Fire Resistant Expanding PU foam. A high performance, polyurethane expanding foam, fire rated and tested to BSEN 1366-4 and BS476 Part 20 (the most stringent test for fire rated products).
Selfex Expanding Foam Tape 5-35mm x 4.6mtr | DIY at B&Q
BlazeBrake™ Foam - Ramset
12 x B1 Fire Rated Foam Soudal Gun Grade Expanding Insulating PU Foam 750ml : Amazon.co.uk: DIY & Tools