Agriculture Reference
In-Depth Information
In general, cheap greenhouses originate
from the use of affordable and available
materials within the local context.
In the tunnel greenhouses of the
Mediterranean area, the use of galvanized
steel prevails, normally with tubes of circle
or oval sections, for widths of up to 10 m
and heights of up to 4 m.
For multi-tunnel greenhouses, metal-
lic structures prevail (a predominance of
galvanized steel, due to the high cost of
aluminium; Photo 4.5) or a mixture of
materials (wood-wire, steel-wire, steel-
wood,
4.6.4
Covering materials
Types of covering materials
Until the introduction of plastic materials,
glass was the only greenhouse cladding
used. The rigidity of glass limited the use of
curved shapes in greenhouses so the pre-
dominant roof geometry was that of gable-
roof greenhouses.
The availability of flexible films or
semi-rigid panels broadened the range of
possibilities in terms of greenhouse design
choices and notably decreased the carrying
weights (1 m 2 of 4 mm thickness glass
weighs approximately 10 kg, whereas 1 m 2
of PE of 0.2 mm thickness weighs 0.2 kg).
The lower construction costs of plastic
greenhouses, especially those covered with
flexible film, allowed for an extension of
greenhouse cultivation to many regions of
the world.
A usual classification of greenhouse
cladding materials is that indicated on
Table 4.2. Glass was, as already mentioned,
the first material used to cover commercial
greenhouses. Glass, as a greenhouse cover,
is a material of excellent optical and ther-
mal characteristics (Table 4.6). It bears age-
ing and pollution well and it is not
steel-concrete)
are
used
over
wooden structures.
Steel structures, which are normally
more expensive than wooden structures,
allow for a reduction in the number of
interior pillars (relative to wood), easing
the interior manoeuvrability (passage of
machinery, implementation of thermal
screens) and creating fewer shadows than
wood, increasing the available light. In
addition, steel structures are easier to
assemble than wood, have more accessible
roof ventilation mechanisms and are more
airtight, although the higher heat conduc-
tion of metal weakens these advantages.
Reinforced concrete structures are not
common.
Photo 4.5. The steel structure is used in multi-span greenhouses.
 
 
 
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