Why does compost steam? ( Is it dangerous )

  • By: composthq
  • Date: November 5, 2022
  • Time to read: 7 min.


Have you ever wondered why compost often seems to steam? If you’ve recently built a compost heap and you’ve seen clouds of white droplets rolling off it, you might be panicking, and wondering whether you’ve done something wrong, or if this is dangerous. It can be a pretty concerning sight, especially with a large compost heap.

Compost heaps steam because an enormous amount of heat is generated as the organic waste inside them breaks down. The compost is generally also wet, since food scraps are wet, and as this water gets heated up, it turns into steam. This is normal, but it is something you should pay attention to, as a really hot heap can be a problem.

In this article, we’ll explore why compost tends to steam and what you should do if your compost heap is steaming. We’ll also look at how to prevent a steaming compost heap, and whether your compost is likely to catch fire. Armed with this information, you’ll be a composting pro in no time!

Why Does Compost Steam?

Compost heaps steam because they are both wet and hot. As the moisture inside the heap gets heated, it turns into vapor, just like the steam from your kettle. This vapor works its way to the top of the heap (since steam rises) and spills out into the surrounding air, causing the heap to steam.

This is fairly normal, but let’s spend a bit of time figuring out why it happens. For starters, why does compost get hot?

When your compost is broken down, it is being digested by microbes. These feed on the waste in your compost bin to sustain themselves, and in doing so, they “burn” it using a process called oxidization. This process is what makes food scraps and garden clippings into rich compost that can be reused, but it also produces heat.

A compost heap is also a wet place. It contains things like food (which is full of water), grass clippings, and other greenery, and these may be combined with rain and ambient moisture if you have an open heap. It might surprise you how much moisture is in the average compost heap, and this is needed for it to function healthily.

When the heat interacts with the moisture, it will turn it into water vapor, which is why your compost heap will sometimes steam, especially if the sun falls on it and adds to the heat already being generated by the microbes.

Is Steam A Good Sign?

This depends on a few different factors. First, let’s clarify that heat often is a good thing in your compost heap. Many microbes like a warm environment because this helps them to break down waste and reproduce more quickly. You’ll end up with more microbes, working faster, and generating more heat.

That means you’ll get compost much more quickly – which is what most gardeners want. Even tough items should disappear fairly fast in a hot compost heap. However, there is a limit to this, because too much heat will kill the microbes you need, and could even set the heap on fire. Obviously, that’s dangerous and not something that you want to happen.

Compost fires are relatively rare in little backyard compost heaps, as they aren’t large enough to generate the sort of heat that will start a fire. They tend to happen in commercial settings. However, it isn’t unheard of for home compost heaps to catch fire in the height of summer, especially in hot parts of the world.

Even if your compost heap doesn’t catch fire, you don’t want it to get too hot, because if all the microbes die, it will stop composting. It may eventually restart again, because the heap will cool down once the microbial activity ceases, but this will still slow everything down and should be avoided if possible.

What Causes Fires?

In general, for a compost heap to catch fire, it must have a lot of dry material (think straw, sticks, egg cartons, dead leaves, etc.) and it must also be turned regularly. Turning your compost heap introduces oxygen, which massively increases the activity of the microbes in the pile.

Again, this is generally a good thing, but only up to a point. If you keep introducing oxygen and your compost heap contains dry, combustible materials, it could be dangerous. This is particularly true if it is near a wooden fence, a shed, or any other building that a fire could spread to.

Secondly, adding a lot of nitrogen can make a compost heap heat up very fast. Nitrogen refers to the “green” ingredients of your compost. Most food scraps count as nitrogen, as do things like grass and green clippings from the garden. Think before you dump the contents of your lawnmower basket on your compost heap, especially if the weather is hot!

How Hot Should A Compost Heap Be?

Most healthy, active compost heaps will be around 104 to 122 degrees F, although this can vary depending on your location and the things you add to the heap. They can reach this sort of temperature surprisingly quickly, within just a few days, especially if you are giving them plenty of oxygen.

However, heaps can be considerably colder or hotter and still functioning. The types of bacteria you find will depend on the temperature.

Between 50 and 115 degrees F, your compost heap will support a kind of bacteria known as mesophilic. These break compost down at a good rate, and may be able to break some foods and garden clippings down in just a few months. However, you can safely get compost hotter than this.

“Hot composting” is a term for composting done between 115 and 160 degrees F. This temperature range is hospitable to thermophilic bacteria, and they will break the compost down significantly more quickly. Hot composting is ideal if you need to get rid of waste fast, or if you want compost that’s ready to use on your garden (though you will still have to wait a few weeks).

At colder temperatures, you’ll probably have psychrophiles in the compost heap. These also break down waste, but considerably more slowly. They can operate in temperatures as low as 28 degrees F, but they prefer higher temperatures of about 55 degrees F. If it gets any hotter, you’ll start getting mesophilic bacteria.

You don’t want your compost to rise above 160 degrees F, or there is a risk that the bacteria will be wiped out entirely, and you’ll have to wait for them to re-form before the composting process can continue.

It’s worth noting that the temperatures given here are estimates and you might see slightly different ranges in other places. Composting isn’t an exact science, and there are a lot of variables that will affect your heap.

Are High Temperatures Good?

In general, a high temperature in your compost heap is good. As mentioned above, this speeds up the composting process, and it has the secondary benefit of killing pathogens and weed seeds. That means the compost is semi-sterile when you come to use it, which is certainly a big benefit, especially if you have a lot of weeds in your garden.

However, you should take action if you think your compost heap is getting too hot, both for the sake of your microbes and to reduce the risk of a fire starting. There are several things you can do to bring the temperature down.

The first is to take off the lid or remove any cover so that heat can escape. Next, stop adding new green ingredients, and instead add some browns. You should stop turning the compost heap regularly, but you may want to lift the top off the pile so that heat can escape.

If you’re really struggling to lower the temperature, watering the compost heap will help. The moisture will quickly cool it, and will also minimize the risk of fire. However, you do need to thoroughly soak the heap for this to work. A light sprinkle will not solve the problem.

Few people have problems with their compost piles getting too hot, but if you have an active heap that you turn frequently, especially in a hot climate, you might run into this issue. Use a thermometer to check the temperature regularly, as this is the best way to tell.

Steam on its own does not mean that your compost heap is getting too hot, so don’t worry if you see water vapor rising from the heap, especially on a cold morning. Check the temperature if you’re concerned, but this is generally fine.


Compost heaps steam because they are both hot and moist. The heat turns the moisture into vapor, and the vapor rises from the pile and escapes into the air. This is normal and not dangerous; it shows your compost heap is functioning well. However, you should keep an eye on your compost’s temperature, especially during the hotter months, to check it’s not getting out of control.

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