The S40 is double walled stainless steel, which provides the boiler with good thermal insulation.
Additional insulation applied to the outside of the boiler is not required, and is not recommended.
The reason for this is that there are electronics which are integrated into the boiler and are influenced by the temperature of the air in the cavity between the double walls. Adding extra insulation to the outside of the boiler can increase the temperature in this cavity, resulting in electronics being subjected to temperatures higher than designed for.