Controlled Environment Agriculture Coverings and Glazing Materials

Introduction

Controlled environment agriculture (CEA) operations utilize various types of structures, and each is enclosed in different materials. Indoor vertical facilities are typically constructed of materials such as poured concrete, prefabricated concrete walls, steel, and insulated metal wall and roof panels. Shipping containers used for CEA are typically made of steel (often Corten steel) that may be uninsulated or insulated. In these cases, the components that cover or enclose the facility are typically not transparent and do not allow outside light from the sun to enter. Therefore, in these facilities, all of the light for plant growth is supplied by artificial light sources.

In contrast, greenhouses for CEA crop production often have a structural support framework made of wood, steel, or aluminum. The greenhouse is then enclosed with a translucent covering, often called glazing, that seals the structure but still allows natural sunlight to enter. There are many types of greenhouse glazing materials, and each has specific characteristics. The characteristics of each type of glazing dictate its best uses and limitations.

Important Characteristics of Greenhouse Glazing Materials

Many factors need to be considered when selecting a glazing material. The life of the material, its strength, its weight, initial cost, light transmittance, thermal conductance, maintenance issues, and flammability are all important factors to consider.

Light transmittance

The higher the light transmittance of a glazing, the more sunlight that can penetrate the glazing and potentially enter the greenhouse. In northern climates and in the winter, light is often the limiting factor for photosynthesis. Therefore, the highest possible transmittance level is desirable. Sometimes, such as in summer or in southern or equatorial locations, the amount of light entering the greenhouse is above optimal levels. In these situations, a shade cloth, a retractable shading system, or a shading compound may be used to temporarily reduce the amount of light entering the greenhouse. When light levels drop below optimal, the shading material is retracted or removed.

Light transmittance of a glazing is not constant. As glazings age, they tend to have a reduction in light transmittance due to scratching from dust and debris and aging or "yellowing" of the glazing material due to ultraviolet (UV) exposure. Additionally, light transmittance passes through a new, clean, unobstructed panel or sheet of glazing positioned perpendicular to the light source. The light levels entering the greenhouse and reaching the plants might be much lower than the transmittance of the glazing because of light being blocked by the supporting structure, effects from the angle of the sun to the structure, and any debris on the glazing. For example, in a reported example in a glass-glazed CEA greenhouse, the percent of light reaching the plant canopy was only 56% of the outside light level. In a double polyethylene greenhouse, the percent of light reaching the plant canopy was only 45% of the outside light level.

In addition to the amount of light passing through the glazing, the wavelengths of light that pass through can be important to the plants inside. A glazing that screens out different wavelengths of light (e.g, red light or far-red light) can impact the morphological development of the crop. Many plastic glazings reduce the amount of UV light that passes through, which can impact disease development as well as pigment formation (e.g red coloration from anthocyanins) in the crop.

Thermal conductance

Thermal conductance refers to heat loss from inside the greenhouse through the glazing to outside of the greenhouse. In other words, how well does heat energy move through the glazing.

Thermal conductance can be expressed as Btu loss/ ft²/hr/(oFinside - oFoutside). However, when evaluating glazings, it’s most common to compare the “U” and “R” values of a glazing to determine its heat loss potential. The “U” value is the overall coefficient of heat transfer and includes all elements of construction. It is basically a measure of heat loss from a structure that is glazed with a particular glazing. It is important because different types of glazings are mounted to the greenhouse differently, and these mounting materials can also conduct heat. The lower the “U” factor, the lower the rate of heat loss from the greenhouse structure. The “R” factor is a measure of the resistance to heat flow. The higher the “R” value, the more resistant to heat flow and the better insulated the glazing.

In most situations with greenhouse glazings, the “U” and “R” values are inversely related. It is usually desirable in greenhouse glazing to have as low a “U” (heat loss) factor and as high an “R” (resistance to heat flow) factor as possible.

Strength

The stronger the greenhouse glazing, the more resistant it is to breakage from debris or weather events such as high winds and hail. Therefore, the higher the strength, the lower the probability of breakage and the associated costs of replacing the glazing. However, often glazings with a high level of strength are not very flexible.

Weight

The heavier the glazing material, the higher the dead load on the structure. To account for the increased dead load, a stronger support structure is required. This results in increased construction costs and may reduce greenhouse light levels due to an increase in obstructions created by the supporting structure (e.g., trusses blocking light).

Life span

A short glazing life span means frequent replacement. Therefore, the initial cost of the glazing may be low compared to other glazings, but after the glazing is replaced several times, it may become less economically attractive than one with a higher initial cost and a longer lifespan that spreads the cost over many years.

Scratch resistance

Dust, soil particles and other debris can scratch the glazing. Scratching reduces the glazing's light transmittance, thereby lowering light levels inside the greenhouse. This, in turn, may require more frequent replacement of the glazing and increase costs. Some glazings can be treated with a compound that improves scratch resistance.

Cost

All aspects of the cost of a glazing need to be considered. These include the initial cost of the glazing material, structural support costs, the glazing's lifespan, and the glazing's thermal conductance. A glazing material with a high initial cost compared to others may be more economically attractive if it has a long lifespan or low thermal conductivity, thereby reducing heating costs.

Common Greenhouse Glazing Materials

Glass

Many types of glass are available, including floated glass, insulated glass, low-iron glass, and safety glass. Different thicknesses are also available. Typically, standard single-layer glass used for greenhouses has a light transmittance of 88% to 94% when used as a single layer and 77% as a double layer (Table 1). Double- strength glass has a light transmittance of approximately 88%, and insulated glass has a transmittance of approximately 78%. Low-iron glass will have the highest light transmittance levels. Glass-glazed greenhouses may have relatively higher air infiltration rates due to the space between glass panels. Therefore, glass tends to have a higher thermal conductance than many other glazings. Also, because of the higher air exchange rate, glass-glazed greenhouses typically have lower relative humidity levels than greenhouses glazed with other types of glazing materials. Glass is resistant to heat, UV light, and abrasion but has a relatively low impact resistance. Glass can be relatively expensive to purchase and install and requires special supports to hold the glass panels in place and support their weight. However, glass has a long life span, often exceeding 25 years. Most commercial greenhouses no longer use glass as a glazing material because of the high weight and cost. However, safety glass is often used in botanical centers and conservatories.

Polyethylene film

Polyethylene film is a common greenhouse glazing material, particularly adaptable to Quonset or arched-roof structures due to its flexibility. It is low-cost, lightweight, and easy to install. Typically, standard polyethylene film has a light transmittance of 85% to 87% for a single layer of film and 74% to 77% for a double layer (Table 1). The thermal conductance of polyethylene film is high but varies across polyethylene brands and between single- and double-layer configurations.

Additives may be included in the film to increase lifespan, reduce condensation, or reduce heat loss. These additives may be sprayed onto or incorporated into the film through a process known as coextrusion. During coextrusion, three layers of polyethylene are laid down to form a single sheet of polyethylene film. Each layer may include materials that alter the film's properties.

Polyethylene is short-lived in comparison to other glazings. Without additives, polyethylene will last only one to two years before needing replacement. This is because it is very susceptible to degradation by UV light.

However, if additives are included during coextrusion to make the material more resistant to UV light, the polyethylene glazings might have a lifespan of three to four years or longer.

As mentioned above, polyethylene film has a high thermal conductance. However, some brands of polyethylene films have an IR (infrared) reflector added to the inside layer of the film, which reduces heat loss through the glazing. This is common in northern climates to reduce heating costs in the winter.

Another problem with polyethylene glazing is that of condensation and dripping. Because of the temperature difference between the inside and outside air, water vapor tends to condense on the surface of the polyethylene film inside the greenhouse. Because the film is very hydrophobic, water tends to bead and collect on the surface until large enough drops form that they fall from the glazing onto the plant materials below. This dripping of water from the glazing onto the plants can increase disease incidence and reduce crop uniformity. An additive may be sprayed onto the film or incorporated into it and essentially acts as a wetting agent. This prevents water beading and allows smaller droplets to form that run down the glazing and onto the floor rather than dripping onto the plants.

Usually, a 6-mil (0.15 mm) thick film is used for greenhouses when a single layer is used. If a double layer is used, 6 mil is on the outside and 4 mil (0.10 mm thick) on the inside. In a double polyethylene glazing system, a small squirrel cage fan is used to force air between the layers. This provides a "dead" air space that serves as insulation and decreases thermal conductance.

Ethylene Tetrafluoroethylene film

Ethylene Tetrafluoroethylene film (ETFE) is produced in translucent sheets or rolls much like polyethylene film. It is strong, hail-resistant, and UV-resistant. It comes in various thicknesses, from 2 mm to 20 mm, but the materials used to glaze greenhouses are typically 1.10 mm to 1.60 mm thick. It may be used to cover greenhouses using a single, double, or triple layer. Properties vary based on film thickness and any additives that may be applied to the film for purposes such as increasing diffusion of light or preventing condensation and dripping. Generally, a single- layer greenhouse ETF will have a light transmittance of 87% to 94% and an R-value of approximately 1.0 (Table 1). Two layers of ETFE will have a light transmittance of 81%- 88% and an R-value of approximately 2.0.

Three layers of ETFE have a light transmittance of 77% to 83% and an R-value of approximately 3.0. Light levels will vary between standard ETFE and diffuse ETFE. Unless an additive is applied to the film to reduce UV transmission, ETFE allows approximately 94% of UV to pass through the material. Therefore, ETFE might be a good glazing choice where higher UV levels are desired in the production environment. Because of its strength and durability, ETFE film has a lifespan of 25 years or longer, depending on the specific brand. ETFE sheets require specialized installation and repair expertise, and retrofitting to a standard greenhouse is often possible with proper expertise.

Fiberglass Reinforced Polyester

Fiberglass-reinforced polyester (FRP) panels are relatively strong, lightweight, and low-cost. The panels are rigid and usually corrugated. New single panels have a light transmittance of up to 90%, while double panels have a light transmittance of 60% to 80% (Table 1). Panels can be easily attached to metal or wooden frames with screws and rivets. However, FRP is highly susceptible to UV degradation. Exposure to UV light causes yellowing of the panels and a reduction in the light transmittance (in some cases after 1 or 2 years for untreated panels).

New types of FRP are treated with a UV inhibitor to minimize yellowing and increase lifespan. Whereas traditional panels had a lifespan of only about 2 to 3 years, treated panels can last 10 years or longer. Another serious problem with FRP panels was that they were highly flammable. Some new FRP panels are treated with a flame retardant. However, FRP panels are no longer commonly used as greenhouse glazing in commercial greenhouses, though they are sometimes used in homeowner or hobby greenhouses.

Acrylic

Acrylic panels may come in various forms. They may be single panels or bi-wall panels. The thickness of the actual material, the thickness of the overall panel (and thus the airspace), and the distance between the flutes (the supporting cross sections within the panels) may all be varied.

These changes in the panel affect strength, flexibility, thermal conductance, light transmittance, weight, and cost.

Typical acrylic bi-wall panels have a light transmittance of 87%-93% (Table 1). Acrylic panels are relatively strong, rigid, and lightweight. Panels are resistant to UV degradation and experience little reduction in light transmittance for 10 years, but the typical effective life span of acrylic bi-wall panels is 20 to 25 years. Panels may be treated with materials to increase their UV resistance and reduce condensation inside the greenhouse. Acrylic panels are easily scratched, are flammable (less so than FRP), and have a high degree of thermal expansion and contraction; therefore, they require special anchors on the greenhouse frame. The initial cost of the panels is high compared to polyethylene and FRP, but their high light transmittance and long lifespan make them a popular choice for greenhouse glazing.

Polycarbonate

As with acrylic panels, polycarbonate panels may come in various forms. They may be single panels, bi-wall panels, tri-wall panels, and panels with crisscrossed supports (flutes).

Single-panel and bi-wall panels are most commonly used for greenhouse glazing. The thickness of the actual material, the thickness of the overall panel (and thus the airspace), and the distance between the flutes might all be varied. These changes in the panel affect strength, flexibility, thermal conductance, light transmittance, weight, and cost.

Typical polycarbonate bi-wall panels have a light transmittance of around 83%, whereas typical single-wall panels have a light transmittance of around 94% (Table 1). Panels are relatively strong, rigid, and lightweight.

Panels are resistant to UV degradation and experience little reduction in light transmittance for 10 years. Panels may be treated with materials to make them more resistant to UV and reduce condensation inside the greenhouse; thus, the typical effective life span of polycarbonate bi-wall panels is 20 to 25 years. Polycarbonate panels are easily scratched but are far less flammable than acrylic or FRP. As with acrylic, polycarbonate panels have a high degree of thermal expansion and contraction and therefore require special anchors on the greenhouse frame.

Shading Materials

Sometimes, in subtropical and tropical locations, heating is not required, and the amount of light entering the greenhouse exceeds optimal levels. Additionally, high temperatures may be an important concern. In these cases, a shading material made from polypropylene or some fabric may be used as the glazing instead of a solid transparent glazing. The shading material reduces the amount of light entering the structure and reduces the ambient temperature. It also potentially protects from wind. Various shading materials that reduce light transmittance by 10% to 90% are available. Some of these materials, such as Saran, are usually black, and the amount of light excluded depends on the thickness of the material’s weave. Other types of shading glazing are made from fabric with various thicknesses and dimensions of thin aluminum strips or fibers incorporated to reflect light away from the structure.

Colored Glazing Materials

Certain glazings may have pigments added (polyethylene films) or be filled with colored liquid (polycarbonate bi-wall panels) to adjust the spectral transmittance (light quality) of the glazing. The principle behind such glazings is that by altering the quality of light the plants experience, plant growth can be manipulated. For example, when grown under an environment rich in red versus far-red light, or under an environment high in blue light, plants should grow shorter and stocker and thus have less need for plant growth retarding chemicals. However, in commercial practice, these types of glazing have not been extensively used.

Other Glazing Materials

Other potential greenhouse glazing materials exist. Among these are polyvinyl chloride, weatherable polyester film, and polyvinyl fluoride film. However, either because of their properties or their cost, they are not commonly used for commercial CEA greenhouses.

Maintenance of Greenhouse Glazings

Greenhouse glazing should be periodically washed on the inside and outside to remove dirt and debris that block light (reduce transmittance) and scratch the glazing. Glazings that have reached their maximum life span and begun to yellow should be replaced. The glazing and mounting should be inspected before the heating season to ensure the glazing is properly mounted with no gaps between panels that could allow significant heat loss from the greenhouse.

Calculating Greenhouse Surface Area

It is often necessary to calculate the surface area of a greenhouse to determine the amount of glazing required (e.g., if polyethylene film is being replaced). To calculate surface areas, a greenhouse manager needs only to know several basic geometric formulas.

Circumference of a circle = 2 × π × r Area of a circle = π × r2

Total surface area of a cylinder = (2 × π × r × H)

+ (2 × π × r2)

Area of a triangle = 0.5 × (L × H)

Area of a rectangle (parallelogram) = L × W

Where π = 3.14, r is the radius, H is the height, L is the length, and W is the width.

Using these basic equations, we can calculate the surface area of two example structures.

The first structure (Figure 1) is a Quonset house without extended side walls. The dimensions are 50 feet in length and 20 feet wide. Calculations can be performed by assuming the structure is essentially a cylinder cut in half; therefore, the front and back surfaces are half-circles.

Surface area:

Use the total surface area of a cylinder equation divided in half. From the greenhouse description, we know that H = 50 ft (the length of the greenhouse). The r = 10 ft because the diameter is 20 ft (divide 20 ft by 2 to get the radius).

0.5 × [(2 × π × r × H) + (2 × π × r2)]

0.5 × [(2 × 3.14 × 10 ft × 50 ft) + (2 × 3.14 × 100 ft²)]

0.5 × [(3140 ft²) + (628 ft²)] = 1884 ft²

The second structure (Figure 2) is an A-frame or a free-standing gable greenhouse. The A-frame dimensions are 30 feet wide, 6 feet tall, and 100 feet long. The roof section is also 100 feet long, and each roof section is 20 feet wide. The structure can be broken down into cubes, rectangles, and triangles.

Surface Area:

Top Part of the Second Greenhouse:

Roof (use the area of a rectangle) = L × W

(20 ft × 100 ft) × 2 = 4000 ft² (multiply by 2 because there is a roof section on each side of the greenhouse)

Gable – the triangular end-wall section (use the area of a triangle) = 0.5 × (L × H)

[0.5 × (30 ft × 13.2 ft)] × 2 = 396 ft²

Top surface area = roof area + gable area = 4000 ft² + 396 ft² = 4396 ft²

Bottom Part of the Second Greenhouse:

Use the area of a rectangle = L × W and then multiply by two to account for the two end walls and the two side walls.

(30 ft × 6 ft) × 2 = 360 ft²

(100 ft × 6 ft) × 2 = 1200 ft²

Bottom surface area = end walls + side walls = 360 ft² + 1200 ft² = 1560 ft²

Total Surface Area = 4396 ft² + 1560 ft² = 5956 ft²

Values are educational examples only and are averages developed from various sources. Prices may change over time, and different brands may have somewhat different U and R values.

^a U factor reported in (BTU/hr.ft².oF).

^b R value reported in ft².oF.hr/BTU.