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

Cost

All aspects of cost need to be considered. These include the initial cost of the glazing material, structural support costs, life span of the glazing and thermal conductance of the glazing. A glazing material that has a high initial cost when compared to other glazing materials may be more economically attractive if it has a long lifespan or has a low thermal conductivity.

 

 

Life span

A short life span means frequent replacement. Therefore, the initial cost of the glazing may be low as 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 long life span.

 

 

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 resulting costs associated with replacing the glazing.

 

 

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 costs and may result in a reduction in greenhouse light levels due an increase in obstructions by the support structure.

 

 

Light transmittance

The higher the light transmittance of a glazing, the higher the amount of sunlight that can penetrate the glazing and enter the greenhouse. In northern climates and in the winter, light is often the limiting factor for photosynthesis. Therefore, maximizing the amount of natural sunlight entering the greenhouse is desirable. Sometimes, such as in summer or in southern or equitorial locations, the amount of light entering the greenhouse is above optimal levels. In these situations, a shadecloth or 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 removed.

Light transmittance of a glazing is not constant. As a glazing ages, it tends to have a reduction in its light transmittance due to scratching from dust and debris and aging or "yellowing" of the glazing material due to U.V. exposure.

 

 

Thermal conductance

This is the rate at which heat energy moves through a glazing material and is expressed as Btu loss/ ft²/hr/(^oF_inside - ^oF_outside). Generally, a low thermal conductance is desired in order to minimize heating costs.

 

 

Scratch resistance

Dust, soil particles and other debris can scratch the glazing. Scratching reduces the light transmittance of the glazing and can therefore result in reduced light levels inside of the greenhouse.

Common Greenhouse Glazing Materials

Glass

Many types of glass are available including floated glass, low-iron glass and safety glass. Different thicknesses are also available. Typically 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. Low-iron glass will have the highest light transmittance levels. Glass glazed greenhouses have relatively higher air infiltration rates due to spaces between glass panels. Therefore, glass tends to have a higher thermal conductance (1.1 - 1.3) 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 glazings. Glass is resistant to heat, U.V. light, and abrasion. At $2.00 to $7.00 per ft², glass is expensive to purchase and install and requires special supports to hold the glass panels inplace and support their weight. Glass also has a low impact resistance. However, glass has a long life span often exceeding 25 years. Most commercial greenhouses no longer use glass as a glazing 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 that is particularly adaptable to quonset structures because of its flexibility. It is low in cost ($0.08 - $0.14 per ft²), light-weight, and easy to install. Typically standard polyethylene film has a light transmittance of 85% to 87% for a single layer of film and 76% for a double layer. Thermal conductance is 1.2 for a single layer and 0.7 for a double layer. However, these values may vary by brand (i.e. Tufflite, Standard UV, Tufflite Dripless, Fog Bloc, Sun Saver, Dura-Therm ), because additives may be included in the film to increase life-span, reduce condensation or reduce heat loss. These additives may be sprayed on or included in the film through a process known as coextrusion. During the process of coextrusion, three layers of polyethylene are laid down to form a single sheet of polyethylene film. Each layer may have materials included that alter the properties of the film.

Polyethylene is short-lived in comparison to other glazings. Without additives, polyethylene will last only one to two years before needing to be replaced. This is because it is very susceptible to degradation by U.V. light. However, if additives are included that make the material more resistant to U.V. light, polyethylene glazing may last for three to fours years. In the coextrusion process U.V. inhibitors are added to the outer layer of film to reduce the impact of U.V. light and increase the life span of the film.

Polyethylene has a high thermal conductance. However, some brands of polyethylene films have an I.R. (infrared) inhibitor added to the inside layer of the film. This reduces the heat loss through the glazing.

Another problem with polyethylene glazing is that of condensation and dripping. Because of the difference between inside and outside air temperatures, water vapor tends to condense on the surface of polyethylene film inside of the greenhouse. Because the film is very hydrophobic, the water tends to bead and collect on the surface until large enough drops are formed that they fall from the glazing onto the plant materials below. This dripping of water from the glazing onto the plants can result in increased disease incidence. An additive may be sprayed onto the film (i.e. SunClear) or incorporated into the film that essentially acts as a wetting agent. This prevents the beading of water and allows smaller droplets to form that run down the glazing and to the floor.

Usually a 6 mil (0.15 mm thick) film is used for greenhouses if a single layer is being used. If a double layer is being used, 6 mil is used on the outside and 4 mil (0.10 mm thick) is used on the inside. In a double polyethylene 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.

 

 

Fiberglass Reinforced Polyester

Fiberglass reinforced polyester (FRP) panels (i.e. Excelite and Lascolite) are relatively strong, light weight, and low in cost ($0.85 - $1.25 per ft²). 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%. Thermal conductance for single corrugated panels is approximately 1.2. Panels can be easily attached to metal or wooden frames with screws and rivets. However, FRP is highly susceptible to U.V. degradation. Exposure to U.V. light causes yellowing (after only 1 or 2 years for untreated panels) of the panels and a reduction in the light transmittance. New types of FRP are treated with a U.V. inhibitor. Whereas traditional panels had a life span of only about 2 to 3 years, treated panels can have a life span of 10 years or longer. Another serious problem with FRP panels is that they are highly flammable. Some new FRP panels are treated with a flame retardant. However, FRP panels are no longer commonly used as a greenhouse glazing for commercial greenhouses.

 

 

Acrylic

Acrylic panels may come in various specifications. They may be single panels (i.e. Plexiglass) or bi-wall panels (i.e. Exolite). 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% and a thermal conductance of 0.58 (16 mm panels) to 0.65 (8 mm panels). Panels are relatively strong, rigid, and lightweight. Panels are resistant to U.V. degradation and experience little reduction in light transmittance for 10 years. Panels may be treated with materials to make the panels more resistant to U.V. and to reduce condensation inside of the greenhouse. The typical effective life span of acrylic bi-wall panels is 20 to 25 years. Panels cost $1.50 to $3.50 per ft² so initial cost is relatively high. Additionally, they are easily scratched, are flammable (much less so than FRP), and have a high degree of thermal expansion and contraction, and therefore require special anchors on the greenhouse frame.

 

 

Polycarbonate

Polycarbonate panels may come in various specifications. They may be single panels (i.e. Dynaglass, Lexan Corrugated and Macrolux Corrugated) or bi-wall panels (i.e. Macrolux, Polygal, and Lexan Dripgard). As with acrylic, 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 polycarbonate bi-wall panels have a light transmittance of 83% whereas single-wall panels have a light transmittance of 94%. Polycarbonate bi-wall panels have a thermal conductance of 0.5. Panels are relatively strong, rigid, and lightweight. Panels are resistant to U.V. degradation and experience little reduction in light transmittance for 10 years. Panels may be treated with materials to make the panels more resistant to U.V. and to reduce condensation inside of the greenhouse. The typical effective life span of polycarbonate bi-wall panels is 20 to 25 years. They cost approximately $1.50 ft² for single-wall panels and $1.75 to $2.50 per ft² for bi-wall panels. They are easily scratched but are less flammable than acrylic or FRP. As with acrylic, polycarbonate panels have a high degree of thermal expansion and contraction and therefore required special anchors on the greenhouse frame.

 

 

Shading Materials

Sometimes, such as in summer or in southern or equitorial locations, heating is not required, and the amount of light entering the greenhouse is above optimal levels. In these situations, a solid transparent glazing may not be used. Instead, some type of shading material is used as the glazing. The shading material reduces the amount of light entering the structure and also reduces ambient temperature. Various shading materials that reduce light transmittance by 10% to 90% are available. For Saran structures used in subtropical locations, the shade material serves as the only and permanent, glazing material used.

Other potential glazing materials exist. Among these are polyvinyl chloride, weatherable polyester film and polyvinyl fluoride film. However, these materials are not commonly used in commercial situations.

Calculating Greenhouse Surface Area

It is neccessary to calculate surface area of a greenhouse in order to determine the amount of glazing required and to determine the heating requirements. To calculate surface areas you only need to know several basic geometric equations. If you have not already, you should commit these formulas to memory.

Circumference of a circle = 2p r

Area of a circle = p r²

Total surface area of a cylinder = (2p rH) + (2p r²)

Area of a triangle = 0.5 x (LH)

Area of a rectangle (parallelogram) = L x W

Volume of a cube = L x W x H

Where p = 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 is a quonset house without extended side walls. The structure is 50 feet in length and 20 feet wide. Calculations can be determined by assuming the structure is essentially a cylinder cut in half (therefore the front and back surfaces are half circles). Click here for a picture. Surface area:

0.5 x [(2p rH) + (2p r²)]

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

0.5 x [(3140 ft²) + (628 ft²)]

=1884 ft²

The second structure is an A-frame or a free-standing gable greenhouse. The length of the structure is 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. Click here for a picture. Surface Area:

-- Top Part:

(roof) L x W

(20 ft x 100 ft) x 2 = 4000 ft²

(gable) 0.5 x (L x W)

[0.5 x (20 ft x 20 ft)] x 2 = 400 ft²

Top surface area = 4000 ft² + 400 ft² = 4400 ft²

-- Bottom Part:

L x W

(30 ft x 6 ft) x 2 = 360 ft²

(100 ft x 6 ft) x 2 = 1200 ft²

Bottom surface area = 360 ft² + 1200 ft² = 1560 ft²

Total Surface Area = 4400 ft² + 1560 ft² = 5960 ft²

© 2005, M.R. Evans