What are the requirements for framing for a gas insert in an existing masonry fireplace? See the attached picture that shows the framed opening for the gas insert (circled in red). The insert will project about 5" outside of the existing firebox. My contractor insists that drywall all the way to the insert edge (the red box drawn on the picture) is ok. Is he correct or do we need to use non-combustible material like cement board? BTW, he is going to insulate with Rockwool between the studs. We were also planning to just paint the wall around the insert for a clean, modern look. Is it possible to paint over cement board or other non-combustible material? Is any special surface prep required?
Any advice would be much appreciated.