GREENHOUSE ORIENTATION:
In southern latitudes single bay greenhouses can be oriented east-west
to allow maximum light reception in the late fall, winter and early
spring. For multiple bay, gutter-connected greenhouses the orientation
is usually north-south so that the shadows from the gutters track
from west to east across the crop rather than shading the same areas
all day. In either case, the rows of plants within the greenhouse
should run north-south to optimise equal light to all plants throughout
the day.
WINDBREAKS TO SAVE ON HEATING:
Winds of only 15mph can double the heat loss from a greenhouse.
The wind reduces the thickness and therefore the insulating effectiveness
of the thin air layer (boundary layer) along the greenhouse glazing.
A wind will essentially ‘suck’ heat away from a greenhouse
faster than if the air was still.
Windbreaks, in the form of fences, trees, buildings, etc. can slow
the wind and therefore cut heat losses from the greenhouse.
USE OF DOUBLE VERSES SINGLE LAYER GLAZING:
Double layer glazing, with at least a ¼” insulating
layer of air in between, can reduce the conductive heat loss by
up to 40% over single layer glazing. Using triple layered glazing
or, for example, a double layer of polyethylene over glass, can
further cut heat loss, but it will also reduce solar radiation,
so this is very rarely done.
STRUCTURAL INSULATION:
Insulating materials can be applied to the foundation of the greenhouse,
to the south wall (in the southern hemisphere) and to the walls
up to the height of the plants to reduce conductive heat loss.
Weather stripping and other insulating materials should be added
where ever there are gaps in the structure. This includes around
doors and vents and where glazing panels meet the structural supports.
If the glazing material is cracked (ripped polythene, broken glass
panes or cracked poly acrylic or carbonate) replace immediately
to reduce heat loss.
INFLATABLE TUBE INSULATION:
Polythene tubes (6-18” in diameter) can be hung from the greenhouse
ceiling. When inflated they create an effective insulating barrier
to heat loss through the ceiling (up to 40%). Make sure the tubes
fit snugly along the walls.
Since polythene above the crop will reduce light transmission,
tube systems have been designed to be retractable or removable during
the day.
Though effective, these systems are rarely used in commercial operations.
RETRACTABLE HEAT OR INSULATING BLANKET OR CURTAIN:
Porous, non-porous and aluminised materials are all used as insulation
blankets.
It can be single or multiple layers: more layers giving more insulation.
The material, placed between the ceiling and the crop, must be secured
along the walls to minimise cold air above falling through onto
the crop.
These curtains can be used during the day in summer for shading
as well.
These retractable curtains are perhaps the most cost effective.
IR COATINGS ON POLYTHENE FILMS:
These infrared barrier films allow heat into the greenhouse during
the day (requiring a bit more venting or cooling) but significantly
reduce heat loss at night as much as 30%.
OTHER INSULATING METHODS – EXPERIMENTAL:
Polystyrene beads have been used by blowing them into the air space
between two glazing layers. Energy savings may amount to 60-90%
annually.
Liquid foam (or soaps) can be blown into the air space between
two glazing layers for an energy savings of perhaps as much as 50%.
A disadvantage of this is that most foam breaks down in cold.
Unfortunately, neither of these experimental methods is currently
practical.
EQUIPMENT OPERATION AND MAINTENANCE:
Maintain the heating equipment (check for leaks, valve operation,
thermostats, etc) so that it operates at peak efficiency. Insulate
supply and return hot water/steam pipes. Inspect regularly.
