Cement board to cover gap for small insert stove?

[jameshowison]

We have a small insert stove (Stuv 6 insert). All working great, now figuring out finishing.

The stoves has a 2” metal frame around it. We would like to keep it at that size. Yet the gap from the frame to the sides of the masonry fireplace is closer to 5”. So there is a gap of 3 “ on the top, left, and right.

Currently I have that covered by a Hardibacker backer board, screwed to the masonry. I cut the spot for the stove out of a single sheet. Plan was to tile on top of that backer board, and over the existing stucco fireplace finish.

Life intervened and the backer board has been in place for 18 months. We have had many fires.

Now, though, the backer board has developed a crack from the left hand top corner. When we have a fire the crack opens a little more, then closes. The backerboard is not hard up against the metal of the fireplace, but it is against a recess edge in the stucco finish. I was not careful about whether it is “in plane” against the existing masonry, so perhaps it cracked after being in tension too long?

But perhaps we are dealing with dissimilar expansion and contraction in the materials: the masonry fireplace and the backer board expanding differently? Doesn’t seem a good stable base for tile.

Any guidance about closing this gap around the insert?

Larger metal is an option but not preferred. Perhaps we could fasten the backer board to the 2” stove frame (extending it with tiled board not attached to the masonry?) does different backer board expand differently?

Also is it obvious to those with experience that we need a control joint around the concrete board? We were considering using metal lath/stucco across the whole existing fireplace, and onto the board (either as tile base or as finished surface). But if expansion rates are significant with the heat should have a control joint there?

 

[EbS-P]

Im not a professional but you should double check that cement board can be used as a surround. And you insulated that liner? And what is the space at the bottom center cutout? @bholler @begreen

 

[jameshowison]

Yup. Liner all insulated. Walls are all dense concrete block anyway

Space underneath comes from raising the unit up on top of fire bricks but just under both side of the unit, not in the middle. If I did it again I would definitely use some metal platform that could be raised by screwing something from the front.

But yes, now I realize that I’m basically making a surround and trying to tile it.

Why do people try to maintain access to the insert unit? What sort of frequency is that required? Is it “just in case” or is there some sort of maintenance access regularly used?

I figure that sealing it in makes access messy and destructive (but possible in an unexpected issue).