I'm about to install a Woodstock Soapstone stove in front of my fireplace and wondering what is best way to insulate the inside of the fireplace so heat isn't lost to outside. Up until now, I've had stoves tucked into the fireplace, and I can feel the heat radiating right through the brick to the exterior of the fireplace, since its on the outside of the house. Would sheets of Durock be a good solution to fit behind the stove, against the walls of the fireplace? Or big pieces of slate? I need to get to the back of the stove so can't seal up the front of the fireplace altogether.