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.
Straw bales are a very good natural insulation material. When sourced from 50 kilometers benear your 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.