How can a thin strip around a fire door expand to fill a gap?

A door needs a small clearance around its edge so it can open and close. In a fire-resistant assembly, that gap could also become a path for hot gases. An intumescent seal helps address this changing requirement.

Under ordinary conditions, the seal is a relatively thin strip. When sufficiently heated, its material reacts and expands into a thicker insulating mass. That expansion can fill a designed gap around the closed door.

The response depends on the product’s composition. Some materials use expanding graphite, while others form a foamed char through chemical reactions. These changes differ from merely stretching a rubber strip or melting wax into a space.

The seal is one part of a larger assembly that includes the door, frame, closing mechanism and other details. Separate smoke seals may act before strong heating occurs. Useful protection comes from the tested combination and its proper condition, not from an isolated strip alone.