Thermal Curtains

Figure 16: Thermal curtain partially closed. Source: Scott Sanford

Figure 16: Thermal curtain partially closed. Source: Scott Sanford

In a greenhouse, 70 to 80 percent of the heating occurs during night hours. A substantial reduction in energy use can be realized with the use of thermal curtains. Some growers have reduced heating bills by up to 30 to 50 percent.

A thermal curtain is a fabric sheet and support system that is pulled across the roof area (sometimes walls) of a greenhouse during nighttime hours to reduce heat loss (Figure 16). Curtains work by reducing the volume of the heated space in a greenhouse and provide an additional thermal barrier. High-efficiency thermal curtain materials are designed to reflect the infrared radiation emitted by the greenhouse structure, plants and benches back into the greenhouse. Some curtains can also double as shade cloth in the summer to reduce ventilation and cooling requirements by reflecting unneeded light (heat) out of the greenhouse. There are a variety of curtain materials available to meet different crop needs, with energy savings ranging from 20 to 75 percent [4].

 

Curtain Materials

There are many types of screen materials on the market. Semi-porous curtains made of alternating strips of clear and aluminized polyester or acrylic fabric are best for combined heat retention and shading. The aluminized strips reflect unneeded light out of the greenhouse during summer and reflect heat back into the greenhouse at night during cold weather. Flame-resistant fabrics are recommended and may be required by state or local fire codes. Check with your building inspector to determine local code requirements. Porous fabrics with gaps or open weaves for air circulation have some value for heat retention (about 20 percent) but are principally used only for shading.

 

Installation 

Curtain systems can be installed in almost any greenhouse but some modifications may be required. If plants, lights, heat pipes, air ducts or irrigation lines are hanging from the roof structure, they may need to be moved or supported from the ground before curtains can be installed. Manufacturers of thermal curtains have developed curtain system designs that avoid the need to move some of the items supported by the roof structure. A thermal curtain system can be opened and closed by hand or can be fully automated to open and close based on temperature, solar radiation and /or time of day. All edges of the curtains need to be sealed when closed to prevent a chimney effect where warm air rises above the curtain and displaces cold air. Such an effect can cause localized crop damage as the cold air falls to the floor.

When installed and used correctly, thermal/shade screens can improve plant growth and reduce both heating and cooling costs, with payback times for an average greenhouse at three to five years [3].