What must be established for insulations used in ducts to ensure safety?

The flame spread index is a critical safety measure for insulations used in ducts because it determines how quickly and how far flames can travel along the surface of the insulation material when exposed to a fire. This evaluation is crucial in ensuring that, in the event of a fire, the insulation does not contribute to the rapid spread of flames, which can lead to greater hazards for occupants and damage to property. Materials with a lower flame spread index indicate better fire-resistance properties, making them safer for installation in duct systems.

While thermal resistance, flexibility, and durability are essential characteristics for insulation materials, they do not directly address fire safety as effectively as the flame spread index does. Thermal resistance, for instance, pertains to the material's ability to resist heat flow, which is important for energy efficiency but does not inform fire behavior. Flexibility relates more to the ease of installation and performance under movement rather than fire safety. Durability refers to the lifespan and physical integrity of the insulation under various environmental conditions, which also does not account for how the material will react to fire. Thus, considering the flame spread index is vital for ensuring the safety of duct insulation in the event of a fire.