Last I checked, fiberrock is not to be used for hearth construction or wallshields. It has 15% cellulose content. From the USG data sheet for this product: 5. Panels should not be exposed to sustained temperatures above 125 Â°F (51.6 Â°C). http://www.usg.com/fiberock-aqua-tough-tile-backerboard-panels.html#tab-literatureAndVideos The hearth and the walls can see higher sustained temps than 125F. Don't use this.