What is a common characteristic of intumescent materials in firestop applications?

Intumescent materials are well-known in firestop applications primarily for their unique ability to expand when exposed to heat. This characteristic is critical in fire protection systems, as the expansion process forms a protective char or barrier that effectively seals gaps and openings in walls and floors where fire could spread. When temperatures rise due to a nearby fire, intumescent materials react by swelling significantly, which helps to block flames, smoke, and heat from penetrating through fire-resistance rated assemblies.

This expanding property is essential for maintaining the integrity of fire-rated installations, ensuring that building occupants are afforded maximum safety and that the spread of fire is minimized. The other choices do not represent characteristics typical of intumescent materials, as they are not reflective, are not primarily composed of wood pulp, and do not exhibit high electrical conductivity. The focus on their expanding nature under heat defines their role and effectiveness in firestop applications.