How to insulate gas fireplace
 

I have a gas fireplace that comes in through a sun room and the back side in a mud room. The mud room borders the garage. I have a constant breeze of cold air coming into the sun room. The problem is there is the second pipe that comes into the fireplace. It has a gap all around it of about a 1/4”. The cold air feeds into that and has straight path from the top of the box. I bought rock wool to insulate the box but unless I actually touch the second pipe, I can stop the cold air. Any advice appreciated.

fireproof foam insulation
google is your friend

I've used this. Works like charm.

https://krakenbond.net/products/fireplace-sealant

3M also makes 'fire resistant' foam and caulk. Both colored red.
I've used it to seal pipe penetrations through walls and floors (behind cabinets etc). Required by code.
Not for use on exhaust pipe.

Retort cement, not firestop

I guess you didn't like the answers in your previous thread:
https://forums.pelicanparts.com/off-topic-discussions/1187544-rockwool-insulation.html