Which type of firestop material is primarily used for its ability to flex and conform to varying gaps?

The type of firestop material known for its ability to flex and conform to varying gaps is flexible caulk. This material is specifically designed to accommodate movement within buildings caused by factors such as thermal expansion and contraction or structural shifts. Its flexibility allows it to maintain an effective seal in joints and penetrations that may change over time, ensuring continued fire resistance and preventing the passage of smoke and flames.

Intumescent materials expand when exposed to heat, creating an insulating barrier, but they are not primarily characterized by flexibility. Fire-resistant cement is typically used for construction and provides structural integrity but lacks the adaptability for varying gaps. Ablative materials are designed to degrade and provide thermal protection, yet they do not have the same conforming qualities as flexible caulk. Thus, flexible caulk stands out for its adaptability, making it the best choice for applications that require a material to fill diverse spaces while maintaining effectiveness as a firestop.