If you’ve ever incubated before (or even if you haven’t), you probably know that there are a lot of variables you need to keep track of. There are four primary factors to consider: temperature, ventilation, turning and humidity. The Incubation Specialists at Brinsea are here to explain the possibly the most difficult factor to control and measure: humidity..

What Does Humidity Do?

Eggshells may look solid, but they’re actually porous. While eggs are incubating, they naturally lose weight. It’s important to have your humidity set to the right percentage so that your eggs are losing an ideal amount of weight so that developing chicks can use the available air space to breathe and move around.

If Humidity is Too Low

Low humidity will cause the eggs to lose too much weight, which means the air space will be larger than ideal. A large air space means the chick will be smaller and weaker and might not survive. However, low humidity is typically less of a problem than high humidity.

If Humidity is Too High

High humidity results in not enough weight loss. The air space will be smaller than ideal, and the chick will be larger. A small air space can lead to respiratory problems and make it difficult for the chick to move around and break out of the shell.

If a chick pips in a shell that hasn’t lost enough weight, they can die due to weakness from lack of air or because they can’t maneuver to break out the rest of the way.

Measuring Humidity

Humidity is calculated by measuring the water vapor in the air. One of the easiest ways to measure the water vapor is by figuring out the Relative Humidity percentage, also known as RH%. The other way is with a wet bulb.

Note, humidity isn’t a strict variable; it’s more of an average variable. High humidity at the beginning of incubation can be corrected later with lower humidity and vice versa.

Relative Humidity

When you see the RH% on our incubators, you’re seeing the measurement of water vapor in the air compared with the maximum that could be absorbed at that temperature. The RH% is based on the temperature of the air, so 50% humidity at 70 degrees Fahrenheit is different from 50% humidity at 90 degrees.

Maximum possible water vapor capacity increases as the temperature increases, so raising the temperature in an incubator without adding water will cause the RH% to drop. Therefore, it’s important to note the temperature when measuring humidity.

Wet Bulb Temperature

You can measure humidity with the wet bulb (WB) technique by checking the temperature of a thermometer with a moist cotton wick around its bulb. As the water from the wick evaporates, it cools the bulb.

The WB technique takes the difference between the WB and dry bulb (DB) temperatures to determine humidity. There are only two instances where the WB and the DB temperatures would be the same— when the air has absorbed all the water it can (100% RH), and when the wet wick has dried out. It should be noted that it is very difficult to measure the WB in a still air incubator.

Also WB temperature should not be confused with % RH – 90 degrees WB temperature is 45% RH not 90% RH.

How to Achieve Correct Humidity Levels

What can you do if you don’t have a  hygrometer, or you aren’t sure if your hygrometer is accurate? If you’re hatching in an incubator without a reliable hygrometer, we recommend periodically weighing your eggs to check on their progress.

Most bird species (except for the ostrich family) need to lose between 13% to 15% of their weight from the first day of incubation to the day they hatch. By weighing the eggs every few days, you can accurately adjust the humidity to compensate for too much or too little weight loss.

Altering the Humidity During Incubation

Water surface area and fresh air are the two controllable factors to consider when altering the humidity during incubation. The more water surface area there is inside the incubator, the higher the humidity will be.

All Brinsea incubators have two water pots or channels to give you flexibility in water surface area and evaporation rates. To increase the humidity, all you need to do is put water in both channels and reduce the ventilation. Never block off all the holes in the incubator as the chicks require oxygen to breathe.

Evaporating pads or blocks are available and can help raise the humidity if you’ve put water in both water channels and still can’t achieve the correct humidity level.

The Brinsea EX units come with a built-in hygrometer and humidity pump to automatically control humidity. Similar to temperature control, the humidity level can be adjusted on the incubator and the pump takes care of adding water as necessary.

Ambient Humidity

Ambient humidity will influence the humidity inside the incubator. Factors that can cause the humidity in the incubator to fluctuate include whether there is a humidifier or dehumidifier in the room, if you’re running the heater, turned the A/C off and opened the windows, and others.

Should I Spray My Eggs with Water?

Spraying eggs with water only raises the humidity for a short time before the small water droplets on the eggs evaporate and is therefore not an effective solution to low humidity. It’s also not recommended due to the fact that the water could be at much lower temperature than the eggs, which can cause issues as well.

Humidity During Hatching

For virtually all birds, humidity needs to be higher at hatching than during incubation. We recommend raising the humidity during “lockdown” or the last few days of incubation. If you have been weighing your eggs, the weight loss should be right around 13% to 15%, and raising the humidity at the end won’t significantly affect this.

High humidity is necessary because of the membrane that the chicks must break through to hatch. If the membrane is allowed to dry out (due to low humidity), then it becomes too tough for the chicks to tear. They’re then unable to hatch.

During hatching, the humidity should be at least 60% RH. To keep the humidity stable, always keep the lid on the incubator. If the lid is lifted after a chick has hatched, the humidity will immediately drop which could cause other chicks to become shrink wrapped.