Have you got an insulated compost bin?
How to challenge the marketing waffle and avoid buying a duff ‘hot composter’
This post is one of a series that help explain hot composting and how to choose a hot composting bin that performs in a domestic environment.
We are seeing an increasing number of articles on hot composting and more and more compost bin suppliers describing their compost bin as ‘insulated’ or ‘heat retaining’ to help you achieve faster composting and even hot composting. There is good news and bad in this – good that there is an increasing recognition that hot composting can be achieved at home in your back yard, but also a worry that some of the claims on hot bins are ludicrously misguiding buyers. The basic laws of nature (Newton’s law on cooling and the biological rate of heat production per kilo of food/garden waste) do not support most compost bin designs ever retaining enough heat to rise above ambient (air temperature) never mind get into the hot composting 40-60C range.
If you want to hot compost we suggest you read below and familiarise yourself with science and choose your hot bin carefully. Look for one with a clear application of science to create a design that works. If you do not have the time or expertise to do this then read the article below as our advice is simple. Search for customer reviews and testimonials – but look beyond the ‘it arrived and I have set it up’ type review that appear on many shop sites, search explicitly for “compost reviews 60C”, “compost bin that works at 40-60C”, “I am hot composting” “my temperature gauge reads above 40C”, “I have reached 60C”, i.e. hot temperature in my compost bin”
The HOTBIN team spent 2 years analysing why various compost bin designs failed to deliver hot composting. Once we had the engineering solution for what would work we set about figuring out how to make it cost effectively. There are over 200 compost bins on the market. To date we still have only found 5 domestic compost bins worldwide we believe genuinely offer a realistic chance of hot composting in a domestic environment. They cost respectfully: £900, £550, £250, £185 and £175. Prizs for which is the HOTBIN!
Here’s the science and engineering you need to choose a hot compost bin:
Newton’s law of cooling – hot moves to cold until a balance (equilibrium) is reached. A cup of hot tea always cools to your room temperature. Leave it outside in the snow and it will eventually freeze. A compost heap follows the same law – all heat produced by the bacteria moves from the hot centre out to the cooler outside air. You cannot stop this, all that can be changed is the speed at which the heat lost occurs. Put your tea in a well insulated thermos flask and it will stay warm all day. Forget to drink it and return next day – at best it will be luke warm.
A very thick insulated material will slow heat loss – but this is very different to using the term “insulated” because it uses a thin insulating material (plastic). Know your facts – challenge the supplier – as for their scientific evaluation (evidence) and ask for customer reviews that prove they are hot composting (i.e. achieving 40-60C)
Here’s a quick comparison of several compost bin materials and how thick they need to be to have equal insulation value:
|U ValveMeasure of resistance to heat flow per m2
U valve is used by the building industry to stop you getting ripped off when buying roof insulation
|50 mm wall of expanded polypropylene (EPP)
||This is what the HOTBIN is made from – 50mm thick EPP walls
|To achieve the same insulation (U Value) – ie to slow conductive heat loss to the same level –below we compare how thick the walls of other compost bin materials would need to be
| 500mm thick of wet wood
||That’s about 5 railways sleepers wide! Pallets are 20mm thick wood.
|= 700mm of compost
||A pallet frame is about 1m wide. 500mm each side of a really big heap. Only the very small centre is likely to stay warm. Be prepared to turn all the outer compost into the middle!
|= 600mm of High density polyethylene
||Most compost bins have HDPE walls 2-3mm thick). Image a bin with 0.6mm thick plastics wall!! (Plastic dalexs keep the waste together and offer some protection against wind and rain. They do not “thermally insulate” your compost heap.
|= 50mm of foamed LDPE
||LDPE foam insulates as well as EPP. Most PE foam is floppy and costs significantly more to mfg. Rigid foamed PE board is available – we have only found compost bins with 15mm thick walls – i.e. 5 times less insulation than the HOTBIN.
|= 50mm of PU foam
||Most PU foam boards are ‘open cell’ which means they absorb rain water or steam/water from compost. They will be wet and cease to insulate within a few days.
If you have a very big garden with lots of waste – build a big compost heap (>2m*2m*2m). It will stay warm in the middle for a period of time. Turn occasionally to move the outer cold, non composted waste into the middle. This is what industrial composters do in their ‘windrows’ composting and it is the system largely designed by Sir Howard back in 1930s that forms the backbone the ‘New Zealand 3 bin rotation system’. Not many have this amount of waste, or the time and energy to turn this much material.
Now you know you need to look at thickness and the thermal insulation of the walls (i.e. obtain the U value). However heat is also lost by convection. Air moves within the compost (at last we hope it does otherwise it will go anaerobic and smelly very quickly!). Moving air transfers heat very fast. Winds can cool things 100 times faster than still air. Go out in a cold wind with no clothes on and you will suffer hypothermia very quickly due to ‘wind chill’. Your compost heap is the same, it loses heat as cool air flows over it and carries warm air out.
We have discussed conductive and convective heat loss and the need to minimise it to have a reasonable chance of achieving hot composting by using insulated walls, sealed lids and airflow valves. There is one other important piece of science we have to adhere to – the first law of thermodynamics: energy and matter can neither be created nor destroyed, all we do is change the form. Focusing this down to the relevant bit; the total energy (heat) going in (from food as calories) must equal the total energy going out (i.e. heat lost).
There is a bit of chemical engineering science that looks at amount of energy available from bacteria composting each Kg of food waste, how much is lost as heat due to convection and conduction. It also defines how much needs to be retained to keep the compost above ambient (i.e. at 60C rather than air temperature at 10C. This law is the bit most compost bin designs convientely ignore and or fail to explain. Most households do not have enough waste (i.e. energy going in) to keep warm due to the amount of heat being lost.
The average house produces 5Kg of food waste a week (and maybe 10Kg -20Kg of green waste during the summer gardening season). The HOTBIN will stay hot with 5-10Kgs of waste a week. The maths are straight forward – most other compost bins lose heat so fast they need 50-100Kgs per week going in to keep warm. If you have this much waste, you can copy the hot system used by industrial composting – build big heaps 2mX2m, get a tractor and turn the waste.
Even the very best hot composting systems designs have a minimum amount of waste that needs adding each week to keep things hot. We find around 20% of HOTBIN customers struggle to hot compost and by far the biggest issue is these users who do not have the minimum amount of waste to hot compost. Garden Organic tested the HOTBIN against a leading plastic compost bin claiming to hot compost. The science explains why many compost bins can be loaded with 200-400 litres of waste in one go, they get hot for a few days and then cool. The science also explains why such bins are unable to heat up or stay warm when 5Kgs per week of food waste is added.
The HOTBIN does what is says on the BIN – it will stay hot all year at 40-60C with typical volumes of waste from a 4 person household (i.e. 5Kgs per week).
HOTBIN – it does what is says on the BIN
HOTBIN believes it has used nature’s laws of science to design a compost bins that work and explain why many others will more often than not fail to warm compost never mind hot compost. We neither want to overstate (or understate!) how the HOTBIN or any other product works.