Rock wool insulation as fire block

I find one mention of rock wool and two of mineral wool in the residential code. I don't know that rock wool is an insulation. At least I have not seen it being used as an insulation.

R302.4.2 Membrane penetrations. Membrane penetrations shall comply with Section R302.4.1. Where walls are required to have a fire-resistance rating, recessed fixtures shall be installed so that the required fire-resistance rating will not be reduced.

Exceptions:
1. Membrane penetrations of not more than 2-hour fire-resistance-rated walls and partitions by steel electrical boxes that do not exceed 16 square inches (0.0103 m2) in area provided that the aggregate area of the openings through the membrane does not exceed 100 square inches (0.0645 m2) in any 100 square feet (9.29 m2) of wall area. The annular space between the wall membrane and the box shall not exceed 1/8 inch (3.1 mm). Such boxes on opposite sides of the wall shall be separated by one of the following:

1.2 By a horizontal distance of not less than the depth of the wall cavity where the wall cavity is filled with cellulose loose-fill, rockwool or slag mineral wool insulation.

R302.11.1 Fireblocking materials. Except as provided in Section R302.11, Item 4, fireblocking shall consist of the following materials.

1. Two-inch (51 mm) nominal lumber.

2. Two thicknesses of 1-inch (25.4 mm) nominal lumber with broken lap joints.

3. One thickness of 23/32-inch (18.3 mm) wood structural panels with joints backed by /32-inch (18.3mm) wood structural panels.

4. One thickness of 3/4-inch (19.1 mm) particleboard with joints backed by 3/4-inch (19.1 mm) particle- board.

5. One-half-inch (12.7 mm) gypsum board.

6. One-quarter-inch (6.4 mm) cement-based millboard.

7. Batts or blankets of mineral wool or glass fiber or other approved materials installed in such a manner as to be securely retained in place.

8. Cellulose insulation installed as tested in accordance with ASTM E119 or UL 263, for the specific appli- cation.

 

The rock wool insulation I have seen has been in century old attics, and is similar to a handful of eggs. Can't imagine how it could be supported in a stud cavity to meet the requirements.

 

I see Rockwool used in a project at least once a month. Residential basement finishes being its top use, old home remodel being second and additions being its third use. I’ve seen it used in minimal commercial application. You can also use it to protect TJI joists.
Rockwool does not retain water, water does not penetration Rockwool like typical fiberglass insulation along with the fire advantage.

As alaskajoe states, friction fit and is available in 16” or 24” wide cavities.