When using flexible liner in rectangular duct installation, when is an additional mechanical fastener needed?

The requirement for an additional mechanical fastener when installing flexible liner in rectangular ductwork is based on ensuring adequate support and integrity of the liner, particularly in larger ducts. When the duct width exceeds a certain measurement, in this case, 8 inches, the increased size makes it more prone to potential sagging or movement.

Mechanical fasteners provide the necessary support to keep the liner securely in place, prevent airflow leaks, and ensure that the duct maintains its intended shape. This is crucial because any gaps or looseness in the liner can affect the system's efficiency and performance, leading to energy losses or reduced air quality.

In contrast, smaller ducts or certain liner thicknesses may not require the same level of fastening for stability, as their size and construction materials may inherently support themselves better.