The standard, minimum clearance for radiant wood stoves is 36 inches, according to the National Fire Protection Association (NFPA). If the wood stove is installed 36 inches away, a wide variety of decorative materials can be used. If the wood stove is placed closer than 36 inches, to be safe and to comply with fire and building codes, the materials must mitigate heat transfer into flammable building materials.
The characteristics you want in firewall backing material or combination of materials is to keep a flammable surface from becoming too hot. That means the outer surface, facing the stove, cannot transfer heat to building materials such as wood framing or insulation. The materials must either reflect the heat back into the room, or be a poor conductor of heat so the heat does not transfer through the material.
Two-Part Systems: Sheet Steel
One of the most efficient ways to allow installation closer than 36 inches is to use a 28-gauge sheet metal with free-flowing air between the sheet metal, floor and ceiling. Metal is an excellent heat conductor. It acts as a heat sink, pulling the heat into the material. Air is a very poor conductor of heat. So, the air behind the metal acts stops the heat transfer. This design allows builders and homeowners more freedom in the materials used behind the metal shielding.
A wide variety of masonry and rock materials are acceptable firewall backing, from river rock to flagstone—brick to cinder blocks. The rock or masonry material should be no less than 2 inches thick in all places. For years, fireplaces have been safely made from adobe and adobe-covered masonry.
Heat and Flammability
When choosing materials for your firewall backing, remember this important distinction between flammable surfaces and heat transfer characteristics. No materials in the vicinity of the stove should have a flammable surface. That means, no lacquered river rocks, painted surfaces or raw wood; one popped ember could ignite them. What is most important is the way in which you install the stove, the distances and how you configure the materials. Conceptually, if they're 3 feet or more away, you have great latitude to use any decorative material you like, though it shouldn't have a flammable surface. Closer than 36 inches, the primary alternative material is ventilated sheet metal to pull the heat in, then stop its transfer. More decorative materials with non-flammable surfaces may be used behind the metal heat shield: glass block, ceramic tile, custom ceramics or clay.