Humidity and Ventilation

The relative humidity of the air within an incubator for the first 18 days should be able 60 percent. During the last 3 days (the hatching period) the relative humidity should be nearer 65-70 percent. Too much moisture in the incubator prevents normal evaporation and results in a decreased hatch, but excessive moisture is seldom a problem in small incubators. Too little moisture results in excessive evaporation, causing chicks to stick to the shell sometimes and hatch crippled at hatching time.

Table 3 (Relative Humidity) will enable you to calculate relative humidity using readings from a wet-bulb thermometer and the incubator thermometer.

During the hatching period, using an atomizer to spray a small amount of water into the ventilating holes may increase the humidity in the incubator. (This is especially helpful when duck or goose eggs are being hatched.)

An 8-inch pie tin or petri dish containing water and placed under the tray of eggs should provide adequate moisture. The relative humidity in the incubator can also be varied by changing the size of the water pan or by putting a sponge in the pan to increase the evaporating surface. The pan should be checked regularly while the incubator is in use to be sure that there is always an adequate amount of water.

Whenever you add water to an incubator, it should be about the same temperature as the incubator so you do not stress the eggs or the incubator. A good test is to add water just warm to the touch.

In the latter stages of incubation (from the 19th day on), condensation on the glass indicates the presence of sufficient moisture. However, the condensation is also related to the temperature of the room where the incubator is being operated. There will be more condensation on the glass if the room is cold, so be sure the temperature in the incubator remains steady.

Using a wet-bulb thermometer is a good learning experience for determining relative humidity. The wet-bulb thermometer measures the evaporative cooling event. If the wet and dry bulb read the same temperature, you would have 100 percent humidity. The greater the evaporation taking place, the lower the temperature reading on the wet-bulb thermometer and the larger the spread will be between the wet- and dry-bulb reading.

To make a wet-bulb thermometer, just add a cotton wick to the end of a thermometer. Then place the tail of the wick in water. The cotton then absorbs the water. As the water evaporates from the cotton it causes a cooling effect on the thermometer.

It is also possible to determine whether there is too much or too little humidity in the incubator by candling the eggs and observing the size of the air cells.

Table 3 Relative Humidity

100°F 81.3 83.3 85.3 87.3 89.0 90.7
101°F 82.2 84.2 86.2 88.2 90.0 91.7
102°F 83.0 85.0 87.0 89.0 91.0 92.7
Percent Relative Humidity 45% 50% 55% 60% 65% 70%

Incubator Temperature Wet Bulb Readings


The best hatching results are obtained with normal atmospheric air, which usually contains 21 percent oxygen. It is difficult to provide too much oxygen, but a deficiency is possible. Make sure that the ventilation holes are open to allow a normal exchange of air.

This is critical on homemade incubators. It is possible to suffocate the eggs and chicks in air-tight container