Broilers in a large tunnel ventilated shed – image provided by the ACMF

Climate in poultry houses

The climate in poultry houses influences the wellbeing and health of humans as well as the birds. Respiratory, digestive and behavioural disorders are more likely to occur in houses in which the climatic conditions are not up to standard. The efficiency with which feed is utilised is related to the health status of the flock. Animals that are not healthy cannot be expected to perform optimally. The younger the animals are or the higher their production level, the more sensitive they become to the climatic conditions in the house. Climate can be defined as the sum of environmental factors which influence the functioning of man and animal.

Climatic factors

The following factors must be measured at animal level.

  • Temperature
  • Relative humidity
  • Air composition
  • Air speed and air movement
  • Light

Factors influencing climatic conditions and the birds’ micro-climate

House climate can be influenced by insulation of roof, walls and floor, ventilation, heating, cooling and lighting. The climate directly surrounding the birds is called the micro-climate (for example, chickens in a brooding ring). In fact, the micro-climate is the only thing that is of importance for the birds. It is possible that the climate in the house is acceptable but the climate at bird level is unsuitable. For example CO2 is a heavy gas and CO2 levels at bird level can be much higher than at 2 m height. Another example is the brooding ring. The use of brooding rings means that the temperature of the house can be lower as long as the temperature at chicken level (under the brooder) is correct. This principle is applied in order to save on heating expenses. The advantages should be weighed against the disadvantages i.e. with brooding rings you can save on energy but often the labour to make and manage the brooding rings is more.


Layers are warm blooded (homeothermic) i.e. within a certain range, their body temperature is quite constant. On average, the body temperature of birds is between 41°C and 42.2°C. Body temperature is kept quite constant and is regulated by part of the chicken brain (the hypophyse). This part of the brain is comparable to a thermostat. Contraction and widening of blood vessels and the speed of respiration influence heat emission and retention which consequently influence the body temperature. It takes some time before heat regulating mechanisms start functioning in newborn animals and therefore they need a higher ambient temperature than adult animals do. Furthermore, the ratio between the surface area and weight of young animals is unfavourable and they do not have any fat reserves.

Temperature zones

The comfort zone is defined as the temperature zone in which the birds are able to keep their body temperature constant with minimum effort. This temperature zone also depends on the feeding level and housing conditions. Behaviour of birds will change when temperatures rise to above the comfort zone as they will start to pant and change their body position. When temperatures are below the comfort zone birds will also change their body position and huddle together.

The thermoneutral zone is defined as the temperature zone in which the birds are able to keep their body temperature constant with the help of physical heat regulation . This temperature zone depends on feeding level and housing conditions of the birds and other factors. The lowest temperature in the thermoneutral zone is called the lowest critical temperature (LCT). If temperatures fall to under this temperature the bird will start to use feed energy to warm itself (i.e. maintain its body temperature) and will consequently consume more feed. The highest temperature in the thermoneutral zone is called the highest critical temperature (HCT). If the temperature rises above this temperature the birds can no longer dissipate their heat. They will start to consume less feed and production will drop as a result.

The highest and lowest critical temperature depend very much on:

  1. Age
  2. Body weight
  3. Housing system
  4. Feeding level
  5. Relative humidity
  6. Air velocity
  7. Health

Physical heat regulation

When temperatures are not within the comfort zone, birds have several mechanisms which enable them to keep their body temperature constant without having to produce extra heat. This is referred to as physical heat regulation and factors that influence physical heat regulation include:

  • Tissue insulation – if birds have a layer of subcutaneous fat, they can afford to let their skin temperature drop. Only if the animals are fed properly can they deposit a subcutaneous fat layer when temperature decreases.
  • Feathers – feathers have an insulating effect and decrease the amount of heat that is lost to the environment.
  • Changing body position and huddling – birds can effectively regulate heat loss through body position. Heat loss can be minimised by huddling close together. In hot weather, on the other hand, the birds increase their body surface as much as possible.
  • Vaporisation of water – if temperatures are high, or extremely high, sensible heat loss is minimised and almost all heat will have to be lost as insensible (latent) heat. Latent heat loss is the heat lost from the body through the elimination of respiratory moisture.
  • Flow of blood through skin and mucous membranes – the flow of blood to the skin and mucous membranes can be controlled through the contraction and widening of blood vessels. The larger the flow of blood is, the more heat is lost.

Chemical heat regulation

Another way in which poultry can regulate their body temperature is chemical heat regulation. When the ambient temperature is not within the thermoneutral zone the birds can:

  1. Increase feed intake when the temperature is below thermoneutral zone
  2. Decrease feed intake when the temperature is above the thermoneutral zone.

Measuring and assessing temperature

The best instrument for measuring temperature is the animal itself. Assessing the temperature by observing the birds themselves should only be done when the animals are at rest, not when they are active or eating. Obvious indicators of unsatisfactory house climate are:

  • Behaviour of the animals
  • Abnormal body position
  • External abnormalities
  • Abnormal plumage may point to mistakes in house climate
  • Coughing/sneezing frequencies
  • Activeness

Measuring the temperature is the most common way of assessing the climate in a house. Such a measurement can give a lot of useful information and is not expensive or hard to do. There are several ways of measuring the temperature:

  1. Minimum/maximum thermometer (in every house or section of a house)
  2. Temperature sensor (computerised climate control)
  3. Thermometers (alcohol, electronic)
  4. Infrared thermometers – electronic thermometers

Location of the thermometer

The temperature in a house is not uniform and therefore, there are several places where the sensor should not be placed (i.e. it should not be hung close to the wall or behind something which hinders the air flow) and should not be hung too high in the house. Furthermore, the location of the air inlet and heating equipment is important in determining the best position for the temperature sensor. It is best to place it as close to the animals as possible and in such a way that the fresh air passes the sensor before it reaches the animals.

Recommended temperatures for layers and broilers

The critical temperature for layers is 20°C. For every 1°C lower than 20°C, the birds require an extra 1.5 g of feed per day. The most efficient temperatures for layers are between 20 – 24°C. When temperatures rise above 24°C, shell quality and egg weight will reduce. The critical temperature for broilers and rearing birds is highly dependant on age.

The recommended house temperatures for poultry are given in the following table.

Table 1. Recommended temperatures for broilers 

First day 32-34°C
1st week decrease 30°C
2nd week decrease 26°C
3rd week decrease 22°C
4th week decrease 20°C

Note: These temperatures are recommended temperatures and should be adapted to local situations as necessary.

Relative Humidity

The following concepts are used to measure the humidity of air in poultry houses:

  • Absolute humidity = grams of moisture present in 1 m3 of air.
  • Maximum humidity = maximum grams of moisture that can be present in 1 m3 of air at a given temperature.
  • Relative humidity = the relationship between the moisture content of the air and the maximum moisture content at the current air temperature expressed in percentages.

         Example of relative humidity %
If the air temperature is 10 °C and contains 5.7 g of moisture, the relative humidity is 5.7/9.5 x 100 = 60%. (See the table 3 on absolute moisture content in g/m3 of air for the moisture content in air with a temperature of 10°C). If the same air is heated without adding moisture until it reaches a temperature of 20°C, the relative humidity will be 5.7/17.5 x 100 = 33%. So it can be concluded that heating air results in lower relative humidity. Conversely, cooling the air will result in a higher relative humidity e.g. if the same air was 4°C the relative humidity would be 5.7/6.4 x 100 = 89%. This demonstrates that the warmer the air, the greater its capacity to contain moisture.

         Measuring humidity
Relative humidity in poultry houses is measured to determine whether respiratory disorders are due to too high or too low relative humidity. If the relative humidity is too high, condensation can accumulate in the house. This has a direct effect on the growth of micro-organisms.

Measuring and controlling humidity

There are several ways to measure the moisture content of the air in a poultry house, with the most common being the psychrometer dry/wet bulb or the mechanical hygrometer. Measuring the moisture content in the air may be useful, however there are higher relative costs involved in the measurement of the humidity compared to measurement of temperature alone. Due to this, the moisture content of air is not commonly measured.

Humidity is controlled by the intense heating or cooling of house air in response to the temperature outside the house. When outside temperatures are low, relative humidity in the house is low, which often results in dry dust circulating in the air within the house. If the relative humidity is too high, this may result in wet litter. The ideal relative humidity for poultry is 60-80%.

Air composition

The most important components of air are nitrogen (N2, approximately 79%) and oxygen (O2, 20.3%). In addition to these main components there are several other gasses such as carbon dioxide (CO2), and water (H2O). Birds inhale O2 and exhale CO2 and H2O. True ‘lack of oxygen’ does not occur in poultry houses because animals can inhale sufficient oxygen even if the oxygen levels in the air are substantially lower than normal. What is called ‘lack of oxygen’ in practice is, in reality, often a combination of high CO2 concentration, high temperatures and high humidity.

Harmful gasses in poultry houses are:

  • Carbon dioxide (CO2) – The carbon dioxide in poultry houses largely originates from air exhaled by the birds. The CO2 content of the air is used to measure the effectiveness of ventilation.
  • Ammonia (NH3) – Ammonia is a product of bacteriological processes in the manure. It is easily bound to water. Ammonia is lighter than air and thus it rises in the air. The ammonia content of the poultry house air depends on ventilation, temperature, relative humidity and stocking density. High ammonia concentrations irritate the mucous membranes.
  • Hydrogen sulphide (H2S) – H2S is released when organic matter (protein) in the manure decomposes. It has an offensive smell (rotten eggs) and is a very dangerous gas. When the manure is stirred or removed from the pit, the H2S is released into the air. Even low concentrations of hydrogen sulphide in the air can be fatal for humans and animals. This is why it is important to ventilate at maximum capacity while stirring or removing the manure.
  • Carbon monoxide (CO) – Carbon monoxide is an odourless, very dangerous gas. It is the result of incomplete combustion due to a lack of oxygen (O2) in gas heaters (clean filters).
  • Sulphur dioxide (SO2) – Sulphur dioxide develops when oil is used as fuel. The cleaner the oil, the less SO2 is formed. The Maximum gas concentrations allowed in European poultry houses are in the table below.

Table 2. Gas Standards for European poultry houses

CO 0
CO2 <2500 ppm = 0.25 VOL%
NH3 <25 ppm = 0.0025 VOL%
H2S 0.0
SO2 0.0

(1 volume % = 10,000 ppm)

Measuring gas content of air

A gas detector can be used to measure the gas content of the air. All measurements should be done at animal level. The device consists of a pump and its most important components are the tubes which are necessary to determine the gas content. The tubes are filled with a chemical substance that changes colour when air which contains the gas being measured passes through it. There are special tubes for determining the CO2, NH3, H2S, SO2 and CO contents of the air.

Measuring and controlling dust particles

Dust is harmful to the health of humans and animals and has a negative influence on the house climate. The functioning of equipment may also be seriously hampered by dust, including heating, lighting, and ventilation, and dust has also been shown to carry micro-organisms. The dust in poultry houses mainly consists of skin particles, feathers, feed particles, litter and dried manure.

The amount of dust in poultry houses is seldom measured. It can be measured in many different ways, however the processes are cumbersome and often require a multiple pieces of equipment as it is not known what is being carried in the dust each time the measurement is taken. It is currently difficult to give practical advice on how to measure the amount of dust, and what to measure for.

The amount of dust in a house depends on many different factors. These include temperature, relative humidity, type and age of the animals, type of litter used, feeding system, hygiene, etc. Proper maintenance of poultry houses and regular cleaning creates more comfortable conditions for animals and better working conditions for humans.

Air movement and airspeed

Whether or not birds are comfortable is very much influenced by air velocity and air temperature. Young animals are more sensitive to these factors than older, heavier animals. Taking into consideration the recommended temperatures, the air velocity at animal level is allowed to vary between 0.1 and 0.2 m/second. If house temperatures are low, the animals experience higher air velocities as a (severe) draft which can lead to disease. A simple way of determining the (negative) effect of drafts is the ‘draft value’.

The draft value is the temperature difference between the house air and the incoming air (in degrees Celsius) multiplied by the airspeed in m/sec ( D= (OT – IT) X S D = Draft value OT = Outside Temperature IT = Inside temperature S = airspeed at bird level in m/sec). The standard for the draft value is a value less than 0.8. If the draft value is more than 0.8 there is risk for drafts to occur in the poultry house. If temperatures are higher than 25-30°C, air velocities of higher than 0.1-0.2 m/second will actually have a positive effect and help to cool the animals.

The air movement pattern within a house is easier to control in this way as the influence of air velocity and outside temperature are less. It is not possible to give rules for the air movement pattern within a house because the air movement patterns depend on the ventilation within a house, the house width, the slope of the roof and the way the house is organised.

Measuring air velocity

Air velocity can be measured using an anemometer. The air movement pattern within a house can be made visible by using a smoke generator or smoke powder.


  • Original article by Jan Hulzebosch – Trainer, PTC, The Netherlands.
  • Effective heating systems for poultry houses – World Poultry, Vol.22, No.2, 2005.