Do I need to insulate my compost bin?

Thursday, 18 April 2013  |  HOTBIN Composting

Do I really need to insulate my compost bin?

Yes if you want to hot compost in a UK garden (or other temperate climate) and you have relatively small amounts of waste (under 10Kgs per week). You need to effectively insulate your heap to prevent heat loss.

How do I insulate a compost heap?

There are two routes to create an insulted and hence hot composting heap:

  1. Protect and keep the inner heap hot by using the outer layers of waste as ‘insulation’. You need a large mass (typically 1mX1mX1m (ie 1m3), and have to accept the outer layer (approximate 0.5-0.75m) remains near ambient temperature. You will need to fork/turn it over at least 2-3 times to exchange and move the outer material back into core.
  2. Use insulation materials to protect the heap. A 50mm expanded polypropylene board provides the same insulation as a 750mm thick layer of compost.

Can I use any type of insulation material?

If only it were that easy.  You will see many advice sites saying insulate with straw bales, tarpaulins, cardboard, sheep wool etc. Once your insulation material gets damp or wet it loses its insulation property. Unless you find a way to keep the rain and steam (from the composting) out of the insultion, all the above plus PU foams, cellotax types boards, rock wool are hopeless for insulating a compsot heap.

Can I use straw bales?

They work to an extent. They are very thick and tend to shed rain water - although steam will penetrate. The main benefit from straw bales is they reduce convective heat loss by forming an effective wind barrier. The downside is straw is compostable and you will need to regularly replace them - which can be very expensive unless you have a free source.

If I use a waterproof insulation will that work? 

If only it were that easy! Once you have stopped conductive heat flow, you then need to limit convective (hot air gas flow). And this normally requires a sealed container with a valve - think pressure cooker!

To insulate your heap, you need to prevent all three forms of heat loss - conductive (via solids), convective (via liquids or gases) and radiant (from surface of solid).The table below compares some typical thermal conductivity values (ie transfer through solid materials) used to make compost heaps.

 

Material

Thermal Conductivity (WK-1m-1)

50 mm expanded polypropylene (HOTBIN) 0.03
Compost 0.4 (typically 60% moisture)
Soil 0.46 (typically moist/wet)
Wood (dry) 0.2
Wood (wet) 0.4
LDPE plastic (most dalek cones & bins) 0.4

 

To put this into context, these are the thermal conductivity values of moving air.

 

Thermal Conductivity (WK-1m-1)

Moving air, breeze 0.24
Moving air, windy 2.4