Basic Naturally-Ventilated House Construction for Hot-Humid Climates

 

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.