Chimney pipe through block should I fill the cavities with insulation?

[Dajolu]

I running chimney pipe through a block wall. Should I stuff some rock wool insulation in the cavities or just leave them alone?

 

[stoveliker]

Up thru the wall?
Does class A fit thru that?

What do the class A specs as f your brand say about clearance to combustibles and whether any air space is needed?

 

[PaulMcKenzie]

If it’s just a block wall I’d probably leave the cavities alone. The block itself isn’t combustible. The bigger thing is making sure you’re using the proper Class A pass-through or wall thimble and keeping the required clearances. Check the chimney manual for your brand before packing anything in there.

 

[Dajolu]

I am not sure of the brand yet. I just put in a Selkirk thimble. The hole is big enough to fit the OD of a 6” chimney pipe. The way I have it, the stud on the right will have the required 2” air gap. On the other side of the block is the outside. Hardi board and tile going in the inside. I should be good with all clearance requirements. I’m just wondering if I should stuff the fender block cavities with anything

 

[stoveliker]

I would read the instructions/specs of the thimble.

But given that a thimble is meant to guide a pipe to the outside and keep the outside there (no air coming in), it's meant to have contact with (non-combustible) material.

Therefore I think it's fine to put rockwool in there. Bit less heat loss.

I'm no expert tho.