Section 1: Important Characteristics of Greenhouse Glazing Materials

The greenhouse glazing generally refers to the translucent (allows light through) greenhouse covering. There are many types of greenhouse glazings and each has specific characteristics. The characteristics of each glazing dictate its best use and its limitations.

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 very important factors.
 
Light transmittance
The higher the light transmittance of a glazing, the higher the amount of sunlight that can penetrate the glazing and potentially enter the greenhouse. In northern climates and in the winter, light is often a limiting factor for photosynthesis. Therefore, maximizing the amount of natural sunlight entering the greenhouse is desirable and in most situations, the highest possible light transmittance level is desirable in a greenhouse glazing.  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 or shading compound may be used to temporarily reduce the amount of light entering the greenhouse (discussed in more detail under the “Lighting” learning unit). When light levels drop below optimal, the shading material is 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 U.V. exposure. Additionally, light transmittance values (such as those listed in Table 1) are the percent of light that passes through a clean unobstructed panel of the glazing positioned perpendicular to the light source. The light levels actually entering the greenhouse and reaching the plants will be lower than the listed transmittance of the glazing because of light being blocked by the supporting structure (as well as affects from the angle of the sun to the structure and any debris on the glazing). For example, in a glass-glazed 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.

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

Thermal conductance may be expressed as Btu loss/ ft²/hr/(^oF_inside - ^oF_outside).  However, when evaluating glazings, it is most common to compare the “U” and “R” values of a glazing to determine their 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 will be form 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 more well insulated the glazing.

In most situations with greenhouse glazings, the “U” and “R” values are inversely related. It is desirable in a greenhouse glazing to have as low a “U” factor and as high a “R” 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 resulting costs associated with 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 costs and may result in a reduction in greenhouse light levels due an increase in obstructions by the supporting structure (i.e. trusses blocking light).

Life span
A short glazing 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 longer life span.

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. This in turn may require more frequent replacement of the glazing and increased cost.
  
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, 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.

Section 2: 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 about 77% as a double layer. Double strength glass has a light transmittance of approximately 88% and insulated glass as transmittance of approximately 78%. Low-iron glass will have the highest light transmittance levels. Glass-glazed greenhouses have relatively high air infiltration rates due to spaces 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 many other types of glazings (depending on how well mounted the glazing is and the "tightness" of the greenhouse). Glass is resistant to heat, U.V. light, and abrasion but has a relatively low impact resistance. Glass is 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 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, 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 74% to 77% for a double layer. Thermal conductance of polyethylene film is high but varies between specific brands of polyethylene (i.e. Tufflite, Standard UV, Tufflite Dripless, Fog Bloc, Sun Saver, Dura-Therm ) and single versus double layers.

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 two to three years before needing to be replaced. This is because it is very susceptible to degradation by U.V. light. However, if additives are included during the coextrusion process that make the material more resistant to U.V. light, polyethylene glazings may have a life span of three to five years.

As mentioned above, polyethylene film 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 which reduces 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 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.

Fiberglass Reinforced Polyester
Fiberglass reinforced polyester (FRP) panels (i.e. Excelite and Lascolite) are relatively strong, light weight, and low in 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%. 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 to minimize yellowing and increase life span. 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 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 a greenhouse glazing for commercial greenhouses but are sometimes used for homeowner or hobby greenhouses.

Acrylic
Acrylic panels may come in various forms. 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%. Acrylic panels are relatively strong, rigid, and lightweight. Panels are resistant to U.V. 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 make the panels more resistant to U.V. and to reduce condensation inside of the greenhouse. Acrylic panels are easily scratched, are flammable (less so than FRP), and have a high degree of thermal expansion and contraction, and therefore, require special anchors on the greenhouse frame. The initial cost of the panels is high compared to polyethylene and FRP but the high light transmittance and long life span make them a popular choice as greenhouse glazings.

Polycarbonate
Just as for acrylic panels, polycarbonate panels may come in various forms. They may be single panels (i.e. Dynaglass, Lexan Corrugated and Macrolux Corrugated), bi-wall panels (i.e. Macrolux, Polygal, Lexan Dripgard and Lexan Thermoclear), tri-wall panels and panels with crisscrossed supports. Single panel and bi-wall panels are most commonly used as greenhouse glazings. 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 about 94%. 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 and 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 required special anchors on the greenhouse frame.

Section 3: 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 in commercial greenhouse situations.

Colored Glazing Materials
Certain glazings may have pigments added (polyethylene films) or may be filled with colored liquid (polycarbonate bi-wall panels) to adjust the spectral transmittance (light quality) allowed to pass through the glazing. The principle behind behind such glazings is that by altering the quality of light the plants experience, plant growth and development 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 stockier and thus have less need for plant growth retarding chemicals. However, in commercial practice, these types of glazings have not been extensively used. More discussion of colored glazings and light quality effects on plants may be found under the “Lighting” learning unit.

Section 4: Other Considerations

Shading Materials
Sometimes, such as in subtropical and tropical locations, heating is not required, and the amount of light entering the greenhouse is above optimal levels. Additionally, high temperatures may be an important concern. In these cases, a shading material made from polypropylene or some type of fabric may be used as the glazing instead of a solid transparent glazing (such as those discussed above). The shading material reduces the amount of light entering the structure and also reduces ambient temperature (it also potentially provides protection 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 of the thickness of the material's weave. Other types of shading glazings are made from fabric with various amounts and dimensions of thin aluminum strips or fibers incorporated that actually reflect light away from the structure.

Maintenance of Greenhouse Glazings
Greenhouse glazings should periodically be washed on the inside and the outside to remove dirt and debris that block light (reduces transmittance) and scratches 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 heating season to insure that the glazing is mounted properly without gaps between panels which can allow for significant heat loss from the greenhouse.

Calculating Greenhouse Surface Area
It is often necessary to calculate surface area of a greenhouse in order to determine the amount of glazing required (for example if polyethylene film is being replaced). To calculate surface areas, a greenhouse manager only needs to know several basic geometric equations.

Circumference of a circle = 2πr or πd
Area of a circle = πr²
Total surface area of a cylinder = (2π rH) + (2π r²)
Area of a right (90°) triangle = 0.5 x (LH)
Area of a rectangle (parallelogram) = L x W
Volume of a cube = L x W x H
Sides of a right triangle determined by a² + b² = c²
Where π = 3.14, r is the radius, H is the height, L is the length and W is the width. Further a is the length of one leg of a right triangle, b is the other leg and c is the hypotenuse (long side).

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).

Surface area:

0.5 x [(2π rH) + (2π r²)]
0.5 x [(2 x 3.14 x 10 ft x 50 ft) + (2 x 3.14 x 10 ft²)]
0.5 x [(3140 ft²) + (628 ft²)]
=1884 ft²

The second structure is an A-frame or a free-standing gable greenhouse. The structure is 30 feet wide, 6 feet tall (side walls) 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:
(roof) L x W
(20 ft x 100 ft) x 2 = 4000 ft²

The gable is a little more difficult to calculate. Because we do not know if the top angle is a right triangle we can't multiply the sides (20 x 20) and must we split the gable into two parts that we know are right triangles by drawing a line from the peak down to the top of the wall portions (splitting the gable in half). This gives us a right triangle with the bottom leg being 15 feet, the hypotenuse being 20 feet and the upright leg is unknown in its length. We can calculate the unknown leg using a²+b²=c² (Pythagorean Theorem!).

With this we have:
a²+15²=20² (all in feet)
a²+225=400 ft²
a²=175 ft²
a=13.2 ft²

So we have a right triangle (half of the gable) that has one leg (bottom) of 15 feet and the other (upright) is 13.2 feet. The area of the right triangle is 0.5 (13.2 x 15) = 99 ft². We have 4 of the right triangles (two per gable). So the total gable surface area is 396 ft².

Top surface area = 4000 ft² + 396 ft² = 4396 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 = 4396 ft² + 1560 ft² = 5956 ft²

© M.R. Evans, 2008, 2009, 2011, 2014