material isessentially the size of the floor of one gutter-connected house. In a truss-to-trusssystem, the panels are wide enough to span the distance between one truss andthe next. In either configuration, each panel of curtain material has astationary edge and a moving edge. The drive system moves the lead edge backand forth to cover and uncover the curtain while the stationary edge holds thepanel in place.

How does the gutter-to-gutter system work? The curtain panels are pulled flat across the widthof the greenhouse at gutter height. This configuration minimizes the volume ofgreenhouse air below the curtain that must be heated. These systems requireless installation labor than a typical truss-to-truss system, but guttersystems are not ideal for every greenhouse. If unit heaters or circulation fansare mounted above gutter level, the curtain will block them from heating orcirculating the air under the system where the crop is. Though the volume ofgreenhouse space that is heated is reduced, the amount of cold air ismaximized. This makes it harder to mix and reheat the air above the system whenit uncovers in the morning. Retrofitting can also be a problem if the gaslines, electrical conduits and heating pipes are mounted at gutter level.

How do truss-to-truss systems work? With a truss-to-truss system, the panels of curtainmaterial move across the distance between trusses. There are three ways toconfigure the truss-to-truss system. First, it can be flat at gutter height,minimizing heated areas and making installation easy. Second, it can beslope-flat-slope, where the profile of the curtain follows each slope of theroof part way up the truss with a flat section joining the two slope segments.The benefit of the slope-to-slope curtain system is that it can be installedover equipment and mounted above the gutter. The third is slope-to-slope, wherethe profile of the system parallels a line drawn from the gutter to the peak ofthe truss. This configuration minimizes the amount of cold air trapped abovethe curtain.

What type of material are shade and retention curtainmade of? Covering materials for shade andheat retention include knitted white polyester, non-woven bonded whitepolyester fiber and composite fabrics. White polyester has largely beensuperceded by composite fabric made of alternating strips of clear andaluminized polyester or acrylic held together with a finely woven mesh ofthreads. These panels outperform polyester because their aluminized stripsreflect infrared light out of the greenhouse during the day and back into it atnight. Composite fabrics include : chemical stabilization against breakdown byUV light, flame-resistant fabrics to meet building code requirement and fabricswith gaps for air circulation.

What type of materials are blackout curtains made of? Blackout curtains include polyethylene film andcomposite fabrics where all the strips are either aluminized or opaque. Mostblackout materials attempt to reduce heat buildup where the curtain system iscovered by day-length control in the summer. Knitted polyester is availablewith aluminum reflective coating bonded to one surface. Polyethylene film is byfar the least expensive blackout material, but it is impermeable to water andwater vapor. If the greenhouse leaks when it rains, water can build up inpockets of the film, and the weight can damage the curtain. Polyester knits andcomposite fabrics are porous and allow water and water vapor to pass through,reducing the chance of water-weight related damage and offering a longer life.

What types of exterior curtain systems are available? There are three types of exteriors curtain systemsavailable. A motor and gear driven shade system can be mounted above thegreenhouse roof to reduce the amount of heat and light that enters thestructure. A dark colored or aluminized mesh can be stretched over thegreenhouse roof and left in place for the duration of the high light season.The curtain system can serve as the greenhouse roof, uncovering for maximumlight and ventilation and covering for weather protection.

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