What is the air leakage rating of a firestop system when tested called?

The air leakage rating of a firestop system, when tested, is referred to as the L rating. This rating specifically measures the air leakage or the amount of air that escapes through a joint or opening in a fire-resistance-rated wall, floor, or ceiling assembly. The L rating is crucial because it helps to ensure that firestop systems not only impede the passage of flames and smoke but also limit the infiltration of air that can facilitate the spread of fire.

Understanding this distinction is vital for both fire safety and energy efficiency in building systems. In contrast, the F rating indicates the fire-resistance performance of the assembly, while FS rating pertains more broadly to firestop systems but does not specifically address air leakage. The S rating is often associated with sound transmission class. Therefore, the L rating is the most relevant and correct designation when discussing air leakage in firestop systems.