In temper or cold climate the digester needs to be insulated because the Archea need to stay at the temperature optimum of about 37°C. There are several types of insulation arround.
Spray foam is a simple type of insulation. All the digester is covered in it. The insulation value depends on the material used. There are natural materials around made from Soy.
Straw bales are a very good natural insulation material. When sourced from within a 50 kilometers radius around your home / digester there is nearly no carbon footprint on it. With an k-Value of about 30 this natural material is perfect for a sustainable digester.
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Wrapping an IBC with two layers of stretch foil with an airspace in between to increase internal temperatures of the tank.