In my experience cement board makes a good heat shield and in at least one jusisdiction with strong codes and enforcement fairfax county va it is accepted as a heat shield material when properly installed.
Cement board wood stove heat shield.
I used cement board in that same fashion under stove and as a back wall for one year.
The simplest masonry heat shield is a cement board that is separated from the wall by one inch ceramic spacers.
Constructing the heat shield itself is a relatively easy process.
Step 1 use an electric stud finder to find the studs in the wall behind the position that you will place the wood stove.
Specific requirements for heat shielding your stove may require more than one piece of concrete backer board to meet the required dimensions.
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Modern wood stoves can be installed closer to the wall because of backer boards.
Generally the heat shield must extend a minimum of 20 inches beyond each side of the wood stove and 20 inches above it.
1 heat shield spacers are available online or sometimes at your local hardware store for this purpose.
Backer board is a heat shield that is found behind wood burning stoves.
Click to enlarge any image.
These shields are constructed with a sheet of 24 gauge or thicker sheet metal or 1 2 or thicker cement board with 1 of air space behind the shield and around the perimeter to allow free air flow.
The manufacturer of one brand of cement board says it is suitable for heat shielding go to the faq.
It got extremely warm almost hot on the wood floor and the wall board in back of the stove got hot but not much of a concern.
Hardi cement board is also designed to have ceramic porcelain or stone tiles installed over it to cover and beautify the heat shield after it is installed.
The back of the stove was about 18 to 24 from the wall.