Intumescent sheet material 26 comprises a resilient, flexible intumescent sheet comprising from about 20 to 65 percent by weight of unexpanded vermiculite flakes, such flakes being either untreated or treated by being ion exchanged with ammonium compound such as ammonium dihydrogen phosphate, ammonium carbonate, ammonium chloride or other suitable ammonium compound; from about 10 percent to 50 per