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Alternative covering materials
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The greenhouse covering,
or glazing as it is referred to in the industry, represents the
greatest decision in selecting the design for the greenhouse. There
are a number of different materials to choose from, each with their
own inherent advantages and disadvantages. The glazing will most
drastically affect the amount and type of sunlight to reach a crop.
The glazing will also determine heat loss of the structure. Of the
various glazings available the most common include polyethylene
films, polycarbonate structured sheets, and glass panels. It's advisable
to review all of the different choices before selecting one for
a new enterprise.
Light Transmittal Properties
Greenhouse glazing protects plants from excess cold, hail, and rain.
In doing so however it still has to allow sunlight to pass through
for photosynthesis. Light transmittal is the amount of light that
passes through the covering. A percentage of the sun ray's will
be reflected from the covering, some with be absorbed, and the remainder
will be transmitted through. All coverings are compared to glass
which transmits 90% of the photosynthetically active radiation (PAR;
in the visible spectrum).
Polyethylene Films
According to the National
Greenhouse Manufacturers Association (NGMA) poly film is still
the first choice for glazing, primarily because of its low cost
(1). Air-inflated double-polyethylene film greenhouses can represent
up to 80% of new greenhouse construction the United States (2).
Growers often start out with poly coverings and latter move up to
more rigid glazings. Poly film is porous to both
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Quonset greenhouse
covered with 2 layers of 6 millimeter polyethylene plastic,
kept apart from each other by a layer of air, created by an
inflation blower.
Note the binding tape which is tied across the outer poly
layer in order to help protect the plastic during heavy winds.
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carbon dioxide and oxygen, thus allowing for plants to survive
within a tightly sealed structure such as a greenhouse. It typically
ranges in thickness from 3-8 mils for greenhouse applications (1
mil is 0.001"). Normal installation involves two layers (4-6
mil for the outer, and 2-4 mil for the inner), which provides a
total thickness of 6-10 mil (3) Air is blown between the layers
to inflate them to provide air for insulation. Polyethylene film
is available with life expectancies of one to four years, depending
whether it has been treated with UV inhibitors. With a UV stabilizer
the plastic will last longer before it yellows and cracks. Poly
film can also be ordered with an anti-drip coating, which will reduce
condensation damage on the plants below.
In terms of heat conservation, a double layer of poly will reduce
heat loss during the winter by 30-40% in comparison to glass, thus
lowering fuel expenses. As for light transmittance a double poly
glazing will reduce light transmittance by 15%. Inside a poly covered
greenhouse the sun's rays will be scattered and thus can be received
by the plants from all directions. With this advantage of diffuse
light growers can hang flowering baskets over greenhouse benches
without the lower tier of plants being heavily shaded.
Polyethylene film is very light in weight. Thus the film does not
require a structural support system. The poly covering simply needs
to be stretched to all 4 edges of the greenhouse structure and securely
attached. However due to its tight seal, growers will have to watch
that relative humidity levels do not build up to the point that
the plants begin to suffer from plant diseases. The ease of replacement
of poly glazing is an important factor in reducing the man-power
requirements for greenhouse maintenance. It is estimated that 8
workers can cover 1 acre of gutter-connected greenhouses with double
film poly tubes (both inner and outer layer tied together).
Poly glazing is considered to have a moderate resistance to hail
damage (4) If it does tear small holes can be filled with silicone
sealants. Larger gashes will require the use of special greenhouse
tape, liberally sealed with silicone. In terms of flammability,
polyethylene is considered only moderately susceptible. Lower flammability
rates can help reduce insurance premiums. However, due to its shorter
life span the fire rating is often over-looked.
Glass
Glass glazing was commonly used up until the 1950's when other coverings
(poly, fiberglass) were developed. Originally glass panels were
relatively small: 16" by 18". Today tempered glass for
greenhouse construction is available in sizes of up to 39"
by 65". The larger size panels are actually less fragile. If
the aim is to build a long-lived structure that is visually appealing,
especially to the public, glass may be the solution. In all of the
choices for glazing glass still has the highest life expectancy
(25+ years). As glass is inherently resistant to ultraviolet radiation,
it does not degrade over time and only needs to be replaced due
to loss caused by cracking and chipping damage. The support structure
required for glass still surpasses that required for other glazing
materials. Safety glass is available, and municipalities often have
regulations for the type of glass permitted in roof in roof applications.
As for flammability, obviously glass will not burn.
The percentage of direct radiation transmitted though glass is the
highest of all greenhouse glazing options. Its high clarity, though,
does lead to shadowing within a structure, as well as an amplified
heat loss. Glass is often used in propagation houses where high
levels of light are required from plant growth. In retail garden
centers the heat load on shoppers beneath can become un-comfortable,
thus requiring the use of shade panels on the hottest days.
In order to reduce heat loss inherent to glass during the winter
months, themopane glass can be used. When the 2 panels are separated
by a ¼" space of trapped air the total insulation value
of the house is increased significantly. In Europe glass greenhouses
are quite common in both Holland and Britain, especially where sites
are windy, but snow is un-common.
Rigid plastic glazings
Rigid plastic coverings include corrugated and flat fiberglass (FRP),
corrugated and structured polycarbonates, and structured acrylics.
Glass fiber-reinforced (FRP) panels were first introduced in 1947
for greenhouse glazing (4) Like poly film, FRP panels can scatter
sunlight throughout the greenhouse. Light transmittal through FRP
is considered very good (80-90% of clear glass). Most panel manufacturers
warrant their product for 10 years. FRP received poor ratings during
its early life as it tended to turn yellow and later darken to tan
and brown. More recently a clear PVF film known as Tedlar has been
developed which can preserve the light transmittance of FRP.
The biggest advantage of FRP is its impact resistance, without shattering.
FRP panels can with-stand severe hail impact without breaking like
glass or other rigid plastic panels. It is often used in greenhouse
end walls because of its superior strength.
Structured polycarbonates
These semi-rigid sheets are available in several standard sizes
and configurations. The two primary products are acrylic (polymethyl
methacrylate) and polycarbonate. Both of these come in two primary
configurations: single layer and multi-layer sheets. The layer configurations
refer to the number of walls separated by layers of air. Greenhouse
applications generally see single layer (no air layer), double layer
(two walls separated by air), or triple layer (three walls and two
air separations). Architectural applications have a variety of other
configurations available.
The air between the sheet layers helps reduce heat loss. However,
as the number of sheet layers increases the amount of transmitted
light through the panels' decreases. The grower must decide which
is more important -- light transmission or insulation.
An advantage to polycarbonate structured sheets is their 10-year
life span, which most polycarbonate manufacturers guarantee. Polycarbonate's
longer life expectance will result in a higher price per square
foot than polyethylene film. Light transmitted through polycarbonate,
when compared to polyethylene film, has a higher percentage of direct
radiation vs. diffused radiation. Polycarbonate panels have low
flammability rates, and very high impact resistance.
Acrylic panels are often bonded together to form a sandwich construction
0.6" apart. With this high energy-efficient twin-wall design,
panels can be purchased 4' wide by as long as 12' long. They have
excellent clarity and light transmission, they are flame retardant,
possess high impact resistance, they are UV stabilized, and have
a textured surface which diffuses light thus preventing condensation
drip. In terms appearance they are considered highly attractive.
In retail garden centers they can be tinted solar bronze and shades
of translucent white, which greatly helps to reduce the heat-load
beneath them making for a much more enjoyable shopping experience
(5).
Because the sheets are rigid and come in specific widths and lengths,
installation on a greenhouse requires more components than polyethylene
film. The sheets need to be supported horizontally from underneath
at intervals that are specified by the polycarbonate manufacturers.
Extrusions and point fasteners are required to attach the sheets
to the support structure. Depending upon the layer configuration,
splices are used between the sheets to form a continuous covering
for the roof and / or walls. Expansion and contraction has to be
considered as well.
References
1. Counting on key components. 2001. Matt Hopkins, managing editor
for Greenhouse Grower
magazine.
2. Greenhouse
covering systems. Gene Giacomelli and William Roberts, Rutgers
University Cook College, New Brunswick, New Jersey.
3 The Commercial Greenhouse. 1998. James Boodley, Delmar
Publishers, Albany, New York.
4. The Greenhouse and Nursery Handbook: A Complete Guide to Growing
and Selling Ornamental Plants. 2000. Francis Jozwik. Andmar
Press, Mills, Wyoming.
5. Starting a greenhouse business: A commercial growers guide. 1994.
Alan Stevens. Kansas State University.
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