Intumescent is a big word and not familiar to many people. In simple terms, an intumescent material swells with exposure to heat. This swelling increases the volume of the material and produces a char which is a poor conductor of heat. The combination of swelling and the associated reduced heat transfer blocks fire, smoke and hot gasses. Intumescent materials are often liquids, putties, or sealants but they can also be foams and can be applied like a tape which offers a variety of benefits. The foam is available in 4 thicknesses- .18,.24, .34, and .51 inches (4.5, 6.0,9.0, and 13 mm) with a pressure-sensitive adhesive on one side. You remove the liner, and apply the foam tape to the surface. The product is watertight and airtight when compressed 30% and releases no toxic gases when heated.