What characteristic of double wall vent pipe helps prevent fire hazards?

The characteristic of double wall vent pipe that helps prevent fire hazards is the insulation barrier. This insulation is crucial because it reduces the risk of heat transfer from the inner pipe, where the flue gases travel, to the outer environment. By incorporating this barrier, the outer wall remains at a lower temperature, minimizing the chance of igniting nearby combustible materials. This feature is essential in ensuring safe operation in commercial kitchens and within buildings, where fire safety standards are paramount.

While other options like being lightweight, flexible, or made from aluminum might contribute to different aspects of the vent pipe's utility, they do not specifically address fire hazard prevention as effectively as the insulation barrier does. The focus on maintaining lower external temperatures is what makes the double wall vent pipe particularly important for safety in heating applications.