What type of materials is the exception for minimum clearance applied to?

The exception for minimum clearance is applied to cementitious materials due to their unique properties when compared to other materials. Cementitious substances, such as concrete or certain types of fireproofing, have a higher resistance to heat and combustion. This allows for different clearance requirements since they can endure exposure to elevated temperatures without risking structural integrity.

In mechanical systems, especially where heat-producing equipment is involved, maintaining proper clearances is crucial for safety. However, cementitious materials have the ability to mitigate heat transfer more effectively than materials like wood, which can ignite, or metal, which may conduct heat. This makes it feasible to reduce the distance these cementitious materials need to be set away from heat sources, thereby providing flexibility in design and installation, while still ensuring safety and compliance with building codes.