I am building an interior hearth and surround that will ultimately be covered in cultured stone for a woodstove. I have constructed the hearth of 2X6 covered with 1/2" plywood with wonderboard over that. I have questions about securing the wonderboard to the wall for the surround:

-Is it necessary to secure 1/2" plywood to the drywall and then attach the wonderboard to the plywood? I am concerned that simply screwing the wonderboard to the studs will not provide enough support for the stone.

-Is it necessary to use a vapor barrier when installing wonderboard for an interior installation?

Thank You for your help!

I'm a firm believer in strength and 1/2 ply is what I would like for a backer for the wonder board. Also it gives you more screwing surface for the wonder board.

What kind of wood stove? If it is not zero clearance, then check your local building code to ensure that your design can use any wood at all (even if tile covered).

Too much can be costly and a burden... the studs and wonderboard are sufficient enough as far as strength goes. Run a bead of carpenters glue along the studs then mount your board for extra bonding. Always check your local codes before any job is started. And especially the fire codes and ratings per installations.

My local codes require an air space behind the wonderboard