BULLETINS
& ARTICLES
GLASS VERSUS
PLASTIC
PRESS RELEASE
Light transmission under glass versus plastic is
often a hotly debated subject amongst greenhouse owners and constructors.
Each has their own viewpoint and to the grower, both can be convincing.
Light is only one factor in the myriad of needs
that the greenhouse operator needs to consider for their crop, however,
it is without doubt, one of the more important fundamentals.
In a past issue of Horticultural News (1993) it
was reported that research that had been done in both New Zealand and overseas
relating to this topic. This article stated “twinskins of greenhouse
film transmit about 9% less light than a single sheet of glass, however
when the light levels available to the plants in a greenhouse situation
are measured, the reverse is usually the case”. Since this article,
further advancements have taken place in structural plastic house and greenhouse
film technology and we asked Australasian greenhouse manufacturer Redpath
to review and update this information from a poly-house manufacturers perspective……
Take a modern commercial plastic house and a modern
glasshouse and strip them of their cladding material – what do you have
left? You have a steel/aluminium space frame designed to do a specialist
job.
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to provide around 30% floor area lift ridge ventilation
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between 30-60kg (75lb) crop support capacity
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its design and gutter must handle the same expected
climatic conditions of wind and rain
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foundations are similar to meet building regulations
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similar automatic vent drive systems are fitted
Essentially the major difference is the choice
of cladding chosen by the manufacturer and its retention system holding
it to the frame.
Many references to claims of light transmission
are not backed up by the appropriate research. A ‘general claim’ that ‘light
levels under singleskin and twinskin are lower than glass with a corresponding
production loss’ is often not a quantifiable statement.
Under laboratory conditions a singleskin greenhouse
will transmit a very similar amount of light to the crop as horticultural
glass, whilst some types of film will transmit greater amounts. A
research paper completed by Dr John White at the Levin Horticultural Research
New Zealand Centre titled “Multibay Inflated Roof Polythene Greenhouse”
published in the New Zealand Journal of Experimental Agriculture 1982 Vo.
10, 111-116 stated “light transmission was found to be better in the inflated
roof greenhouses than in the glasshouses.” This research is based
on New Zealand conditions and management practices and is supported by
other research and established successful plastic clad greenhouse operators
worldwide.
Further improvements in greenhouse film technology
has taken place since this work was done and it would be reasonable to
assume that plastic is now in an even stronger position. The latest
resin formulas of greenhouses films are providing up to 93% PAR (photosynthetic
active radiation), whilst the latest reliable sources stated by manufacturers
in Holland and New Zealand is that horticultural glass offers 89%.
Manufacturers of the most recently released greenhouse
films’ developed in Canada (Duratough)
are now offering a 4 year UV full replacement
warranty package, plus state that their new films’ will remain ‘clear’
and offer consistent PAR light for at least the warranty time period (
a problem in the past when some films would change ‘colour’). To
arrive at these figures one needs to consider other factors affecting or
impeding light transmission to the crop
In favour of plastic, the structural framing of
a modern poly-house truss offers considerably less solid overhead shading
beams (see diagram below) as the lightweight cover requires less support
at its edges, the individual panels are also considerably larger and are
able to ‘clear span’ from rafter to rafter without the need for light-impeding
glazing support bars. Ventilator design and mechanisms also tend
to require a more frequent support structure in glass, to handle loads
when compared to plastic designs.
Glasshouses commonly require overhead thermal or
shade screen equipment to assist environmental control, which also has
further crop-shading impact when drawn back and not in use. Plastic
has the natural advantage of being thinner and not as reflective as glass,
which assists sunlight in being able to travel throughout the intensively
planted crops dense foliage. The curved shape of the latest cosecant
curved rafter greenhouse truss technology now available from redpath additionally
increases light entry into the structure, due to the fact that, at any
one time during the course of the suns travel through the day, the angle
of entry of light will more likely be at right angles to the roof pitch,
and thus reduce loss light through deflection.( this is particularly important
during winter when the sun is low in the sky.)
Light transmission is also affected by other factors,
- simply cleaning the greenhouse or glasshouse regularly inside and out
will maintain higher and more consistent light transmission levels.
Many plastic clad greenhouse manufacturers offer safe and wide ‘walkthrough’
internal structural gutters making the external cleaning process simple.
Recently developed greenhouse films are anti-static treated which assists
the film in remaining cleaner for a longer period of time by resisting
dirt and dust attraction.
Light transmission to the crop (up to 10%) can
also affected by the build-up of internal condensation that forms on the
greenhouses cladding due to inside/outside temperature differentials.
Traditionally plastic clad greenhouses have suffered more from the affliction
of condensation than glass clad houses. This is due to the fact that
glasshouses typically have a greater air exchange rate via their poorer
internal/external air seal at gutter edges and glass laps. Whilst
this has the effect of making the glasshouse run ‘drier’, the downside
to it is the reduced energy efficiency, in the form of lost generated heat
to the outside environment, which is especially noticeable in windy and
wet conditions common in winter.
With plastic there is less uncontrolled internal
air loss. A good plastic house with seals on doors, and upstand flashings
on ridge ventilator systems will offer considerably better energy efficiencies
with claims being made of up to 30%. Even if we accept a conservative
figure of 15% energy savings, it takes little effort to calculate the potential
cost savings annually to the grower, not to mention the reduced investment
required for the smaller energy system needing to be installed.
A cosecant greenhouse truss design solves the condensation
control issue. This truss type progressively increases the pitch
of the cladding so as to speed up the run off of condensation and does
not allow it to drip onto the crop underneath. The run-off is then
trapped in a small internal gutter and directed outside the building, which
prevents the condensation from being re-circulated within the environment,
avoiding uncontrollable humidity and high saturation points that could
potentially promote crop disease.
Draining condensation quickly and preventing further
reformation, will improve the light transmission levels in this style of
truss plastic house, particularly in low light winter conditions when condensation
is typically more prevalent.
A cosecant rafter also allows the use of singleskin
roof panels without the necessity of isolating the poly sheeting to the
rafter, further improving light onto the crop. Manufacturers of these
designs overseas, claim equal if not greater light transmission levels
to the crop over glass. New Zealand is blessed with a temperate climate
and very high average light levels when compared to some northern hemisphere
countries, so low light levels are typically only an issue during a short
period of the year (6 weeks) whilst for the greater majority of the year,
growers often have a problem trying to reduce rather than increase levels.
Light transmission is only one aspect of a host
of fundamentals required by the crop and its operator, however, new developments
in plastic house structural frames and the plastic film cladding itself
challenges the traditional line of thinking that glass has any advantage
in this area.
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