Which type of firestop material is designed to expand when exposed to heat?

The type of firestop material designed to expand when exposed to heat is intumescent material. This characteristic is crucial in firestop applications, as intumescent materials react to high temperatures by swelling and forming an insulating char. This expansion helps to seal gaps and openings, preventing the passage of heat, smoke, and flames through penetrations in fire-rated assemblies.

Intumescent firestop products are particularly effective because they can adapt to changes in temperature and provide a dynamic response in the event of a fire. When exposed to heat, they typically increase in volume by several times, effectively blocking and protecting areas that could otherwise allow fire and smoke to spread.

In contrast, non-intumescent materials do not expand upon heating, which can limit their effectiveness in certain fire scenarios. Ablative materials may provide thermal protection through insulation but do not exhibit the dynamic expansion properties of intumescent materials. Interstitial materials refer more generally to those used in the spaces between fire-rated assemblies, but they do not specifically indicate the ability to expand under heat. Therefore, the defining characteristic of intumescent materials makes it the correct answer in this context.