Do you need an airlock for brewing beer?

One of the most common pictures we see in homebrewing is one with a vessel (usually a glass carboy) and beer fermenting inside of it with some sort of plastic device on top that generally is bubbling.

That unique piece of equipment on top is called an airlock and is going to be the subject of today’s post.

I’m going to be answering what an airlock is and how it works, what fermentation is, if you need to use an airlock during primary & secondary fermentation, if you can ferment without the use of an airlock, its pros and cons, and much more.

So, without any further ado, let’s get started.

Two fermentations with their airlock

What is an airlock and what is it used for?

An airlock is a piece of equipment  that is used on top of the fermentation vessel. It allows the CO2, generated during fermentation, to leave the fermentor while preventing oxygen and any other particles (even insects) from getting  in. 

They are usually made out of plastic and come in different shapes. The most common one is the one that is shaped like an “s”, while another common alternative are the 2-piece airlocks. 

It doesn’t matter which one you are using because, in the end, the concept is the same; a little bit of liquid trapped inside the airlock that is “pushed” by the CO2 pressure inside the fermentor which, in turn, allows all of the CO2 to leave the fermentor and, at the same time, prevents any oxygen and any other foreign particles from getting inside the fermentation vessel.

Fermentation and how it works

The last step of the brew day is to cool the wort down, transfer to a fermentation vessel and pitch the yeast. After a few hours, yeast will start to eat the sugars in the wort and generate alcohol and CO2. This process is called fermentation. 

During fermentation, the wort, commonly referred to as green beer, will go from a sweet syrup to an alcoholic beverage similar to beer. We still need to give this alcoholic beverage some time to turn into actual good beer and also to allow to carbonate, which will happen during the lagering, or conditioning, process.

Now to the question at hand…

Do you need an Airlock for brewing beer?

You don’t need an airlock for brewing beer, although it is highly recommended to avoid any possible risk of contamination and/or oxidation. The CO2 from the fermentation and the krausen (the foam that forms on top of the wort during fermentation) will act as a natural protection barrier, but the airlock ensures that no strange particle or oxygen makes it into the fermentor.

For many years, beer used to be fermented in open vessels with no lid, and they even used to have slides where the yeast was harvested from the top and then used for other brews. Nowadays, some styles, like hefeweizens, are traditionally brewed in open fermentors. Of course, in sealed rooms with good ventilation and air filtration to avoid any contamination risks. But that’s not our case. 

So, as homebrewers, an airlock is the way to go to prevent any undesired results in our brews.

Do you need an airlock during primary fermentation?

For homebrewers, an airlock is recommended through every single step of the fermentation. If there is a stage during fermentation where an airlock could not be used (again, not recommended) that would be the primary fermentation. The high krausen and the constant CO2 production will protect the young beer from most threats. 

But keep in mind that by the end of the fermentation, the krausen will drop down, leaving the beer exposed to basically everything.

What about secondary fermentation, is an airlock necessary?

After main fermentation is over, an airlock is a must. There is no Krausen to protect the beer and the production of CO2 is low. At this point, without an airlock, the risk of contamination and/or oxidation are high.

Carboy in a secondary fermentation

Can you ferment without an airlock?

Fermenting beer without an airlock is definitely possible. You just have to be sure that you protect your beer after the main fermentation is over. The primary risk is the air itself, since it could be carrying any sorts of wild yeast, bacteria, small insects, pollen, you name it. 

That’s why if you are fermenting without an airlock, try to keep the air inside the room as clean as possible. That means; not leaving the windows open (especially in Spring and Autumn), don’t look inside the fermentor (ever), keep pets away, etc.

There are a few airlock alternatives that will be mentioned below, and for advanced homebrewers, if you ferment under pressure, an airlock is not needed since you will need a spunding valve instead.

Does fermentation need to be airtight?

The only moment when oxygen is recommended in fermentation is right after pitching the yeast. Once fermentation has started, all contact with oxygen should be avoided. 

So basically yes, fermentation should be airtight. Once fermentation has started, oxygen is beer’s #1 enemy. It could carry all sorts of possible contaminants, and it will oxidize the beer when it comes into contact.

Any processes that take place after primary fermentation should be performed as free of oxygen as possible.

Problems you might run into when not using an airlock

As said before, you don’t need an airlock during primary fermentation, but it’s highly recommended. Why? Because if you are not using an airlock that means you are fermenting in an open vessel, and that could lead to contamination and anything that’s in the air could end up inside the fermentor (wild yeast, bacteria, dust, pollen, insects, etc.).

Krausen from the fermentation will act as a barrier, but it will fall down after a few days, leaving beer susceptible to contamination. 

How to use an airlock

Airlocks are really easy to use: You need to stick it in the hole that’s in the fermentor lid and fill it with water or some sanitizer like alcohol or StarSan. There should be some marks on the Airlock telling you how much liquid you should put in it.

Then you just leave it in for the whole fermentation process. You can shortly take it out and wash it, if necessary. But if at all possible, leave it on the fermentor.

Different types of airlocks

One and 2 piece airlocks

There are basically two types of airlock: A one piece airlock, shaped like an “S” and a two piece airlock, which is a small cylinder with a pipe in the middle and a cap that allows the CO2 to get out.

Both models are really efficient and inexpensive, so you can choose whichever you like the best.

Airlock alternatives?

If your airlock is broken, went missing, or you don’t have one, don’t panic. There are still some alternatives that will help you get through the fermentation process without any risks.

  • Blow Off tube: a hose that goes from the lid of your fermentor to a glass or a jar full of water or sanitizer. The idea behind this method is the same as an airlock. CO2 will build some pressure and escape through the water while keeping oxygen and any particles away from your beer.
  • Rubber balloon: Yes, you read that right, a birthday balloon is a great solution when we don’t have an airlock laying around. Just place it on top of your carboy or fermentation vessel and poke a few holes into it. The balloon will inflate a little letting CO2  escape while preventing anything from getting in. The risk attached to this method is that by the end of the fermentation the CO2 amount leaving the fermentor will be too low to keep any oxygen out of it.
  • Loose lid method: This one is pretty much self explanatory. Just leave the lid a little bit loose. This way CO2 goes out preventing anything from getting in. Risks are the same as the ones we encounter with the balloon method. 
Blow off tube

The most recommended method out of these three is the Blow off tube. It’s simple and really effective. It is an airlock but bigger, and it helps if the fermentation is too vigorous and oevrflows.

Common problems you might run into

Main Fermentation going wild.
Airlock overflowing
  • Airlock not bubbling: either the fermentation stopped (it finished or you encounter some other problems) or the fermentor is not properly sealed. Check for some leaking CO2 around the lid. 
  • Airlock overflowing: The fermentation vessel was too full and the Krausen is escaping through the airlock. Just take the airlock out, wash it, sanitize it and put it back on. You may have to do this a few times, but the Krausen will fall off eventually (the blow-off tube method can come in handy here).
  • Airlock losing water: This can either be caused by a a really vigorous fermentation that is pushing the liquid out (not really common) or your airlock is broken. To solve this, fill it with water and try to see what’s going on. If the airlock is broken, change it, or use one of the alternative methods stated above. 
  • Airlock going backwards: This can happen either at the beginning of the fermentation or while cooling down the beer for lagering. At the beginning of the fermentation, the yeast will consume oxygen and this will create a vacuum effect, that’s why you see the airlock going backwards. By cooling or cold crashing the beer you will see a similar effect. The liquid slowly contracts while cooling and will take up less space than at room temperature, which also creates a vacuum effect, making the airlock go backwards.


The best way to protect the beer while fermenting is to use an airlock (blow off tube will work as well). Using an airlock will allow you to relax and take your measuring samples without having to worry about your beer getting contaminated.

It is one of the simplest, most reliable and cheapest pieces of equipment that you can buy. So, get a couple of them and use them, and don’t worry about what model you get since they all work great.

Have a good one!



  • Is an airlock required for lagering or conditioning? Airlocks should always be used for lagering and conditioning because once the Krausen has disappeared, there’s little to none CO2 leaving the fermentor, which means that at this point beer could be really susceptible to contamination. 
  • Should I keep the airlock cap on or off when brewing? Most airlock caps come with little holes in them to allow CO2 to escape. If that’s the case with your airlock, you can leave the cap on. If not you can just take it out.
  • Should I fill the airlock with water? You could definitely fill the airlock with water, but it is better if you use some sanitizer like StarSan or alcohol for sanitizing purposes, otherwise you run the risk of contaminating the beer. 
  • When should I place the airlock in the fermentation vessel? At the beginning of the fermentation process. Once you finish your brew day, the beer is cooled down and the yeast is pitched. Just close the lid and put in the airlock since there’s enough oxygen inside the fermentor for the yeast to consume, therefore you lower the risk of contamination by putting the airlock in right away.
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