Greenhouse covering materials

Master Protected and Precision Horticulture – Notes, Case Studies and Practical Insights – with Rahul

Greenhouse covering materials

a. Glass.

It has high light transmission and good durability.
Glass has the greatest longevity of any glazing material (25-plus years).
Use low-iron glass for the highest light transmission and tempered glass for the greatest strength.
Double-pane glass reduces the relatively high heat loss of glass, although some loss in light transmission results.
Glass must be installed and maintained to prevent air and water leakage between the panes.
Modern glass greenhouses use rubber or vinyl gasket material below the edge-to-edge glass and bar caps over the top of the seam.

b. Fiberglass

c. Plastic film

Plastic film is lightweight, very flexible and the least expensive type of greenhouse covering.
A single layer has high light transmission, but it also has high heat loss in cold weather.
Many Quonset-shaped greenhouses have a double layer of plastic with an insulating layer of air between them.
This reduces light transmission by about 10 percent, but it also reduces heat loss by 35 percent.
They have additives that slow deterioration from ultraviolet light, are stronger and have infrared inhibitors that reduce heat loss.
Greenhouse-grade plastics will last 4 to 10 years while other plastics will last only one growing season.
Some materials also have anti-drip surfaces that reduce droplet formation from water condensing on the interior of the plastic film.
They are of following types:

i. Polyethylene:

Commonly used plastic for greenhouse covering is thermoplastic.
The basic characteristic is they soften on heating and hardens with cooling and the process is reversible.
They are stiff, robust, resilience to resist loads and deformations.
Polyethylene used for covering year round production have UV inhibitor in it otherwise last only one heating season. Standard length 30.5, 33.5, 45.7, 61.0 and 67.0 m.
A polyethylene covering is colder than air inside greenhouse in winter due to which when warm air inside come in contact no time the water falls as beads over the plant.
The wet foliage causes diseases and also the constantly wetted soil becomes waterlogged and oxygen deficient.
With the water dripping problem, condensation also reduces light intensity within greenhouse usage of antifog surfactant is recommended.

ii. Polyvinyl chloride film:

UV resistant vinyl films of 0.2 and 0.3 mm thickness are guaranteed for 4-5 years respectively.
This extended life of polyvinyl film is advantageous as compared to polyethylene which has only 1 or 2 years.
But the advent of 4 year polyethylene the advantage of vinyl film has gone.
The cost of 0.3 mm vinyl is 3 times that of 0.15 mm polyethylene.
The vinyl films tend to hold a static electric charge due to which it attracts and holds the dust that reduces the light transmittance unless the dust is washed away.
In Japan 95 % greenhouses are under plastic and within the group 90 % are covered of vinyl film.

iii. Polyester film:

Polyesters offers long life and are strong.
Films of 0.13 mm thickness are used for roofs will last for 4years, while 0.08 mm films are used on vertical wall have life expectancy of seven years.
Although polyester having the higher cost as compared to polyethylene it offers the extra life expectancy.
The advantage include light transmittance equal to that for glass.
Polyester is still frequently used in heat retention screens because its high capacity to block radiant energy.

iv. Tefzel T2 film (Ethylene tetra fluoro ethylene):

It is the recent addition of plastic covering. This film was earlier used for transparent covering on solar collector. The life expectancy is 20 years or more.
The light transmission is 95 % and is greater than that of any other covering material.
Double layer will have about 90 % transmittance.
It is more transparent to IR radiation so that less heat is trapped inside due to which the cost of cooling will be reduced.
Disadvantage of Tefzel film is availability only in 1.27 m wide rolls which requires clamping rails on every 1.2 m.

v. Fiberglass reinforced plastic

Fiberglass is a rigid plastic that will bend around gently curved surfaces.
Most popular material in past.
Life period varies with grade. Some grades give 5-10 years while better grades can last up to 20 years. Corrugated panels are used because of greater strength.
It has high light transmission but comes only in single sheets (flat or corrugated); it also has a high heat loss.
Fiberglass-reinforced plastic is lower in cost than other rigid plastic glazing, but it does require more maintenance since it must be recoated every few years.

vi. Polycarbonate:

Polycarbonate is a rigid plastic covering that comes in sheets that have enough flexibility to bend around gently curved surfaces such as the Gothic-arch greenhouse.
It has high light transmission and is low maintenance, very strong yet lighter than glass and is a fire retardant.
Polycarbonate is available in single- and double-layer sheets.
The double sheets lose some light transmission but reduce heat loss to a greater extent
Double sheets also have greater longevity than single one
Polycarbonate sheets must be installed to allow for some contraction and expansion of the material with temperature changes.

vii. Acrylic:

Acrylic is a rigid plastic glazing that is clearer than polycarbonate, although it may slowly yellow with age
It is available only as a double-layered sheet that lacks flexibility and must be installed on flat surfaces.
Light transmission and heat loss are equivalent to double-layer polycarbonate.
Acrylic has the longest life expectancy of any glazing except glass, but compared to polycarbonate it attracts more dust and dirt, scratches more readily and is more flammable.