GREAT STUFF Gaps & Cracks is a polyurethane-based insulating foam sealant that fills, air-seals and insulates small gaps (less than 1 inch) inside or out. But you probably guessed as much from the name. What you might not have guessed is how easy it is to use (thanks to the straw applicator) or how it expands to take the shape of cracks and voids to form a durable, airtight, and water-resistant bond that eliminates unwanted airflow and helps reduce condensation.