As the first foam sealant to be recognized as a fire block, GREAT STUFF PRO™ Gaps & Cracks inhibits the spread of flames through concealed spaces. It's a minimally expanding, single-component polyurethane foam sealant for general purpose building envelope air sealing. It fills gaps up to 3" wide and expands to take the shape of cracks and voids, forming a permanent, airtight and water-resistant bond to vinyl, wood and metal surfaces.