Shade screens in a greenhouse. Notice gaps between
the screens for moisture and air exchange.
Jessica J. Prenger
Research Engineer
Peter P.
Ling
Assistant Professor
Curtains and screens are commonly used for shading and photoperiod control, and they have found wide use as thermal insulation in recent years. In the following discussion, design and implementation of curtains and screens will be explained and related to the prevention of condensation on plants. Very high humidity and condensation on plant material provide an ideal environment for pathogens like Botrytis to grow. One way to prevent condensation on the leaf surface is to keep its temperature above the dew point of the air surrounding it. Thermal insulation provided by screens helps trap warm air nearer the crop. Screens must be managed carefully, but their flexibility makes shading and thermal screens an important part of environmental control for greenhouses.
Though curtains and screens were first developed to shade sensitive crops and to manage the photoperiod of others, the insulation supplied by thermal screens helps keep heat near the plants, and prevents the loss of radiative heat to the cold night sky during the evening. In addition, thermal screens maintain better heat distribution under the screen (Bakker et al. 1995). The substantial savings for this energy conservation practice are demonstrated in Table 1.
| Table 1. Heat loss reduction from increases in thermal resistance. | |
|---|---|
| Covering System | Daily Heat Loss Reduction (%) |
| Single layer lapped glass | 0 |
| Single layer lapped glass plus thermal blanket | 36 |
| Double skin plastic structural panel | 46 |
| Double layer polyethylene | 27 |
| Double layer polyethylene plus thermal blanket | 57 |
| From Greenhouse Engineering NRAES-33. Northeast Regional Agricultural Engineering Service, Cooperative Extension. | |
Growers can create a shading/insulation system that is appropriate for their operation from the variety of curtain materials available. The material is the least expensive part of screen system construction and installation, so single and multi-layer systems have similar costs. Thus, many designs have screen layer(s) with excellent shading or blackout properties and additional layer(s) for thermal insulation during the night and winter. Moreover, a screen system is most cost effective when it has multiple purposes (Bakker et al. 1995). Screens used for photoperiod control are often made with materials having excellent thermal insulation properties (Table 2). On the other hand, screens specifically for thermal insulation (typically polyethylene film) deployed during the day must also allow acceptable light transmission.
| Table 2. Heat transmission values and heat loss reductions for various thermal screen materials. | |||
|---|---|---|---|
| Blanket Material | Heat Transmission Value (Btu/hr-¢XF-ft2) |
Heat Loss Reductiona (%) | Applications |
| Mobile air curtain (double layer polyethylene film) | 0.68 | 20 | C/E |
| Stationary air curtain (aluminized PE tubes) | 0.54 | 36 | E |
| White-White spun bonded polyolefin film | 0.51 | 40 | C/E |
| Grey-White spun bonded film (heavy weight) | 0.43 | 49 | S/E |
| Grey-White spun bonded polyolefin film (lt. wt.) | 0.56 | 34 | S |
| Clear polyethylene film | 0.45 | 47 | C/E |
| Black polyethylene film | 0.48 | 44 | B/E |
| Aluminum foil-clear vinyl film laminate | 0.40 | 53 | S/E |
| Aluminum foil-black vinyl film laminate | 0.63 | 26 | B |
| Aluminized fabric | 0.39 | 54b | E/S |
| Double layer spun bonded polyester (tobacco shade cloth) | 0.53 | 38 | B/E |
| Applications: B-black out; C-climate control; E-energy
savings; S-shading (a) Compared to single glass for the same greenhouses for night time heat loss. (b) Average of four years of test data. From Greenhouse Engineering NRAES-33. Northeast Regional Agricultural Engineering Service, Cooperative Extension. | |||
Other design considerations for shading systems include providing an adequate seal around the outer edge of the screen. The screens should prevent air drafts above and below the curtain except in specific controlled situations, which will be discussed in the next section. Another concern is the location of the screens when they are not being used. The screens and conveyance system should not hinder lighting and cultural practices when they are not in use.
Managing thermal blankets is a challenge when humidity and condensation are taken into account. In the winter months, humidity can build up under the screen during the night because of the plants' transpiration in a heated environment (de Graaf, 1985). A grower can alleviate the problem by establishing a "moisture gap" between the curtain and the greenhouse covering, allowing some movement of air between cold attic space above the curtain and warm humid space below it (Bakker et al. 1995). To create air movement through plastic film screens, they can be perforated much like fan-jet poly tubes.
A large temperature difference between the attic space and space under a thermal screen can cause condensation to deposit on the material. This may lead to a shower of condensation onto the plant canopy when the screen is moved. Another threat occurs when blankets are opened after a cold night. "Cold shock" happens when the cold air trapped in the attic space drops onto the plant canopy. To prevent damage to the plants and condensation on the leaf surfaces, the blankets should be opened incrementally.
Thermal blankets provide excellent insulation, keeping the plant environment warm and reducing energy costs. Using the curtain for shading or photoperiod control as well as energy conservation can have a tremendous impact on energy consumption and improved production. However, screens must be managed carefully to avoid excessive humidity and condensation problems on the material itself. Selecting and controlling these screens appropriately can be a successful component of an integrated disease control strategy, and can make a healthy contribution to the bottom line.
Aldrich, R.A. and J.W. Bartok, Jr. 1989. Greenhouse Engineering. NRAES-33. Northeast Regional Agricultural Engineering Service. Cornell University, Ithaca, NY.
Bakker, J.C. et al. (Eds.) Greenhouse Climate Control-An integrated approach. Wageningen, The Netherlands: Wageningen Pers. 1995.
de Graaf, R. 1985. The Influence of Thermal Screening and Moisture Gap on the Transpiration of Glasshouse Tomatoes During the Night. Acta Horticulturae 174:57-63.