The invention relates to a fire valve made of a body (100) consisting of a frame (50) and a tunnel (10) in which a butterfly damper (200) is pivotably mounted between a closed position, separating an inner side (I) from an outer side (E), and an open holding position. The body (100) and the butterfly damper (200) formed by a blade (210) are each produced by assembling cut and pasted refractory sand-lime plates (11, 51, 211). The assembly forming the body (100) and that forming the butterfly damper (200) are provided with steel reinforcements directly attached to the plate assemblies. The reinforcements (60, 70) of the body (100) are attached onto the outer side of the tunnel (10) and of the frame (50) and the reinforcements (240, 250) of the butterfly damper (200) are attached onto both sides of the blade (210). The butterfly damper is locked by the expansion of the metal elements.