2. Learning objectives
4.1 Describe the factors that affect light levels in protected
structures, including shape of structure; site factors;
orientation; type and condition of cladding materials.
4.2 Describe how the temperature can be maintained in
structures, including heating by gas, oil or electricity; heat
distribution using circulating water and air; cooling by forced
or natural ventilation; evaporation; and shading.
4.3 Describe methods of changing the relative humidity (RH) in
a protected environment, including the effects of ‘damping
down’, ventilation and temperature changes.
4.4 Describe manual and automated methods of irrigation,
including the use of watering cans, hose pipes, capillary
systems and ‘drip’ systems
4.5 Describe how light levels can be manipulated, by the use
of supplementary lighting and shading, including blinds and
shading paints.
3. Factors affecting light levels
Site – needs an open site with no overhanging
trees or shading buildings
Orientation – north/south gives even light
distribution; east/west best for winter light
Glazing materials – compromise between light
transmission and cost and insulation values.
Shape of building
Use of shading or supplementary lighting
4. Light levels – shape of
building
Light will pass through a transparent material if it hits
the surface at 90°. Otherwise it will be reflected at a
corresponding angle to the angle of incidence.
No material however is perfectly transparent so some
light will always be reflected from impurities etc within
the material.
The angle of the light changes with the season and
the latitude.
So the angle of the roof and sides of the greenhouse
can make a radical difference to the amount of light
that passes through the glazing.
5. Light Levels – shape of
building
With a traditional straight sided
greenhouse with a roof at
about 30° light from a low sun
(winter and spring) passes
through the sides but is mainly
reflected from the roof.
Dutch Light greenhouses have
sloping slides which increase
the light transmission but have
the same issue with the roof
6. Light levels – shape of
building
With this Mansard
greenhouse each pane of
glass is at a slightly
different angle. Whatever
the time of year some part
of the glass is at 90° to the
light. This is very
expensive to build. Poly-
tunnels have a similar
response to light.
7. Controlling light – blinds and
shading
Reducing light intensity within the greenhouse
will protect plants from scorching and may
reduce temperature. Black out shading can
also be used to create artificial ‘night’.
Materials used include blinds (external or
internal), shading paint and shade netting.
External shading is most effective at reducing
temperature.
Factors to be considered include cost, life
span, work involved and ease of adjustment.
8. Controlling light –
supplementary lighting
The light source chosen must include PAR
Commonly used are High Pressure Sodium
and compact fluorescent lights.
HPS generates heat and is more expensive
to run but gives more light than compact
fluorescent lights.
Artificial lighting can be used to add to
existing day light in winter and/or to extend
the ‘day’ to mimic light conditions at other
times of year.
9. Controlling temperature -
heating
Possible sources of heat – electricity, paraffin or
gas or use stored heat from the sun.
Electricity is more controllable but expensive to
install. Unlike gas and paraffin no need to
ventilate.
Gas and paraffin are less controllable and produce
condensation but are cheaper than electricity.
Solar thermal can buffer temperatures in a cool
greenhouse but a very elaborate set up would be
needed for any greater level of heating.
Insulation is vital to avoid wasting heat.
10. Controlling temperature -
cooling
Use of external shading – blinds or shade
netting most effective, shading paint has
some effect
Evaporation of water – ‘damping down’.
Energy used to turn the liquid to gas is taken
from the heat in the air or in the greenhouse
floor etc.
Ventilation – natural or forced. Hot air moves
upwards and draws cool after it. Top and
bottom vents must be open for natural
ventilation.
11. Water - irrigation
Methods – by hand using a can or hose; by
capillary watering systems; by drip or spray
watering systems.
Aim is to maintain water content of the
growing media at the ideal level.
Drip or spray systems can be automated and
easily adjusted to conditions.
Capillary systems can look after themselves
while you go away for a few days.
Hand watering allows precise control and
plant monitoring.
12. Water – Relative Humidity
A measure of the amount of water vapour in
the air relative to the maximum it could hold;
this varies according to the temperature.
Ideal levels vary according to the plant and
its maturity.
Relative Humidity can be increased by
misting or damping down and reduced by
ventilation.
13. Question sheet answers
1. Shade the outside of the greenhouse, damp down
in the morning or early afternoon, open the side
and top ventilators in the morning and close at
night.
2. Blinds allow easy adjustment on cloudy days and
provide good reduction of temperature on hot days.
3. It removes the need for daily watering and
increases humidity in hot weather
4. Benefits: precise amounts applied, plants are
checked daily. Limitations: hard work in large
areas and needs someone to do it.
5. Because light is reflected back through the glass
by the inside surface of the water droplets.
14. Learning outcomes
4.1 Describe the factors that affect light levels in protected structures,
including shape of structure; site factors; orientation; type and condition
of cladding materials.
4.2 Describe how the temperature can be maintained in structures,
including heating by gas, oil or electricity; heat distribution using
circulating water and air; cooling by forced or natural ventilation;
evaporation; and shading.
4.3 Describe methods of changing the relative humidity (RH) in a protected
environment, including the effects of ‘damping down’, ventilation and
temperature changes.
4.4 Describe manual and automated methods of irrigation, including the
use of watering cans, hose pipes, capillary systems and ‘drip’ systems
4.5 Describe how light levels can be manipulated, by the use of
supplementary lighting and shading, including blinds and shading paints.
