Cover within bottom with dirt or sand to dissipate hot and cold temperature. Doors could be made to provide anywhere from 30 to 60 minutes of protection against fire. This has to be one for the twentieth century's greatest products. It really isn't the same with any other door. http://E-Hp.info/mitsuike/4-bbs/bbs/m-123y.cgi?id=1&post=1&&comment=000587