What is an important characteristic of insulation used on boilers?

Low thermal conductivity is an important characteristic of insulation used on boilers because it determines the material's effectiveness in reducing heat loss. Insulation with low thermal conductivity minimizes the amount of heat that escapes from the boiler, thus improving energy efficiency and maintaining optimal operating temperatures. This characteristic is crucial for ensuring that heat is retained within the system, contributing to performance and reducing energy costs.

The other options, while they may have their own applications and benefits in different contexts, do not align with the primary purpose of insulation in a boiler setting. High flexibility is not typically required for boiler insulation, as rigid or semi-rigid materials are more common to withstand the operating conditions. High water absorbency would be detrimental, as moisture can compromise insulation performance, leading to inefficiencies or even damage. Lightweight construction, while sometimes desirable for ease of handling, is secondary to the primary function of heat retention and efficiency that low thermal conductivity provides.