Can 4-pound density mineral wool be substituted in a UL system that specifies 6-pound density mineral wool?

The correct answer, indicating that substitution is not allowed, is based on the critical importance of maintaining the specifications outlined in UL systems, which are developed to ensure safety and performance in firestop applications. Each material used in these systems is meticulously tested for its fire-resistance properties, including things like density, thermal performance, and structural integrity. Therefore, substituting a 4-pound density mineral wool for a 6-pound density variant could compromise the integrity and performance of the firestop system, potentially leading to failure under fire conditions.

Adhering strictly to the specified materials is paramount because they are selected based on rigorous testing to meet fire safety standards. Changing one component, particularly one as significant as the density of mineral wool, could affect how the system performs—impacting factors like thermal resistance and fire containment. This is why substitution is typically not permitted unless expressly stated otherwise by the manufacturer or the testing agency.

While there may be instances in other contexts where material substitutions could happen with proper approval or when certain conditions are met, in firestop systems, maintaining compliance with specified materials is crucial for both safety and regulatory adherence.