Greenhouse Glazings
What’s New Under the Sun, Under the Glazing, and at the Plant Canopy

Dr. Gene A. Giacomelli¹
Department of Bioresource Engineering
Cook College Rutgers University
New Brunswick, New Jersey

The types of greenhouse coverings currently available are dominated by plastics.  However, these range from traditional glass to the polymer plastics, such as thin films or multi-layer rigid thermoset plastic panels.  Enhancements that are available include: ultra-violet radiation (UV) degradation inhibitors, infrared radiation (IR) absorbency, and anti-condensation drip surfaces, as well as, unique radiation transmission properties.  The selection of a specific covering is no longer an easy task, as the numerous alternatives have implications for the greenhouse superstructure and the enclosed crop production system.

Considerations for Designing and Selecting a Covering System
The selection of the greenhouse cover material or glazing influences the amount and type of solar radiation received at the plant canopy.  This directly affects plant growth.  In addition, the microclimatic factors, such as air humidity or carbon dioxide concentration, are indirectly affected by the glazing system.

For a proper discussion of greenhouse coverings, it is not only necessary to evaluate their capabilities and shortfalls, but also to consider their relationship within the overall greenhouse crop production system.  The design must consider (1) the type of crop and its associated cultural demands, and (2) the production system which will support the growth of the crop.    

Even when a material offers strength, consistency, durability, manufacturing quality control, and safety, other factors should be considered.  These include the transmission of solar radiation and energy conservation, and how these interact with glazing/superstructure of the greenhouse.

There are three general categories of coverings typically used for greenhouses: glass, plastic films and rigid plastic panels.   Modern plastics have provided alternatives to traditional glass for covering the greenhouse.   Plastic glazings include rigid plastic structured panels, such as fiberglass reinforced polyester  (FRP),  polycarbonate  (PC), acrylic (PMMA, polymethylmethacrylate), and polyvinyl chloride (PVC) panels. Thin film coverings include low-density polyethylene (LDPE), polyvinylchloride (PVC), and ethylene vinyl acetate copolymer  (EVA).   These materials have been used in single, double and even triple layers to cover the greenhouse.

Glass is quite inert, in contrast to plastic, and can provide effective use for many decades.  Most plastic coverings are affected by weathering.  Glass is non-combustible, resistant to UV radiation and air pollutant degradation, and it maintains its initial radiation transmission.  The most predominant drawback of glass may be its vulnerability to catastrophic losses caused by hail.

Glass may be tempered to greatly increase its strength and size.  Traditionally small in size, new glass panes are now available with dimensions up to 6 ft by 12 feet.  Cleaning the glass to maintain  maximum  radiation transmission, and sealing the edges for reducing energy losses are all the maintenance that is regularly required.

Polyethylene film greenhouses have been developed so that they are reliable, and usually have a lower initial cost than most other greenhouse glazing systems.  Low air infiltration rates resulting from the continuous film cover have improved energy savings but contribute to high greenhouse air humidity conditions.  Moisture condensation, especially on flattened arch-shaped roofs, promotes dripping on the crop below.  Fan ventilation is generally required for cooling.  It has traditionally been difficult to install ridge vent openings, thus effective natural ventilation has not been available, especially for large, gutter connected.  Naturally ventilated polyethylene film covered structures have only recently become available.  The potential of open-roof greenhouse structures has only begun to be developed, as currently there are several greenhouse designs available where the entire roof can be mechanically opened and closed.

Selection of the type of covering material to use on new construction or on renovation projects requires many practical considerations.  The flexible and forming properties of the film simplify the covering process compared to rigid plastics or glass.  The attachment procedures for plastic film range from the simplicity of wooden nailer strips to the reusable aluminum extrusion inter-locking strips.  The need for replacing the film every two or three years requires that the recovering process be rapid and easy.  A means of recycling or disposing of spent film must also be considered.  Glass or rigid structured plastic panels require the more elaborate aluminum extrusions for their attachment to the greenhouse structure.  These must be designed for the longer life of these covering materials.

Ridge and furrow or gutter-connected structures designed for a double-polyethylene covering should use a continuous tube of plastic film, as an alternative to applying two individual sheets.     Both glazing layers are applied simultaneously as the tube is unrolled, greatly simplifying the task.     The distance between adjacent gutters should be less than 20 to 21 feet on an arch shaped roof greenhouse.  This assures that a 25 foot tube, currently the largest manufactured, will span the arch between the gutters. The lock-down devices at the roof gutters must be easily operated, reusable and free from maintenance.  Covering gutter-connected greenhouses with double film layers from a  tube has been accomplished at rates as much as 1 acre per day with eight workers.

Rigid plastic structured panels made of acrylic, polycarbonate, PVC and FRP, are initially more expensive as a cover than polyethylene film, but they require less maintenance and provide a longer useful life.  They can be used on new construction and on glasshouse renovations.  Re-glazing systems for acrylic and polycarbonate panels use fewer, stronger support elements which are  spaced wider apart. This has effectively reduced the amount of structural shading typically associated with glass.  The strength of these wider panels (compared to glass) comes from their double-walled cross section depths, which range up to 0.63 inch.

The greenhouse design with rigid plastic structured panels must carefully consider climates where heavy snowfall is likely.  The insulated double-walled panels reduce the rate of snowmelt, thereby allowing for large amounts of snow to accumulate.  This is unlike double-layer polyethylene glazing where the layers collapse together when loaded with snow, thereby increasing the melting rate.

Ultra violet radiation promotes photochemical degradation processes in all plastics and is generally the major cause for their replacement.  Temperature extremes and their duration can weaken film coverings.  This can typically become a problem where the film contacts the greenhouse structure.    Air pollutants also reduce the usable life of plastic coverings.  These may be from sources external to the greenhouse which are attracted to the outer plastic layers and reduce radiation transmission.    They may also come from internal sources such as chemicals used for pest control, which can cause premature failure of the plastic.

Plastic Films - PE, PVC & EVA
Polyethylene [PE] film is the most common greenhouse covering film in the United States, and currently there is a great selection of high quality greenhouse film from which to choose.  All greenhouse grade polyethylene film has a minimum useful life of 24 months in most areas of the United States.  However, three and four year films are now available.  These films, normally a co-polymer of PE and ethyl vinyl acetate [EVA], are manufactured with the addition of 1 to 5% vinyl acetate.  This formulation has significantly improved the physical properties of PE, including its resistance to cracking in cold temperatures, and its tearing strength (particularly where folded).

Degradation of the physical properties of PE by ultra-violet radiation remains the primary cause for its limited life.  The addition of inhibitors during manufacture is necessary to slow the degradation process.

Techniques of co-extruding films and multi-layering of films during the extrusion process have been developed.  These films offer even further opportunities for improved performance of  the covering material.  The most recently developed films have included an infrared (IR) barrier, condensate control, and/or wavelength selective properties.

Rigid Structured Plastics - FRP, PC, PMMA, PVC
Modern rigid structured plastic coverings such as FRP [fiber-reinforced polyester, “fiberglas”], PC [polycarbonate], PMMA [acrylic], PVC  [polyvinyl chloride] are normally corrugated or have multi-layered cross sections for strength.  They are strong, have a long life (10-year guarantee with minimal light transmission reduction) and the double-walled panels improve energy savings.

Acrylic and Fiberglass panels have potential fire problems.  Polycarbonate and PVC panels have less of a fire problem because burning will cease after removal of the flame source.

FRP has good resistance to hail damage.  FRP panels have a tendency to degrade on the surface directed to the sun and expose the reinforcing glass fibers.  These exposed fibers become dirty, thereby reducing light transmission.  Various treatments are available to eliminate this problem.     One of these adds a thin film of Tedlar to the surface of one side of the panel during manufacture.

Acrylic panels constructed into a double-walled channel cross section provide good structural strength, and the heat saving advantages of double-glazing.  Panel dimensions include widths of 4 ft and lengths from 8 to 16 ft.  Fewer structural elements are required for mounting the panels than are needed for the smaller and heavier traditional glass panes.

Polycarbonate panels are similar in cross section construction to the acrylic panels.  They are manufactured in a variety of sizes, with thinner cross sections able to bend to an arch roof shape.     Panel dimensions range from widths of 4 to 8 ft and lengths from 8 to 32 feet.  Polycarbonate panels are adversely affected by UV radiation and will discolor if not protected.  Panels are either co-extruded with acrylic or use an acrylic coating for UV protection.  They can also be manufactured in a corrugated, single-layer cross section.  The channels of the cross section can cause some problems with condensation and subsequent algal growth within the double-wall structural panels.

In general, all of the described glazing materials can perform quite well, and depending on the desired application will be satisfactory.  Many factors determine the optimum choice for a given situation, such as the type of crop grown, the costs of heating and the need for mechanization.

Dark Winter Conditions  -- Can they be overcome?
No, not completely!  But their negative impact can be reduced, or enhanced!  There are choices to be made!

The short-duration, cloud-covered days of the late fall, winter and early spring contribute to the primary cause for reduction of plant growth, and subsequent difficulties in crop production from diseases, nutritional deficiencies and reduce product quality.  That primary cause is too little solar radiation, or “light”.   Short of erasing the clouds, changing location, waiting for spring, what can be done to reduce the effect of low light conditions?  Surprisingly the first remedy may not be glazing related, but more associated with the greenhouse structure, its orientation, and it siting.

Whatever the amount of natural solar radiation available for the plant, it must first reach the greenhouse, then pass through the glazing and around overhead structural framework before reaching the plant canopy.  Therefore it is important to consider a southerly exposure which is free from nearby buildings, groves of trees and other obstructions to light. Also consider an obstruction-free northern exposure as well, since on cloudy, diffuse days, a significant portion of light for plant growth does enter the greenhouse from the north.

A freestanding, single-bay greenhouse [ground to ground, or Quonset-style], generally provides more light than a gutter-connected, multi-bay greenhouse.  There is less overhead structure in the single-bay greenhouse, and the relatively narrow span allows for more glazing area [light reception] for the crops, than the gutter-connected greenhouse. However, all of these good intentions can be lost on greenhouses cluttered with non-essential overhead structures or devices, which can cause localized shading and significant reduction of total light within the growing area.

Greenhouse compass orientation dramatically affects the total light entering each day, and its distribution within the greenhouse.  An east-west oriented ridge will have a large south-facing wall and roof area for collecting the low sun angle winter sunlight.  Thus it will provide the most total daily light during the winter season.  However distribution of the light may not be uniform to all plants within the greenhouse.     Those on the north side may receive less light.  This is especially a problem for tall crops, especially if they are grown in rows which are aligned with the east-west ridge.  A long, narrow [less than 25 feet wide] freestanding east-west greenhouse for bedding and potted plants, or hydroponic lettuce will offer the best light conditions for the winter months.     However, a tall crop, grown within a gutter-connected, multi-bay greenhouse should be oriented with its gutters [or ridges] in the north-south direction.  The reduction in total light entering the greenhouse [described above] is offset by an even more important factor, the improvement of light uniformity to all locations within the growing area.  The benefit of the north-south ridge orientation is the “movement” of the shadows caused by the overhead structures as the day progresses from an eastern to western sun location.     No single location of the greenhouse remains in shadow throughout the day.  A non-moving shadow will occur within an east-west oriented greenhouse, and it is most pronounced during the winter months.  Non-uniform light patterns causes irregular plant growth.

Transmission of light is directly affected by the number of glazing layers.  The number of layers of glazing is generally more important than the type of glazing material [those described above] in determining the amount of light entering the greenhouse.  Double-layer materials always reduce light transmission more than single layer.  This is true whether the measurement is observed with the sun directly overhead, or at low sun angles to the glazing surface, because there are two surfaces which the light must pass, each offering the chance for the light to be reflected or absorbed. Low sun angle is the angle of the sun relative to the surface of the greenhouse glazing. Rigid double-layer materials reduce transmission more than film double-layer glazings for low sun angles, that is, whenever the sun is not directly above or nearly overhead.  This is primarily caused by the additional interior surface used to create the channels of the double-walled rigid panel glazings.  This essentially becomes a third surface for the light to have to pass before entering the greenhouse.

Major factors which combine to modify the amount of solar radiation transmitted into a greenhouse structure include:

day of year, and hour of day

latitude

local weather conditions

predominance of direct or diffuse solar radiation

waveband of the radiation [PAR or “Total”]

cover material properties (at installation and as affected in time  by weathering, air pollutants, moisture  condensation, and dust and dirt accumulation).

Radiation transmission is also influenced by the physical structure of the greenhouse, including:

angle and shape of the roof

the number and width of spans (distance from gutter to gutter, if multi-span or ground to ground, if single span)

height of end walls

length to width ratio of the structure, and

compass orientation

Condensation on the Glazing
Condensation may seem to be an unwanted event in the greenhouse, however, it is an important result of the environment responding to plant transpiration of moisture into the greenhouse air.  It should not be prevented, and essentially it cannot be stopped, but its potential negative effects can be minimized.  Transpiration, the evaporation of water from the plant leaves to the greenhouse air, is critical for natural leaf cooling, nutrient uptake and growth of the plant.  The plant will not even stand upright unless it continually transpires water.  Condensation of water vapor from the warm moist air onto the cool surface of the greenhouse covering material is the primary means of reducing the greenhouse air humidity whenever ventilation is not occurring.  Water vapor in the air is converted to liquid water on the glazing.  It is also a method for heat loss from the greenhouse.    The change of water vapor to liquid results in a release of energy called latent heat, which is then lost through the glazing.

Water droplets remaining on the glazing is an undesirable situation.  They will fall to cause potential crop damage by localized over-watering or encourage disease infestation.  In addition, droplets remaining on the glazing become another surface for light to pass [or to be reflected away], prior to entering the greenhouse.  Efforts to incorporate inhibitors to droplet formation during manufacturing, particularly on plastic coverings, have somewhat reduced this problem.     By modifying the surface tension of the glazing, the water moves downward along the sloped glazing and collects at a desired location or within a drip gutter.     

Water vapor from the air can condense inside the multi-layer film and rigid panel glazings.     Where a small fan is used for double-polyethylene film greenhouses, it is very important to use outside air, not greenhouse air for inflation.  The fan should be mounted such that it forces outside air between the two layers, as the highly humid inside air when cooled would cause water to condense within the two layers.  Water vapor can pass through the rigid multi-layer panels and condense inside, because they are made of water-permeable materials.  Sealing both ends of the channels does not eliminate this problem as the polycarbonate (and acrylic) materials are not completely impervious to water vapor.  If the channels are not sealed at the bottom, condensate can drain from the panel.  A fiber-covered aluminized tape can be used to allow for controlled drainage.

Infrared [IR] Barrier Glazing
Radiation is one means to lose heat from the greenhouse, and it is dependent on the glazing surface area of the greenhouse.  It is not exactly the same effect as leaving the door open, but the “door” is now the entire surface area of the greenhouse glazing, so it can become significant.  The heat losses by radiation are directly related to the physical properties of the cover material.  The cover is generalized as a thermal glazing if it reduces radiation heat loss.  Thermal PE films reduce radiation heat loss more than non-thermal PE films, because they contain an additive which can absorb and return the heat which tries to escape.  In this case, it is not the visible light radiation [PAR] which is of concern, but infrared [IR] and thermal radiation [longwave].

The net radiation of the greenhouse is important for evaluation of the greenhouse energy situation, and the potential energy savings with an IR glazing.  Net radiation is the difference between the energy received and energy lost in the greenhouse by radiation.  During the day, the sun which generally provides a large amount of radiation assures a net gain of energy, because the heat losses from the greenhouse are much smaller.  This net gain of energy causes a subsequent greenhouse air temperature rise, and cooling is needed.  However, at night, the warm materials within the greenhouse (earthen floor, concrete paths, metal benches, plants, etc) have significant radiation heat losses to the colder outdoor environment, if the glazing allows.  The net energy loss is caused by transmission of infrared  and thermal radiation through the cover [as well as emission of radiation from the cover to the cold sky].  The amount of this radiation energy loss depends, not only on the properties of the cover, but also on the temperature of the cover, and the atmospheric conditions (water vapor/clouds, carbon dioxide, and ozone content).  Generally thermal glazings have proven cost-effective where heating energy costs are a significant operating expense, and even for well-designed and maintained greenhouse operations.  Furthermore, the addition of a thermal blanket or screen between the crop and the glazing can greatly reduce the heat loss by radiation, and in combination with a thermal glazing can offer additional energy savings.

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¹NJAES Paper No. I-03130-14-99  (ghGlazingSEghVegGrowersFL.doc).  Portions excerpted from "Greenhouse Covering Systems" by Gene A .Giacomelli and William J. Roberts,  Department  of Bioresource  Engineering,  Cook College,  Rutgers University .  NJAES Paper No. D-03130-17-92  (hortglaz.pap) .     Printed in HortTechnology Vol 3(1):50-58. 

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