What goes behind firebrick on back wall of fireplace?

[BeerChemistWhiskey]

I am in the process of rebuilding the back wall of my firebox, preparing to install a wood burning insert to enjoy this winter. After pulling out the fire bricks (literally, I could pull them all out) I found that behind them was filled in with what seemed to be loose chunks of concrete or mortar.

I've done some searching online but I can't find a definitive answer on what I should use to properly fill in the space behind the firebrick when I set it all back up. Some sources have said to fill in with mortar, vermiculite, or "rubble". One Canadian source (if I remember correctly) said to either 1. Leave an airspace or 2. Use a compressible insulating material.

I'm thinking I wouldn't mind some extra insulation for the fireplace, as my chimney is on an exterior wall. So I'm thinking I could place a few batts of Roxul in the void behind the fire brick?

How is this "supposed" to be done?

 

[bholler]

If it is on an exterior wall noncombustible insulation. If it is interior you dont need anything

 

[Simonkenton]

On an exterior wall, at the least, nothing. At best, insulation.
If they filled that space with concrete/mortar rubble, that was a screw up as it would readily conduct heat to the outside wall.
And, when the fireplace was not running, would readily conduct outside cold into the living space.

I wonder if they filled that space, or if that is just rubble that rained down from a failing fireplace.

 

[bholler]

On an exterior wall, at the least, nothing. At best, insulation.
If they filled that space with concrete/mortar rubble, that was a screw up as it would readily conduct heat to the outside wall.
And, when the fireplace was not running, would readily conduct outside cold into the living space.

I wonder if they filled that space, or if that is just rubble that rained down from a failing fireplace.

It is typically filled with rubble and what ever mortar falls in there. We find all kinds of stuff thrown in there. But yes it should be insulation or nothing. Never mortared in solid.

 

[Simonkenton]

That is weird, that the space is typically filled with rubble etc. Because, most fireplaces are on an exterior wall.
If nothing is put in there, then the air space there is a dead air space, and is in and of itself, insulated.
If filled in with some type of mortar and rubble, the cold air on days when the fireplace is not running, will run straight from the outside into the living space.

 

[bholler]

That is weird, that the space is typically filled with rubble etc. Because, most fireplaces are on an exterior wall.
If nothing is put in there, then the air space there is a dead air space, and is in and of itself, insulated.
If filled in with some type of mortar and rubble, the cold air on days when the fireplace is not running, will run straight from the outside into the living space.

I agree it is not right but neither is much of anything else with traditional fireplace construction or design.