A safe or other insulated chamber is constructed with permeable plaster inner walls and door, which can be kept damp and cool in case of fire by fluid pumped through the walls from a self contained reservoir. Evaporation of water from the porous plaster provides a cooling effect, and venting of the moisture through openings from the walls permits the evaporate to escape without a buildup of moisture in the chamber. The circulating pump is activated by heat.