This is my friend's super compost bin. Look at the thickness of the insulation. EPP or expanded polypropene has a themal conductivity of 0.035W/mK. As you'll see below, the temperature in the actual compost itself was about 70 degrees Celsius and a little lower in the trapped air space above. That would give a temperature difference in excess of 50 degrees at the moment. The thickness of the insulation is about 4cm. This means that 0.07Joules per second get through the insulation. Then I went searching for a U-value. I couldn't find one but I did find that U-value = thermal conductivity/thickness = 0.035/0.04 = 0.875 W/m^2.K So by finding the surface area we can calculate the rate of energy loss of the whole compost bin.
Rate of loss of energy = surface area x U-value x temperature difference = 2 x 0.875 x 50 = 88Watts. That's not far off what is given off by a resting adult human.
The micro-organisms responsible for the decomposition release energy by respiration, which is an exothermic reaction.
