When I started this post, I thought about calling it something like 17 ways to get your compost hot.
But in reality, when you build a compost heap, you (mostly) don’t provide the heat. Instead, you provide the conditions for heat to be generated.
(I say mostly because you can help kick-start compost bins through hacks like using a hot water bottle.)
As long as the conditions are right, mesophilic compost bacteria become active. As they do so, they generate heat. The heat creates the right conditions for thermophilic bacteria, which generate more heat again.
This leads to faster heating and helps with pathogen elimination.
However, heat in compost can easily be lost, especially from the outer layers.
That means the composting process takes place more effectively in the core of the pile, and more slowly in the outer layers.
As the diagram above shows, one way to help this process along is by providing insulation.
The insulation traps some of the heat, leading to more even decomposition.
Let’s take a look at some of the ways we can provide that insulation.
Build it large
The larger the compost heap, the more insulation the outer layer will provide for the interior.
If you want to generate heat without using any other form of insulation, I’d recommend a minimum height, width and depth of 3 feet. Even then, the compost will only truly get hot at the center of the pile.
You don’t have to build this all at once in a couple of days – I’ve found it still gets hot even when built over a few weeks.
Of course, it will get going faster if you put it all together in one go, but I think that’s probably unrealistic for the average gardener.
Still, you don’t want it TOO big, or the compost won’t draw enough air in. Cornell University recommends limiting the width to 10-15 feet and the height to 6-8 feet, which is easily done for most gardeners!
Combine compost heaps
If you have more than two compost heaps that are roughly at the same stage of decomposition, you can combine them together to create more insulation.
My father did this recently. He has a plot on a community allotment, with a shared compost heap. The compost heaps were not getting hot, so he pulled them apart, mixed them up and put them back together.
After he did so, they started to generate heat. This will partly be because of mixing air into the heap again, but the increased size will also help insulate that heat.
If you are constructing your heaps from scratch, I would always recommend building one large compost heap instead of two small ones.
However, if you do have two smaller heaps that are not generating heat, combining them is a good solution.
Remember, it’s not all about size
As we’ve seen, a compost heap needs to be at least 3 feet by 3 feet. Even that might not be enough to get your compost to thermophilic temperatures.
However, before you give up hope because you don’t have enough organic material in your garden, it’s worth remembering that the reason for the larger size is primarily for insulation.
That’s why, when you turn a compost heap, you:
- Turn the inside to the outside, as the core should be more decomposed.
- Turn the outside to the inside, as the outer layer is less decomposed.
However, once we understand that the size is important because of insulation, we can look at other ways to insulate compost.
Using loose straw
One method I have used in the past, and which I picked up from the Humanure Handbook, is straw.
Place a base of straw at the bottom, and build it up around the sides. Place your compost on top of the straw, and cover it with another layer.
Each time you add organic material, simply draw the side apart, add your compost material and cover it with straw again.
(If you have smellier material, dig a hole in the compost too, place your material there, and cover it with both compost and straw.)
Straw acts as a biofilter, helping to trap any unpleasant smells. It’s also pretty effective at trapping oxygen inside.
You do have to buy the straw, and I found I needed two bales to complete a single pallet bin.
However, as it’s on the outside of the bin it doesn’t decompose as quickly as the inside of the compost, and you can use at least some of it on your next pile.
Still, I am fortunate enough to live in a farming area, and I can pick up bales very cheaply. If you’re gardening in a city, using straw might not be as feasible for you.
Do note that because of the amount of space straw takes up, it does reduce the amount of material you can add to a bin. Whether that matters to you will obviously depend on how much material you need to process.
Using straw bales
Another option is to build your compost heap out of straw bales.
While more expensive than using loose straw, this provides fantastic insulation – and removes the need for a compost bin. It also helps to provide a windbreak, which reduces moisture loss.
As straw bales decompose over time, they will eventually need replacing. This is both a pro (more organic matter for your garden) and a con (more work and expense!)
Another method I use is placing a piece of old carpet on my newest pile.
Obviously, this only insulates the top of the compost, so it’s not as effective as straw.
One thing that has been on my mind recently is whether carpet can leach any toxic chemicals. I’ve seen mixed reports on this, and it seems to be a controversial area.
Just for safety, though, I also cover the top of my compost with some leftover plastic, so if anything does leach it will run over the plastic and into the ground around my compost. This also helps keep excess rain out of the compost.
You can also put another layer of plastic above the carpet to keep that dry too and help it maintain its insulating properties.
Top tip: Place sticks between the top of your compost and any insulating cover to trap air inside.
Using an insulated bin
Most bins will provide some level of insulation, but the best, in my opinion, are insulated bins.
These are designed to keep in warmth, and usually come with air vents to ensure that air can still access the compost.
If you get other factors right, it’s astonishing how effective they can be, and I’ve achieved hot composting with these bins in the middle of winter.
There are many options here – I currently use a HotBin, which uses a 50mm expanded polypropylene board to provide the same amount of insulation as 750mm of compost.
This is not always available outside the UK, but a similar alternative is the Squeeze Master Thermal Compost Bin.
Also see our guide to the different kinds of compost bins available, and the factors you need to take into account when choosing one.
Line your bin with cardboard
When I haven’t got straw, I sometimes line the side of my pallet bin with cardboard.
Cardboard has a low thermal conductivity, which means it doesn’t conduct heat easily.
It also has the advantage of keeping materials like grass and sawdust neatly in the pallet bins, instead of spilling out everywhere like they usually do.
Corrugated cardboard works even better, due to its double lining and the trapped air instead. You could also double-line your bin with cardboard to increase the insulating effect.
As mentioned elsewhere, this works best when you keep the cardboard dry.
Using a compost jacket
Another option is to use a compost jacket that goes around your compost bin to keep it warm.
The only bin I have seen that currently provides this option is the Green Johanna.
Mark from Great Green Systems pointed me to an experiment where using the jacket increased the temperature of the compost from 86°F – 140°F / 30°C to 60°C – even in winter.
Above: The Green Johanna in Mark’s garden.
You can also make your own jacket. In Rapid Organic Compost, Ken Powers describes how he first wraps a dalek bin in a fitted radiator reflector and then covers that with a black sack containing 8-inch fiberglass.
If you choose to use this method, do ensure you don’t block any air vents.
Use solid walls for your compost bin
Another alternative is to build solid walls. Some people choose to build solid wooden walls, while I have also seen concrete walls used in stables.
There is a trade-off here. Compost heaps draw some air in from the sides of the compost heap, and you’ll be blocking this off by using a solid wall.
However, as long as you use plenty of bulking agents you should be okay. I’ve certainly seen people achieve excellent compost when using solid walls.
Put a bin in a greenhouse or polytunnel
If you have a large greenhouse or a polytunnel, you can also put a bin in it.
This is surprisingly effective in spring and autumn, but you do have to be careful in summer as it can get too hot.
Compost bins can also provide heat for greenhouses. Some gardeners have used a pallet bin to heat a greenhouse over winter, while others use it to generate enough heat to germinate seeds in late winter/early spring.
This is less effective in polytunnels, which rapidly lose heat when the sun is out.
Use a lid
Using a lid is not just about insulation – it’s also about moisture control, and it ties in with other factors mentioned here.
As you’ll know from times when your clothes have got wet, when an insulating material gets wet it loses some of its effectiveness.
Instead of keeping all the heat in, the moisture helps conduct the warmth out of the compost heap.
Simply by keeping the rain off, a lid can make the insulation you are using more effective while adding some insulation itself.
However, you do want to ensure there is an air pocket between the lid and the compost.
The main con here, at least for homemade bins, is the added time and expense it takes to add a lid.
You can throw a pallet together in less than an hour if you use twine or wire instead of brackets.
However, to make a compost bin with a lid you really need solid sides, which is a bigger and more expensive job.
Still, it’s an option I am considering to replace my pallet bins when my current ones finish falling apart!
Use an insulated compost tumbler
Compost tumblers are primarily designed to make aerating the compost easier. However, some compost tumblers also come with superb insulation, which helps keep the content warm.
The Jora Compost tumbler is perhaps one of the best examples of this, as it uses heavy-duty insulation to allow the temperature to rise to up to 160°F/ 71°C.
(That’s actually a little too hot – I’d recommend trying to keep your compost below 149°F / 65°C.)
They also keep the contents off the ground, which helps prevent cold in the soil from drawing out moisture in the winter.
Five More Ideas
Want more ideas? Here are 5 more suggestions:
- Use a layer of compost or sawdust over your compost (this also acts as a biofilter to trap odors)
- Add insulated foam, bubble wrap or styrofoam to the inside of your bin (bearing in mind this will reduce access to air)
- Use a layer of leaves (you can usually re-use these at least once, as they are slow to break down due to their high lignin content)
- Build a complete shelter around your compost bin
- Move a bin into a garage or shed
You can also bury the bottom of the bin into the ground, but bear in mind this will restrict airflow.
Other factors that keep your compost hot
It would be a mistake to think that insulation is the only factor keeping your compost bin hot!
Here are some other factors to keep in mind:
- Size of the compost materials: Heaps with smaller materials get hotter fast.
- The mix of high carbon and high nitrogen materials: See C: N Ratio.
- Air: See 11 Ways to Aerate Your Compost.
- Moisture levels: See Moisture In Compost.
- Where you put your compost bin – shade will cool it down, sun will heat it up. See Siting Your Compost bin.
It’s also a great idea to monitor compost temperatures, as this can help you take action if your compost is not hot enough, or indeed if it gets too hot!
One of the problems with some guides online is they make the process of composting seem very difficult.
For example, some guides tell you to turn the compost every other day, precisely calculate the carbon-to-nitrogen ratio, build your heap in two days, and build a huge heap to maintain warmth.
While these are all ways to get superb compost, the reality is that most people don’t have the time, garden size, or inclination to do this.
In fact, by making the demands so high these guides are likely to put people off more than encourage them.
So if there’s one takeaway from this article, it’s that there are many ways to provide insulation. Pick the ones that work for you – and don’t be afraid to experiment!
It depends how precise you want to be! One way is to insert a stick into the compost, then check it with your hand to be sure it is getting hot. The more precise way is to use a compost thermometer guide. Here’s a guide to using compost thermometers.
If your compost is insulated, you may not need to turn it as often as an uninsulated compost pile. A lot depends on the oxygen and moistures level in the compost heap. Try to use bulking materials in the compost heap to ensure you create air pockets.