The Plastic Greenhouse - Greenhouse Technology and Management - page 83

The development of quality regulations
adapted to the regional or national condi-
tions of use is necessary for an appropriate
transparency (in this case in the economic
sense) of the market.
The use of double layers to reduce the
thermal losses is not limited to alveolar
panels, because the idea is also used in the
case of glass (double glass) and flexible
films, using a double layer (which plays the
role of thermal screen) in some cases or, in
others, as an inflated double layer (by means
of air pressure, coming from a fan).
It is worth highlighting the recent
expansion of screens (nets) as cladding
material of greenhouse structures (Montero
et al , 2009), when temperature increases
are not the goal, but rather protection
against wind or shading or improving the
ambient humidity. The structural require-
ments of these greenhouses are lower than
for those of film, as the screens offer less
resistance to the wind and are permeable
to water.
The so called 'screenhouses' are effec-
tive and economical structures for shading
crops, protecting them from wind and hail,
improving the temperature and humidity
regimes, saving irrigation water and exclud-
ing insects and birds (Tanny et al ., 2006).
There is a relationship between the
porosity of a screen and its transmission of
solar radiation, but other parameters also
influence diffusion effects on the incident
radiation and, consequently, on shading
and transmission levels (Sica and Picudo,
2008). Screens contribute to increasing the
diffuse fraction of the transmitted solar
radiation through them at different levels,
depending on the structure, texture and col-
our of the screens (Abdel-Ghany and
Al-Helal, 2010; Moller et al ., 2010).
Dust deposition on the screen can
widely alter its light transmission (Santos
et al ., 2006). Screenhouses can save around
30% of the annual irrigation water required
for outdoor conditions, without any loss of
yield and even improving quality (Tanny
et al , 2006).
The use of coloured screens, instead of
the conventional white or black screens, to
manipulate the vegetative growth of the

crop and to improve the yield and quality,
has been recommended (Oren-Shamir et al. ,
2001). In coloured screens, the spectral
manipulation is aimed at specifically pro-
moting desired photomorphogenic/physio-
logical responses, while light scattering
improves light penetration into the inner
canopy (Shahak et al ., 2009). In order to
limit the visual environmental impact of
screenhouses, the colour of the material
should be chosen carefully (Castellano
et al. , 2008a).
Net or screens are characterized, among
others, by their porosity (which influences
their shading effect) and their permeability
to the air (Castellano et al ., 2008b). Porosity
is determined by the diameter and physical
characteristics of the thread and the density
of the screen (number of threads per centi-
metre), which also affects its durability,
overall weight, strength and elasticity
(Castellano et al ., 2006). The most common
prime raw material used for making agricul-
tural screening is high density polyethylene
(HDPE). In Europe, screens are character-
ized by the number of threads per centime-
tre in each direction (e.g. a 10 × 20 screen
has ten threads in one direction and 20 in
the other) (Teitel, 2006). See Chapter 8 (sec-
tions 8.4.6 and 8.4.7).

Assembly of the cover

In the case of flexible films, the mechanical
characteristics of the film and its assembly
determine the life of the film and the degree
to which the greenhouse performs properly
(how airtight it is, thermal losses, etc.).
The form in which the film is assem-
bled depends on the dimensions and the
characteristics of the ventilation system
(roof, side, mixed) and is limited in princi-
ple by the width of the film. It must be
assembled at high temperatures (which will
allow the plastic to expand) so the film
remains as taut as possible (Fig. 4.11). The
fixing must avoid, if possible, nailing or
drilling the film (which is sometimes very
difficult in artisan greenhouses such as the
parral), because these drillings are the
point of entrance for rainfall water which
will directly drip over the crops, and also