Abstract: A thermally insulative fire block sheet having an elastomeric layer is provided which has a surface flammability flame spread index of zero at 20 minutes of exposure in accordance with ASTM E-162 and which is capable, in a thickness of 1/16 inch, of blocking a flame of 1900.degree. F. for more than an hour. The sheet comprises at least two layers. One layer, facing the direction of the expected flame, is a silicone foam rubber comprising the reaction product of a vinylterminated polysiloxane containing --Si(CH.sub.3).sub.2 O-- units and an organohydrogensiloxane polymer containing Si(H)(CH.sub.3)O-- units. The silicone foam rubber also contains alumina trihydrate. The other layer is an adherent flame resistant support layer for said foam, preferably a glass cloth layer.