Greenhouse Glazing  

(Reprinted from Horticultural Engineering, a publication of the Rutgers Cooperative Extension, Volume 17, No. 1, January 2002.)

With the spring growing season just around the corner, it seems a good idea to review some of the issues surrounding glazing materials. Let's first look at some light (radiation) terminology. The radiation spectrum can be divided in several specific wavebands, which are defined by their range of wavelengths or energy content (e.g., radio and TV radiation, microwave radiation, visible light, etc.). The higher the wavelength, the smaller the energy content. Typically, the wavelength of light used by plants is expressed in the units of nanometer (nm; one billionth of a meter; a human hair is approximately 200,000 nm thick). Not all components of sunlight (approximately 280-2,800 nm) are useful for plant growth and development. In general, ultraviolet (UV; less that 380 nm) and excessive infrared (IR; above 770 nm) or heat radiation can be harmful to plants and should be avoided. Plants use Photosynthetically Active Radiation (PAR; 400-700 nm), as their energy source for the process of photosynthesis. Therefore, greenhouse structures and especially the glazing material should have a high transmittance for PAR radiation. Note that the terms light and radiation are used interchangeably and that visible light is not exactly the same as PAR. Visible light (the colors in a rainbow; ROYGBIV) consist of wavelengths that cover a slightly larger part of the radiation spectrum (380-770 nm). Since light is the driving force for photosynthesis, small changes in light intensity have an immediate effect on the rate of photosynthesis. Plants respond to changes in light intensity very rapidly.

Direct and diffused radiation

To understand the impact of greenhouse glazings on crop production, we have to investigate how light interacts with these cladding materials. Based on physical properties, surface orientation (angle of incidence), and the number of layers of the glazing material, portions of the incoming light are either transmitted, reflected and/or absorbed.

On a cloudless day, most sunlight travels in a straight path through the Earth's atmosphere. Under these conditions, the incoming light is termed direct radiation. On a cloudy day, the sunlight is diffused by the many water vapor particles in the moisture-laden air. This light is called diffuse radiation. It is important to understand that diffuse radiation reaches the greenhouse surface from many different directions other than the direction of its source (the sun). This phenomenon can actually be an advantage for greenhouse crop production. Diffuse light is capable of reaching deeper into the plant canopy because it can penetrate from many different angles. This results in improved plant growth. However, the light intensity from diffuse light is usually much lower than the intensity from direct light.

In addition to the interaction between incoming light and the greenhouse cladding material, structural elements such as posts, trusses and equipment (e.g., overhead heating pipes, shade curtains and supplemental light fixtures) reduce the amount of light that reaches the top of the plant canopy. It is not unusual for a greenhouse structure to reduce the amount of light that ultimately reaches the plant canopy by an average of 40-50 percent compared to the amount of light available outside the greenhouse. Therefore, the need for maximum light transmission should be one of the main criteria during the design of greenhouses and overhead equipment, and in selecting glazing materials.

Types of glazing materials

The most common greenhouse glazing materials are glass, rigid plastics and plastic firms.

Glass has the highest light transmission, lasts the longest (30-plus years) and is the most expensive. Tempered glass is recommended because it is stronger which allows for fewer support bars, and it increases the safety for people working underneath in case of breakage. Most glass greenhouses are clad with a single layer resulting in a relatively high heat-loss coefficient (see Table 1).

Rigid plastics (e.g., polycarbonate and acrylic) are less expensive than glass and last seven to 20 years. They are usually manufactured as twin-walled sheets. The air space between the two walls acts as an insulator. Light transmission through rigid plastics is very good, although it usually decreases over time as the plastics age and turn yellow due to the amount of UV radiation contained in sunlight. The large sheets are much lighter than glass and require fewer support bars to attach them to the greenhouse frame. However, these rigid panels are not so easy to install on curved roofs.

Plastic films (e.g., polyethylene) are the cheapest greenhouse cladding material, but they usually last only three to four years. Plastic films, normally 4-6 mils thick, are almost always installed in two layers that are inflated by a small fan. This provides some strength to the greenhouse surface and the air space between the layers acts as an insulator, significantly reducing the heat loss from the greenhouse. Air-inflated greenhouse surfaces experience approximately 60 percent of the heat loss compared to similar surfaces clad with a single layer of glass or plastic. It is important to always use outside air to inflate the two layers of film because this will significantly reduce potential condensation between the layers. A common additive to the film material (the so-called IR films) helps reduce the heat loss from greenhouse during cold outside conditions. Some films are manufactured with a special surface treatment to prevent condensation droplets from falling on the crop (so-called no-drip films). Instead, the condensation water channels along the film and runs off to the side.

Table 1. Light transmission through various greenhouse glazing materials.

Material	Transmittance PAR (%)	Transmittance Infrared (%)	Transmittance Ultraviolet (%)	Life (years)
Glass	90	Less than 3	70	30+
Acrylic*	86	Less than 5	44	20
Polycarbonate*	83	Less than 3	18	7-10
Polyethylene **	Less than 80	50	48	3-4

*twin walled, **double layer

 

Table 2. Heat loss coefficients (U-values) for greenhouse glazing and
construction materials.

Material	U (Btu per hour per ft2 per oF) =(1/R)
Single (double) layer glass	1.1 (0.7)
Single (double) layer polyethylene	1.1 (0.7)
Double layer + energy curtain	0.3-0.5
Twin walled acrylic	0.6
Twin walled polycarbonate	0.6
½" Plywood	0.7
8" Concrete block	0.5
2" Polystyrene	0.1 (R = 10)

 

Energy Conservation Strategies for Greenhouses

There are many parameters which contribute to the efficiency or the inefficiency of a greenhouse heating system. These include the type of glazing, the crop being grown, and the physical configuration of the greenhouse. The following table lists some parameters used in normal greenhouse design and how they affect the energy consumption of the greenhouse.

Table 3. Illustrates the effect of changing greenhouse design parameters on fuel consumption.

Design	Gutter Height	Root U-value	Wall U-value	Temperature set point	Gallons oil per sq. ft.	Difference in cost
1	8 ft	1.2	1.2	60oF	1.49
2	10 ft	1.2	1.2	60oF	1.57	+$0.08
3	10 ft	1.2	0.8	60oF	1.43	-$0.06
4	10 ft	0.8	0.8	60oF	1.04	-$0.45
5	10 ft	0.5	0.8	60oF	0.75	-$0.74
6	10 ft	0.5	0.8	55oF	0.55	-$0.96

The data in the Table 3 are for a greenhouse that is 96 feet wide and 100 feet long with eight 12-foot bays. It is at a location with 5,016 degree days with an outside design temperature of 0°F. The last column shows the difference in cost from changing the parameters which are emboldened and underlined in Table 1. The U-values are heat loss coefficients in Btu/hour per square foot per degree Fahrenheit.

The difference between designs 1 and 2 shows that it costs approximately $0.08 per square foot more to have a 10-ft. high sidewall as compared to an 8-ft. high sidewall. The higher sidewall is very desirable for humidity control in the greenhouse and yields a very small increase in energy consumption giving an increased cost of $768 per year using oil at a price of $1.00 per gallon.

The design change in design 3 is accomplished by double glazing the side and end walls of the greenhouse yielding an energy savings compared to design 1 of $0.06 per square foot or $576 and a savings of $1,344 compared to design 2 with the elevated side wall.

For design 4 the roof and all the walls have been double glazed providing an energy savings of $0.45 per square foot or $4,320 annually.

Installing an energy screen in design 5 produced a savings of $0.75 per square foot or $7,200 per year.

For crops growing on the floor using a floor heating system, and aerial set point temperature reduction of 5° F is possible with little adverse effects on the crop. By installing floor heating, a savings of $0.96 per square foot is possible resulting in an annual savings of $9,216.

These data can be used to evaluate the payback for various design changes. Energy screens not only reduce energy consumption for heating but also can be used for summer shading and cooling. Growing on the floor eliminates the cost of benches and saves energy but can be used for specific crops which require no manual labor during the growing period. The increased greenhouse height is important. Most designs today are at least 12 feet to the gutter.