Hearth Framing - Non-Combustible Wall Assembly??? Advice?

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New Member
Dec 30, 2015
Seattle, WA


I am looking to install a wood burning insert into our existing fireplace. Here are some of the specifics and context:
- 1923 house, original hearth is long gone. firebox and chimney are original.
- The hearth was built of lava rock from 1965. This wasn't really the aesthetic we were going for so we've since ripped it all out. What remains is the masonry firebox and chimney and some nice, existing tile on the floor.
- We're interested in installing an insert in lieu of making this a working fireplace.
- We've got our eye on the Morso 5660
- We're considering to do a super simple, clean, modern hearth - basically a white box with an insert installed in it.
Hearth Framing - Non-Combustible Wall Assembly??? Advice?

To achieve this aesthetic, I think the easiest assembly would be to frame out the box and clad it - applying a skim coat (and paint) finish to make it smooth and finished.
Being that we need to build this out of non-combustible materials, I've come up with a wall assembly I believe to be non-combustible and compatible with the requirements of building codes and inserts.
Hearth construction:
- 1/2" Cement Board
- 3-5/8" Metal Framing
- 4" Mineral Wool Insulation
- Seal edges (connecting to existing masonry) with fire resistant mortar
I'm also including some drawings.

Two questions I have:
1. Does this assembly meet non-combustible requirements?
2. If I wanted to install blocking around the firebox opening (or anywhere else in this hearth assembly), what would be the best way to do so? (I was considering to use 2x4 fire treated lumber).

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