Greenhouse materials and structures
4.2 CLADDING MATERIALS
The characteristics of the cladding material determine the quality of the light trans-mitted into the greenhouse, and radiation transmittance can be improved qualitatively and quantitatively (Castilla and Hernandez 2007). The cladding material must have characteristic such as: strength, consistency, durability, manufacturing quality control, safety, transmission of solar radiation and energy conservation. Greenhouse designs vary widely and choosing the correct type of greenhouse is an important decision faced by growers. The glazing will drastically affect the amount and type of sunlight that reaches the crop.
The glazing will also determine the heat loss of the structure. When selecting a cladding or cover for a greenhouse, it is important to consider the next question (Giacomelli 2002):
How much energy (light) does it let into the greenhouse, and how much energy (heat) will go out?
What are the purchase, installation, and maintenance costs?
How well can the grower manage the environment which is imposed by the glazing to produce a quality, salable product for profit?
These questions can be answered with the experience of the grower, the crop pro-duced, the glazing, the local outside environment, and the greenhouse environmental control systems. There are also other variables that affect the cover used for the crop, such as transmittance, dust and dirt, the gases and also salt (Zabeltitz 1990):
High transmittance of visible light, photosynthetic active radiation PAR, with wave-lengths of 400–700 nm.
Low transmittance of long-wave radiation, FIR, in the range of wave lengths from 3,000–20,000 nm.
Low reduction of light transmittance by global radiation (long duration of life).
Low ageing due to UV-radiation.
No drop-wise condensation on the inside of the roof, but condensation as a liquid film (No-Drop properties).
Low accumulation of dirt and dust due to the type reduced light transmittance.
Endurance against wind.
In order to solve the current necessities there are 3 types of coverings typically used for greenhouses: glass, plastic films and rigid plastic panels (Quiminet.com, 2011, AMCI 2008, Yuste Pérez, M. P. 2008):
Glass:
Glass is the traditional covering. It is available in many designs to blend with almost any style or architecture. Glass greenhouses may have slanted sides, straight sides and eaves, or curved eaves.
Glass: The glass that is used as a cover in the greenhouses is always the printed glass or glass “cathedra’’. Its thickness varies from 2 to 6 mm, and the plates measure approx., 60 cm. Its thickness is measured in gauges and sold in coils in variable widths (from 80 cm up to 12 m and thicknesses from 200 to 1200 gauges, i.e., from 0.05 to 0.3 mm).
• Advantage:
Good-quality glass is an attractive, very transparent, and formal (in appearance) covering material.
It is very strong (tensile strength).
• Disadvantage:
It is subject to shattering and can become brittle with age.
It is also very expensive.
Its weight requires sturdier framing support than is required with other covering materials.
Plastic films:
The use of plastic is increasing in the building of greenhouses due to its cost per square foot. It can be heated as satisfactorily as glass greenhouses, crops that grow under plastic are of equal quality to those grown under glass, it is considered a temporary structure and usually carries a low assessment rate for tax purposes, or may not be taxed at all. Plastic structures can be made of polyethylene (PE), polyvinyl chloride (PVC), copolymers of these materials, and other readily available clear films. Plastic film is the most applied cladding material in countries with tropical, subtropical and arid climates (Zabeltitz 1990).
Polyethylene (PE): Permits passage of much of the reradiated heat energy given off by the soil and plants inside the structure. When it is used an Ultraviolet-inhibited polyethylene, it is lasts longer than regular polyethylene. It has an inhibitor that prevents the rapid breakdown caused by ultraviolet light. UV-inhibited polyethylene is available in 2- and 6-mil thickness up to 40 feet wide and 100 feet long. Therefore, a polyethylene type loses heat more quickly than glass both during sunny periods and after sunset. This is an advantage during the day and a disadvantage at night.
These films are reported to reduce 20% of the heat loss from the greenhouse and have become common in today’s industry, especially in Europe.
A newly developed polyethylene film in Israel has been designed to allow very low levels of UV light to be transmitted. There is good evidence that UV blocking films
have an adverse effect on flying insects such as Bemisia tabacci, aphids and thrips (Merle 2001).
• Advantage:
It is low in cost and lightweight.
It also stands well in fall, winter, and spring weather, and lets through plenty of light for good plant growth.
• Disadvantage:
Constantly exposed to the sun deteriorates during the summer and must be replaced each year.
Ultraviolet light energy causes polyethylene to break down.
Low density polyethylene (PE): This is a good choice for home-built greenhouses.
Clear PE is used for growing most plants White PE can be used to reduce light and heat for growing lowlight plants or for propagation. In the majority of the polyethylene films for greenhouse, the point of yielding occurs around a 2.6–3% of stretching.
The most secure point of tension in the middle of the point of yielding is between a 1.3–1.5% of the length of the film that it is taut.
• Advantage:
Less structural support is required.
It costs much less than other materials.
Easy to adapt to all types of structures.
Great flexibility.
It resists well the mechanical effects of the wind and hail and is tear resistant.
Does not deteriorate by chemical substances that can be used inside the greenhouse.
Easy to incorporate additives that improve their performance
• Disadvantage:
PE film only lasts about 2 years.
Degrades more quickly than other materials.
It is a poor conductor of heat.
Any deficiency in their placement may cause tears in the plastic.
It is degraded by UV radiation and Oxygen
Polyvinyl Chloride (PVC): Can be used to construct a Quonset type greenhouse frame.
• Advantage:
It is readily available, fairly cheap and easy to work with.
Up to one inch diameter can be bent to form the hoops over which the Polyethylene film can be stretched.
Ends can be constructed from either plywood or fiberglass sheets.
Retains more heat at night, thus avoiding a possible temperature inversion. *Greater resistance to cracking and tearing.
Ages more slowly.
It can be transparent, translucent, photo-selective, bluish, and black.
• Disadvantage:
Fiberglass sheets can also be screwed to the hoops to form a cover over the frame.
Little used (only 2% of all the covering materials are PVC). You have a greater green-house effect than that of the PE, due to the fact that it is a poorer heat conductor than this one.
The moisture condenses very little.
Dust quire fixed on the surface.
Vinyl sheet: Used in ends of the greenhouse so customers could have a nice, clear view of the facilities. Not intended for use as covering for the top of the greenhouse. The clear vinyl siding is supposed to last about 4 years.
• Advantage:
It is heavier than polyethylene, more durable and considerably more costly if made with a ultra-violet inhibitor.
It can last as long as five years.
• Disadvantage:
Like polyethylene, it has electrostatic properties that attract dust, which clouds the sheeting and therefore cuts down the transmission of light.
Rigid plastic panels:
Polyester: The best known of the polyester films is Mylar, with a 5-mm thickness it is used for greenhouses construction; it has the advantages of being light.
• Advantage:
It is strong enough to resist damage by hail, it is unaffected by extreme temperatures and has light-transmission.
Similar characteristics to glass.
• Disadvantage:
It is however expensive.
It will not be so effective when used on poorly built frames that are rocked by wind.
Fiberglass Reinforced Panels (FRPs): Are rigid plastic panels made from acrylic or polycarbonate that come in large corrugated or flat sheets, their greatest advantage is their high resistant to breakage. This factor, coupled with its good resistance to ultra-violet, means it should last between 10 and 15 years.
• Advantage:
Are durable, retain heat better than glass does, and are lightweight.
The panels are either flat or corrugated.
The light admitted to the greenhouse is soft and shadow less.
• Disadvantage:
Make sure that it is not exposed to flame or extreme heat, because it burns readily and rapidly.
Acrylic Semi-rigid: Usually flat acrylic panels are ideal for greenhouses because of their strength, light weight, resistance to sunlight and good light-transmission characteristics.
• Advantage:
Strength.
Light weight.
Resistant to sunlight and good light-transmission.
Will give good service for many years.
It is resistant to snow, strong winds and even hail impacts.
• Disadvantage:
Scratches easily.
It is very expensive, but can last 20 years, so one manufacturer claims it is cost effective over the long run, especially in commercial greenhouses where heating costs can be reduced up to 30%.
Polymethyl methacrylate (PMMA OR PMM): A transparent and rigid plastic, PMMA is often used as a substitute for glass in products such as shatterproof windows, skylights, illuminated signs, and aircraft canopies.
• Advantages:
Has great transparency.
Allows almost all UV rays.
It has high opacity to the nocturnal radiation.
High breaking strength, tear and aging.
• Disadvantage:
Its broadcasting power is almost zero.
Scratches easily and loses optical qualities.
Polypropylene: Has demonstrated certain advantages in improved strength, stiffness and higher temperature capability over polyethylene.
It has been very successfully applied to the forming of fibers due to its good specific strength.
• Advantage:
It is manufactured with a special non-stick treatment to avoid the dust that accumulates on the surface.
Easy to handle in positioning and fixing to the structure.
• Disadvantage:
It is transparent to long wavelength infrared radiation.
Due to its surface that has a high degree of porosity, it can cause rain water dripping, if the structure does not have sufficient slope.
Ethylene-vinyl-acetate or EVA: It is a thermic film necessary for places where the night temperature drops below the optimum temperature necessary for the crop.
• Advantage:
It has a higher thermal effect than with PE.
It is more flexible and more impact-resistant than PE, as well as more resistant to tearing.
More broadcasting power than PE.
• Disadvantage:
High dilation that results in rain water bags and break wind.
But independently from the material, when choosing a glazing, it is important to consider the following requirements Smith, S. (2002):
Fire resistance
Fiberglass covers some of these requirements, it is one of the best light diffusers;
polyethylene, polycarbonates, acrylics and glass follow roughly, but they are not the best options because it is necessary to consider durability, aesthetics, strength and cost.
Finally it is important to mention that there is no ideal glazing; according to researchers such as Smith (2002) and Zabeltitz (1990), most glazings developed for greenhouses allow satisfactory growth but have advantages and disadvantage:
The light loss depends on the cladding material and on the region (industrial or rural).
The influence of dust on light transmittance can be higher with No-Drop film than with standard PE film.
Cladding materials with special spectral transmittance can have various effects on greenhouse climate, plant behavior and pest control.
Photo selective plastic films and screens will be used as cladding materials for green-and screen houses as well as insect screens in greenhouses.
Furthermore, based on the types of materials and structures, as well as the covering materials a selection material can be performed that will serve to raise the floor and bases that would support the entire structure. This is very important as it helps on the safety and durability of the structure.