Types and characteristic factors of roofing materials for greenhouses

The optical qualities of the light within the greenhouse structure are just as important as the thermal ones. Especially for some crops, let's see what could be the best choice.

For the covering of greenhouses we usually refer to three categories of materials.

1 - Thin plastic films, which include polyethylene, ethylene vinyl acetate (EVA), polyvinyl chloride (PVC), polyvinyl fluoride (PVF) and polyester films.

2 - Plastic panels, which include glass fiber reinforced plastic (FRP), acrylic, polycarbonate, polypropylene, PVC and PETG (modified polyethylene terephthalate glycol).

3 - Glass (for example annealed, tempered and laminated).

Let's see the characteristics of each of them.

Plastic films

Among the flexible plastic films we find polyethylene, ethylene tetrafluoroethylene (Tefzel), polyvinyl chloride (PVC) and polyester. Plastic film is currently the main coating for greenhouses for basically one reason, greenhouses with metal frames for plastic film are lighter and cost less.

Polyethylene film
Polyethylene, sometimes also known as poly, has always been and still is the main choice among plastic films for greenhouses in most of the world. The main advantage of polyethylene film is cost, which is significantly less expensive to purchase and install than glass or any other panel. The polyethylene film is very light. Therefore it does not require a structural support system.

Tefzel film
The newest plastic coating material for greenhouses is Tefzel T2 (ethylene tetrafluoroethylene) film. The light transmission is 95% and is greater than that of any other greenhouse roofing material. A double layer has a light transmission of 90%.

Polyvinyl chloride (PVC) film
Polyvinyl chloride has a number of properties that make it very attractive as a coating. PVC has excellent wear resistance. Oxidation has little effect on PVC, but heat and light depolymerize the PVC film in 2 to 3 years and it breaks down. The PVC film reduces the transmission of long wavelength infrared radiation, resulting in less heat loss at night than using polyethylene.

Polyester film
Although the cost of polyester film is higher than that of polyethylene film, polyester film is known for its durability and longer life expectancy. Other advantages include a level of light transmission equal to that of glass and the absence of static electrical charges that collect dust.

Rigid plastics

Hard plastic coatings include rigid panels made of glass fiber reinforced plastic (FRP), polycarbonates and acrylics. Light transmission through rigid plastic is very good, although it usually decreases over time as the plastic ages and turns yellow due to the amount of UV radiation contained in sunlight. Large sheets are much lighter than glass and require less support iron to secure them to the greenhouse. However, these rigid panels are not that easy to install on curved roofs.

Rigid glass fiber reinforced plastic panel
Rigid glass fiber reinforced plastic (FRP) panel has been used in greenhouse roofing since the 1950s, but its demand has declined in recent years. Fiberglass is available in flat and wavy configurations. Corrugated panels are commonly used for greenhouse roofs, as its wavy shape gives the panels strength and rigidity. Flat panels are usually used for side walls, windows and vents. Although FRP panels are classified as rigid plastics, they are flexible enough to be folded into a curve to accommodate the structure of an arch-type greenhouse.

The polycarbonate
Polycarbonate is one of the most widely used shaped sheet materials in greenhouses today. The light transmitted through the polycarbonate, compared to the polyethylene film, has a higher percentage of direct radiation than the diffuse radiation. Similar to acrylic in heat retention properties, it allows about 90% of light transmission like glass. Polycarbonate has slightly lower light transmission than acrylic, but is considerably stronger and impact resistant, more flexible, and is only flammable when the open flame is kept in contact with the material. Although the initial cost of polycarbonate is high, a lifespan of ten to 20 years can be expected (depending on the manufacturer). Polycarbonate has a high impact resistance, about 200 times that of glass.

Polymethylmethacrylate (PMMA) or acrylic
Acrylic has been used for many years and is considered the best performing clear rigid plastic for greenhouses. Acrylic has excellent transparency and light transmission, is flame retardant, has high impact resistance, is UV stabilized and has a textured surface that diffuses light by preventing condensation dripping.

Fiber Reinforced Polymers or FRP or fiber-reinforced polymeric matrix materials
There are some polyethylene films and rigid panels made of FRP, acrylic and polycarbonate that can be formulated to more effectively control or use the heat and light energy of the sun and also the heat energy radiated from the ground. These forms of energy are part of the electromagnetic spectrum and differ only in wavelength. By using different polymers and additives, it is possible for films to transmit, absorb or reflect different wavelengths. It is therefore possible to create a wide variety of microclimates using different types of films, both as a cover on crops and as mulch on the soil surface.

Anti-condensation inhibitors
The water vapor condenses on the cold surface of the inner cover forming small droplets of liquid water. Condensation is particularly problematic in the morning when a thick layer of droplets form due to the cold greenhouse cover and the humid air inside. This has negative consequences on light transmission. In addition, condensation can fall on the crop, favoring the development of fungal diseases. Anti-drip additives modify the surface tension of the water, eliminate droplets and form a thin continuous layer of water so that it drains sideways rather than forming droplets that drip onto the plants.

Plastics that block infrared (IR)
Polyethylene is a poor barrier to radiant energy. The energy absorbed by objects inside a greenhouse during the day is radiated at night, leading to rapid cooling inside a structure on cloudless nights. However, thermal polyethylene, sometimes referred to by manufacturers as IR film, is available which contains an additive that reduces the re-irradiation of this IR energy.

Anti-UV materials
The ultraviolet rays of the sun depolymerize the polyethylene; after three or four years the plastic becomes quite brittle and prone to breakage. The transparency of the film also changes (becomes opaque) and the amount of light and the color spectrum are significantly limited, which will affect the growth and development of plants. Most polyethylene films used in greenhouses are made with a UV stabilizer to extend their life.

Dust inhibitors
Most polymers are poor conductors of electricity, particularly prone to static electricity build-up when two surfaces are rubbed against each other or when there is friction caused by wind. As a result, most plastics attract dust.

Light diffusers
When the sky is clear with high solar radiation, direct radiation can cause burning of leaves in greenhouse crops on hot days. For this reason, plastic films have been developed to increase the percentage of radiation diffused in the greenhouse.

The glass

Glass is the roofing material of choice for many growers, especially those who they see greenhouses as a long-term investment. Glass can be used on any type of greenhouse. Glass is the most expensive material to purchase and install, and the overall cost of building a glass structure will also be substantially higher due to the increased iron content of the structure; however, depreciation over its long life can make glass a cheaper cover than others. The operating costs of a glass greenhouse are also higher than those of polycarbonate greenhouses.

Advantages and disadvantages of glass
The main advantage of glass is its relatively long service life. Maintenance on glass is similar to other rigid materials and includes washing dust to improve light transmission. Glass is still used, in part due to its superior light transmitting properties and fewer relative humidity issues.

Types of glass
Glass for greenhouses is available in different types, including float, tempered, laminated and frosted or "hammered". The typical glass that is used for glass greenhouses is called float glass. It is a clear glass with high light admission to ensure sufficient light in the greenhouse. Over time, as safety awareness has grown, the application of float glass has declined. Today, new greenhouses are often equipped with safety glass (e.g. tempered or laminated). Tempered glass is four to six times more impact resistant than float glass, and when it breaks, it breaks into small pieces, making them unlikely to cause injury.

To deepen the topic

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Calculate the DLI (Daily light integral) of the greenhouse you have in mind. Starting from the Italian geographical area concerned and knowing the limiting factors, in terms of the reduction of light radiation determined by the structure and material chosen as roofing, you can calculate the DLI of your project.

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