A buffer tank for the heating system, must keep the energy losses at a minimum, so thick insulation is essential. In the tank, especially at the top, the water temperature is mostly above 80 °C, and in winter the temperature can drop down to -20 °C, which means an energy difference of 100 °C.
The standard insulation materials today for buffer tanks are mineral or glass wool or polyurethane, and I therefore recommend an insulation thickness of 40 cm at the top and the sides.
At the bottom, where the water normally is much colder than at the top, I think that you can do with 20 cm of insulation. But since the concrete that the tank stands on is wet when it is raining, you can not have mineral wool in contact with the concrete, so I recommend 10 cm of rock wool closest to the tank, since it can withstand high temperatures very well, and then 10 cm of extruded polystyrene (closed cells) closest to the concrete. The polystyrene prevents humidity from being sought upp into the insulation with heat leakage and rust on the tank.
