What is one of the key characteristics of flexible fiberglass duct insulation?

One of the key characteristics of flexible fiberglass duct insulation is its fire-resistant property. This is crucial because insulation materials are often required to meet specific fire safety standards to minimize the risk of fire spread in buildings. The fiberglass construction contains inorganic materials that do not burn easily, providing significant fire resistance compared to other insulating materials. This characteristic ensures that in the event of a fire, the insulation itself doesn't contribute to the fire's spread, making it a safer choice for use in various applications, particularly in HVAC systems.

While other options may have some relevance to insulation materials, they do not specifically highlight the principal benefit that fiberglass duct insulation provides in terms of safety and compliance with building codes.