Intumescent means tumour forming. What on earth would you want one of those for in DIY?
Intumescent Materials are used for fire protection. On exposure to fire, the items swell up to form a fire and smoke resisting foam that will block fire spread for anything upto an hour.
These are strips that go into grooves routed into the sides and top of a door. These are a common sight in large buildings.
They prevent fire spread from room to corridor, or vice versa, and protect occupants in other rooms from the smoke, which can be toxic.
These hoods sit on downlighters fitted in ceilings. Downlighters penetrate the fire & smoke barrier, and the intumescent hood restores this layer of protection. In a fire the hood swells and blocks the hole.
May be used to add fire resistance where materials are joined.
Oak is intumescent, in a fire it forms a layer of fire resistant char, delaying consumption. This feature is occasionally made use of in building design, and is the reason oak was used for electrical wiring accessory backing plates long ago.