What is the maximum flame spread index allowed for materials exposed to airflow in plenum enclosure construction?

The maximum flame spread index allowed for materials exposed to airflow in plenum enclosure construction is 25. This limit is established to ensure that materials used in these critical spaces do not contribute significantly to the spread of fire or smoke. Plenums, which are often used for air distribution in HVAC systems, can facilitate the rapid movement of fire and smoke if combustible materials within them have a high flame spread index. Therefore, a flame spread index of 25 is considered an acceptable upper limit, balancing safety with the need for practical construction materials. This standard helps to protect occupants and property by minimizing the potential for fire propagation in hidden spaces where access for fire suppression might be limited. Ensuring that materials meet this requirement is a key aspect in maintaining overall fire safety within buildings.