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 when incubating -- temperature, ventilation, turning, and humidity. Out of the four humidity is the most difficult factor to control and sometimes measure, which is why it is commonly misunderstood. If you’ve noticed the conflicting information on the internet about humidity, you’ve come to the right place. The Incubation Specialists at Brinsea are here to answer your questions! 

Check the Basics

Our incubators that feature automatic humidity pumps are designed not to pump if the temperature is too low. So if you’ve just plugged your incubator in and can’t figure out why the pump isn’t working, check to see if it’s reached 99.5°F. If it hasn’t, give it some time to get up to temp before you give us a call. 

What Does Humidity Do? 

Egg shells may look solid, but they are actually porous. You can test whether a supermarket egg is good to eat by floating it in water. Fresher eggs will sink because they weigh more, while old eggs will float because they have much more empty space in them. This is also true of eggs you are trying to incubate, although we don’t recommend a float test for those eggs. 

While eggs are incubating they will naturally lose weight. It is important to have your humidity set to the right percentage so that your eggs are losing an ideal amount of weight. If the humidity is kept at the right level, developing chicks will be able to 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 what is ideal. A large air space also means the chick will be smaller than normal. Small chicks are weak chicks, and weak chicks cannot always hatch on their own, and they may die just before or just after hatching. 

It should be noted, however, that low humidity is typically less of a problem than high humidity, which we will cover next.

If Humidity is Too High

If low humidity equals too much weight loss, then high humidity equals not enough weight loss. In this case the air space will be smaller than normal, and the chick will be larger. A large chick may be a strong chick, but a small air space can affect the respiration and cause problems that way. This also causes a lack of space, making it difficult for the chick to move around and break out of the shell.

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

How Do You Measure Humidity?

Now that you know the dangers of incorrect humidity, let’s dig into how to measure it. We want to mention that unlike temperature, humidity is not a strict variable. If the temperature fluctuates even a small amount it can spell disaster, but humidity is more of an average variable. High humidity at the beginning of incubation can be corrected later on with lower humidity and vice versa. 

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, and we explain both methods below. 

Relative Humidity or RH%

When you see the RH% on our incubators, for example, you are seeing the measurement of water vapor in the air compared with the maximum that could be absorbed at that temperature. Let’s put emphasis on the fact that RH% is based on the temperature of the air. 50% humidity at 70°F is different from 50% humidity at 90°F. 

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. When you’re checking your incubator humidity, therefore, it is very important to also note the temperature. It is also important to take the temp into consideration when checking your ambient humidity.

Wet Bulb Temperature

This is a different way to measure the humidity, and it is done 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, which is similar to what happens when you step out of the shower into a cool bathroom. 

The Wet Bulb technique can be used as a cheap way to measure the RH%, which is done by taking the difference between the Wet Bulb and Dry Bulb temperatures. 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. 

How to Achieve the Correct Humidity Levels

So what happens if you don’t have a hygrometer, or you aren’t sure if your hygrometer is accurate? As we mentioned above, humidity that is too high or too low will affect the weight loss of the egg during incubation. So, in theory, you could simply weigh the eggs periodically to check on their progress? If you’re hatching in an incubator without a reliable hygrometer then that is exactly what we recommend doing. 

Most bird species (with the exception of the ostrich family) will need to lose between 13 and 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. 

If you’re good at math it may be fairly simple for you, but for those of you that aren’t it may be helpful to draw a graph (see below) to keep track. The incubation days will be our x-axis going across, and the weight will be the y-axis going up. You can plot out the ideal weight loss from day 0 through to hatching, which we show here in dark blue. Then every few days when you weigh the eggs, mark their weight along the graph and note where it falls on the blue line. We’ve done that here with the pink dots.