It is often recommended to keep the relative humidity (RH) for day old chicks at a high level, 60-70% or even more. The reason for this advice is not always very clear. Sometimes the argument is used that it will be easier for the birds to lose heat, but also avoiding dehydration is mentioned. And often there is no argument given at all, it’s just stated as a fact, something beyond discussion.

When we look at the literature, there is no clear positive effect of a high relative humidity on the start of broilers, nor on the performance later on. Some effects are reported for older birds, but its not always clear what the cause is, as often more factors are changed: litter quality, ventilation rate etc. In some cases a comparison with human heat index factors is made, but as chicks do not sweat we should be careful with that.

The argument for a better loss of heat probably comes from the fact that moist air has a higher heat capacity: it takes more energy to warm up humid air than dry air, as not only the air but also the moisture in the air needs to be warmed up. This means that if we ventilate with a fixed amount of air, the same amount of heat will warm up the humid air a little bit less. But the humidity in the air does not affect the heat transfer from the body to the air. And if humid air would indeed cool better, warm and humid conditions during the final phase of broiler growing would work better than warm and dry conditions: most broiler farmers will probably not agree with that… However, if we use pad cooling in a broiler farm, we see that due to the evaporation of water the air cools down, and at the same time the relative humidity goes up. But that doesn’t mean that the relative humidity cools the birds, the evaporation of water does.

When birds are overheated, the start to pant to cool down. The panting let them evaporate water which has a cooling effect, but of course also results in dehydration as they lose water. The increase of relative humidity will prevent the evaporation of water and will reduce the dehydration, but it will also limit the ability of the bird to cool itself by evaporation. So it will not dehydrate but it will get overheated.

A possible negative effect of a low relative humidity for day old chicks might occur with spray vaccination. With a low relative humidity the droplets will reduce quickly in size and the vaccine might go deeper into the trachea then intended, resulting in a more severe immune reaction.

But what will actually happen when we try to keep the relative humidity for day old chicks at 60-70% or more? Let’s do some calculations on an imaginary situation.

When we place 40.000 day old chicks in a house, we have to ventilate for removal of the CO2 produced by the day old chicks and by the heaters. How much that ventilation will be will depend on a lot of different factors, but let’s assume we ventilate 1 m3 per kg of bird per hour. If our birds weigh 40 gram at placement, it means we have to ventilate 40.000 x 0.04 = 1600 kg = 1600 m3/hr.

When the outside air is 10oC and 50% RH, it contains 4.7 g of water per m3. If we warm up this air to 35oC, it will drop in relative humidity to 12%. Air of 35oC with 60% relative humidity contains 23.7 grams of water per m3, so if we want to have air of that humidity we need to add 19 grams of water per m3. As we are ventilating 1600 m3 per hour, we need to add 1600 x 19 = 30.4 liter of water to that air, every hour. Part of that water will be coming from the heaters, but the majority has to be added by for instance spraying. But evaporating 30 liters of water will cost energy, so the air will cool down. As a result the heaters have to work more, and will produce more CO2. To avoid too high levels of CO2 we will have to ventilate more, bringing in more cold and dry air. Which needs more spraying to keep the relative humidity at the desired level. So if we are not careful, we get into a vicious circle of more spraying to increase humidity, more heating to evaporate the water, more ventilating to remove the CO2 from the heaters, and more spraying to compensate for the extra ventilation.

Trying to reach a high relative humidity should be done carefully. The positive effect on the birds is questionable, but it might influence the climate in the house, and not necessarily in a favorable way.