How to Build a Hot Compost Bin.

Latest Update 3rd April 2017.

This is my homemade hot compost bin.  Its purpose is to make relatively small batches of compost using high temperature microbial breakdown.  Usually this method needs a compost heap containing at least a cubic metre of material so the outer layers provide enough insulation to heat the heap up to the required 55 to 65 degrees centigrade.

My new bin has a capacity of 400 litres and is better suited to my small suburban garden where space and organic waste are in short supply.  The bins walls and roof are insulated and hold the heat in, easily raising the temperature to the required levels.

Its important to supply the composting (aerobic) microbes with plenty of oxygen, so the bin's contents must be exposed to air periodically during the course of this process.  The hot compost bin is designed to be split into 2 halves so it can easily be moved to a suitable new space and re-assembled.  The compost is left behind, but is then tossed through the air back into the bin in its new location aerating and re-mixing it at the same time.

Its important to regulate the temperature of the compost during each cycle so we know how the break down process is progressing.  I use a stainless steel thermometer with a 500mm probe to monitor the temperature.  Its long enough to reach into the centre of the heap and the dial gauge is easy to read.
The unit's external dimensions are 950 x 950 x 950mm (lid on) and it's assembled from 4 identical wall panels plus a roof panel.  Recycled timber is used to make a simple exterior frame for each panel, and 2 layers of 60mm rigid polystyrene foam are glued together to make a single 120mm thick panel which is then bonded into position within the frame.  

Once the glue is set, both sides of the wall are covered with heavy duty shadecloth stapled to the timber frame to protect the foam from damage.
Each sub-assembly is made by joined 2 panels together overlapping them at their corners.  They are joined top and bottom using galvanised angle brackets on the inside and galvanised flat joining plates on the outside.
Two saddle clamps are screwed into position as shown.  They help to handle the sub-assemblies when they are being separated so the compost heap can be aerated.
When the two halves are brought together, they are secured using small cabin hooks as shown above.

The lid is made from 60mm polystyrene foam and 75% shadecloth. Two layers of the foam are laminated together using "no more nails" glue and covered with the shadecloth to improve impact resistance and add a bit of UV protection.

Two pieces of shadecloth are cut and glued to the outside of the foam block, overlapping at the sides, so they are double the thickness of the top and bottom.

The top piece hangs down past the bottom of the lid on 2 sides so it can be hooked onto the base using 35mm gal bullet head nails as hooking point.
There are 4 of these hooking points holding the lid in position so it wont blow away in windy conditions.  Removing and fitting the lid is very easy, and its light enough for even an old man like me to handle.