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One of the keys to minimizing heat stress in naturally-ventilated poultry houses during hot weather is making sure that outside air can easily flow into and out of the house. The easier it is for outside air to flow through a house the less likely there will be a detrimental build-up of heat within the house minimizing inside to outside temperature differentials. Furthermore, increased air exchange rates tend to result in increased air movement over the birds within the house thus maximizing heat loss due to convection. There are primarily three factors that determine the ease at which outside air moves through a house; house width, side wall opening, and local obstructions. Chicken farming in Africa faces many challenges that other countries do not.
House Width:
Traditionally poultry producers have found that naturally ventilation tends to
be most effective in houses which are 12m in width or less. Wider houses tend to
have lower air exchange rates and significantly less natural air movement
towards the cent er of the house. Since air exchange rates tend to be reduced
significant temperature differences can occur between the upwind and downwind
sides of the house especially during cooler times of the year. Though the use of
interior circulation fans house has been found to mitigate the magnitude of the
problem, it is still advisable that house width should be kept to 12m or less in
most hot climates.
Curtain Openings and Curtains -
All naturally-ventilated houses must be equipped with some type of adjustable
side wall curtains to control the flow of air into the house during cooler times
of the year or when small birds are present. In order to facilitate the rapid
exchange of air during hot weather curtain openings should generally account for
between 50% to 80% of the side wall height. The hotter the climate, the wider
the house, the greater the percentage the curtain opening should account of the
side wall.
It is very important that there are solid portions of wall above and below the
curtain opening. The lower solid wall portion of the wall serves a number of
important functions. First, a solid lower wall helps to reduce the amount of
rain that can enter the house during storms as well as prevent outside water
from running into the house. The also solid wall reduces the likelihood that
direct sun light will enter the house when the curtains are fully opened as well
as provide a surface for the bottom of the curtain to overlap reducing drafts
during cooler times of the year. Last but not least the wall helps to exclude
rodents from the house as well as minimize contact between the birds in the
house and birds outside the house. Though the best height can vary in general it
is best if the wall is a minimum of 40 cm tall.
A solid wall above the curtain opening allows adequate surface for the side wall
curtain to seal against during cold weather. Even in hot climates it is
important to be able to seal a house relatively tightly during brooding periods
to minimize heat loss as well as drafts. Furthermore, it is important to have a
solid wall above the curtain opening to facilitate the installation of side wall
air inlets for use during cooler times of the year or during brooding. In
general, it is best that the wall above the curtain opening is such that it will
allow the side wall curtains to overlap the wall by a minimum of 30cm
The side wall curtain should be constructed of clear, non-breathable material to
limit air exchange during when heat conservation is desirable. A rigid curtain
rod at the top of the curtain opening should be used to more precisely control
the amount of curtain opening during when heat conservation is required. It is
best if the bottom of the curtain opening is also equipped with a curtain rod to
minimize drafts when fully closed. Like the top of the curtain the bottom of the
curtain should overlap the solid lower wall by 30 cm or more.
Typically translucent curtains are typically advisable to allow the maximum
amount of light in the house when the curtains are closed or partially closed.
When black curtains are used to control light it is best that the outside
surface is silver or white in color to reflect solar radiation thus minimizing
solar heat gain.
House Spacing:
House spacing can significantly affect the environment in poultry houses during
hot weather. A structure creates a zone on the downwind side where wind
velocities are reduced. If an adjacent house is placed in this zone it can be
subjected to decreased air exchange rates as well as heat, moisture, dust and
microorganisms emanating from the upwind house.
Though there are a number of factors that determine optimal house spacing
(prevailing wind speed, direction, topography, etc.) a minimum recommended
spacing can be calculated from the following formula (Timmons, Cornell
University):
D = 0.4 X H X L0.5
Where:
D= separation distance (ridge to the closest wall of the next house) in meters
H=Height of obstructing building in meters
L=Length the obstructing building in meters
Vegetation
Vegetation height should be kept to a minimum around poultry houses not only to
discourage rodents but to maximize air flow into the house. Close cut green
vegetation can be very beneficial compared to bare ground due to the fact that
vegetation temperatures can be as much as 30oC cooler than bare ground.
Furthermore, green vegetation reflects less solar heat into the house than does
bare earth.
Tall trees planted next to a chicken house can prove very beneficial during hot
weather. If the tree’s canopy is above a houses side wall not only will it shade
a houses roof from direct sunlight, thus reducing heat gain from the ceiling and
the ground surrounding the house, they can also help to direct air into a house.
It is important so as to not affect air flow into the house that the trees have
no branches below the eave of the house which may impede the flow of air into
and out of the house.
Ridge Openings
Ridge openings for naturally-ventilated houses are typically only effective in
poultry houses with un insulated roofs. Air next to an un insulated roof can easy
exceed 55oC. Since this hot air is much warmer than the rest of the air in the
house it will tend to rise to the peak of sloped roof. If there is an opening at
the ridge this super heated air will leave the house. Without the ridge opening
this hot air would tend to accumulate leading to increase house temperatures. In
a poultry house with an insulated roof air temperatures next to the ceiling are
typically not much warmer than those next to the floor so there is no great need
to make a special effort to rid the house of the air next to the ceiling.
Research has shown that ridge ventilation has very little effect on the overall
air exchange rates in most naturally-ventilated houses
House Orientation:
Naturally-ventilated houses should always be orientated in an east-west
direction. The reason for this is to minimize the possibility of direct sunlight
entering the house. Direct sunlight striking upon a bird can dramatically
increase the effective temperature a bird is experiencing. Direct sunlight can
increase the surface temperature of a bird to well above 38oC creating a heat
stress situation at air temperatures that wound not normally be thought of as
problematic. What is more often is the case when sunlight enters a house is the
birds will move away from the side wall where the sun is entering the house
thereby dramatically increasing the effective density of the birds. The higher
density significantly decreases the amount of air movement over the birds body
as well as puts in direct contact with other hot birds.
Though it is true that orientating a house east-west direction may not take full
advantages of winds blowing from east or west this is typically not a problem
for narrow houses (12m or less) with proper house spacing and curtain openings.
Roof overhang
A properly designed roof overhang helps to reduce the possibility of both direct
and indirect sunlight enter a house during hot weather. For most locations in
the world the sun will travel slightly to the north or south of an house
orientated in the east-west direction. Without a proper roof overhang the sun
would be able to shine directly on to one of the houses side walls. In a
naturally-ventilated house this means the sun will shine into the leading to an
increase in heat stress related problems.
The length of the overhang is a function of side wall height and proximity of
the side wall opening is to the ground. The taller a houses side wall the longer
a houses roof overhang should be to prevent sunlight from enter the house. The
closer the side wall opening is to the ground the longer the roof overhang
should be. Roof overhangs should typically be a minimum of 0.6 meters in most
instances but some houses with taller side walls and large curtain openings
could benefit from roof overhangs of 1.25 m or more.
Roof overhangs can also help direct rain coming off the roof of a house away
from the house as well as keep rain directly from entering a house.
Roof Slope:
Though there are structural considerations related the most desirable roof slope
for a poultry house, the optimal roof slope is more often determined by level of
roof insulation. In houses with un insulated roofs a steep roof slope (45
degrees) is highly desirable for a number of reasons. First, a steep roof slope
tends to collect less radiant heat from the sun than does a flat roof. Second, a
steep roof maximizes the distance between the birds and the hot ceiling which
reduces the amount of radiant heat the birds receive from the hot uninsulated
roof. A steep roof also encourages the super heated air immediately next to the
ceiling to quickly rise towards the peak of the ceiling far from the birds. If
the house has some type of open ridge the heated air will quickly leave the
house. Last but not least, a steep roof tend to create a more “open” environment
making easier for air to flow into and out of the house.
If a ceiling is properly insulated it produces essentially no radiant heat so
the need for steeply sloped roofs is significantly reduced. It is still
advisable in a naturally-ventilated house to have a sloped roof to easy the flow
of air into and out of the house as well allowing the equipment to be raised out
of the way in between the flocks to facilitate house clean out.
Insulation
To minimize heat stress related problems during hot weather it is always
beneficial to insulate poultry house roofs/ceilings. Most poultry house roofs
are fabricated from galvanized steel, which will commonly reach temperatures of
50 to 70oC on a sunny day. The hot roof can not only lead to increased house air
temperatures but can dramatically increases the amount of thermal radiation the
birds are exposed to. Much like direct thermal radiation from the sun which can
increase the surface temperatures of objects to 25 degrees or more above ambient
air temperature so can to a lesser extent the radiant heat produced by a hot
roof. It is not uncommon to find nest systems, cages, and other objects in a
poultry house with an un insulated ceiling to be 1 to 5oC above ambient air
temperature.
The best way to eliminate heat from a ceiling is through insulation. Insulation
acts as thermal barrier keeping heat from the hot roof from entering the house.
There are a variety of methods of insulating a poultry house ceiling.