As the first foam sealant to be recognized as a fireblock, GREAT STUFF PRO Gaps & Cracks inhibits the spread of flames through concealed spaces. It’s a minimal-expanding, single component polyurethane foam sealant for general purpose building envelope air sealing. It fills gaps up to 3″ (75 mm) and expands to take the shape of cracks and voids, forming a permanent, airtight and water-resistant bond to vinyl, wood and metal surfaces.