Chapter 3

Greenhouse Curtain Systems

Greenhouse Curtain Materials

The main types of curtain materials are nonporous, porous, and semi-porous. Greenhouse film is an example of a nonporous material. The disadvantage of nonporous materials is that when condensation drips off the roof onto the curtain, it puddles and can eventually add enough weight to cause the curtain support to fail. Therefore, nonporous materials are not recommended for greenhouse curtains.

Shade and Heat Retention

Energy and shade screen systems can provide 30 to 50 percent savings in heating costs. Often referred to as energy blankets, they save energy by reducing the heat loss surface area, providing an extra insulation barrier and trapping a layer of dead air on both sides of the screen material. Covering materials for shade and heat retention include knitted white polyester, non-woven bonded white polyester fiber, and composite fabrics manufactured specifically for use in greenhouse curtain systems. The white polyester fabrics offers excellent durability. Some systems are still using the original fabric panels after more than 15 years. White polyester has largely been superseded by composite fabrics made of alternating strips of clear and aluminized polyester or acrylic held together with a finely woven mesh of threads.

Blackout Curtains

Blackout curtain materials include polyethylene film, knitted polyester, and composite fabrics where all the strips are either aluminized or opaque (Figure 3.7). Most blackout materials attempt to reduce heat buildup when the curtain system is covered for day length control in summer. So-called black/white poly is a polyethylene film that is co-extruded with white top layer for light reflection, and a black bottom-layer for opacity. Knitted polyester is available with a powdered aluminum reflective coating bonded to one surface.

Light-Emission Prevention Curtains

Specific blackout screens for light abatement are typically single layer curtains that block up to 99 percent of light from leaving the greenhouse. These curtains offer similar energy savings as the thermal curtains as well as help to mitigate humidity build-up.

Thermal Curtains Performing Multiple Functions

Curtain performs multiple functions: (1) heat retention on winter nights, (2) partial sun screening on bright summer days, and (3) for light exclusion during photoperiod manipulation of plants. If all three functions are required in a greenhouse, two automatic screen-pulling systems will have to be installed, which is often done. One opaque curtain material can be used for both photoperiod control and thermal insulation.

Selecting Energy/Shade/Blackout Screen Materials

The best energy screens are those that have a low emissivity (reflect heat) and a low transmissivity (i.e., transmit radiation through the fabric). Screens with aluminized surfaces or aluminum strips meet this criterion and have the lowest radiation loses. Generally, if you install a single screen for energy conservation, select one with a closed weave and aluminum strips. This will provide shading during the day and energy conservation at night. Cracking the screen sections a few inches wide during the day will allow excess moisture to escape through roof vents. In hot climates, shading is important both for plant and air temperature control. In this situation, an open weave material works best. It allows the heat to rise and escape out through roof vents. It also allows the heat in the moisture to escape. The aluminum or white plastic strips reflect the heat before it reaches the plant area.

Edge Seals

All curtain edges must be sealed when the curtain is closed to prevent warm air from moving to the cold side of the curtain and cold air from displacing the warm, creating a chimney effect. Such an effect can cause localized crop damage as the cold air falls to the floor. The seals don’t have to retain any pressure, so they can be as simple as the edge of the curtain material overlapping the edge of another object. The leading edge of the curtain can be sealed in a number of ways. Some companies offer aluminum extrusions with a pocket and rubber seals for the leading-edge tube to rest against when the curtain is closed.

Maximizing Curtain Efficiency

Shade curtains should be selected with the type of crop (and its light and temperature requirements), physical location of the greenhouse, and type of greenhouse cover in mind. All greenhouse structures and cover materials block some light. Glass greenhouses can reduce light by 30 percent before reaching crop level, greenhouses of other materials such as polycarbonate can reduce light 55 percent relative to the outdoors. This should be taken into account when selecting the shade level of the greenhouse curtains, as well as in planning out the supplemental lighting.

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