My home has a gas fireplace that we do not use very often. It is in a "bump out" on the back of the house. In the winter, cold air flows through the vents under the fireplace and if you feel the tile around the fireplace, it is cold. Is there a good method to insulate around it without tearing apart the fireplace? What type of insulation should I use that would not be a fire hazard.
Without a picture of your fireplace, I couldn't tell for sure, but I would look in to fire brick. It is a near perfect insulator since it is a light brick with lots of air bubbles (air insulates).
..or at a local fireplace store.
I have done a little more research regarding this topic. Apparently, a lot of people have this problem. The most recommended solution I have found is to apply metal duct tape around the opening for the gas line. That is where the air is blowing through on my fireplace. I have applied this fix, but I need to wait for winter to see if it helps.
The metal duct tape helped somewhat, but I would still have temperatures in the mid 40sF on the lower grate of the fireplace when it was in the 20's outside. I recently discovered the insulation underneath was not sufficient. I could get to that and added additional information. The temperature on the lower grate is now in the 50s when it is in the 20s outside. Obviously, this still need improving, but I am not sure what else I can do without tearing out walls.
If the buildout is raised off the ground then siding or other sheathing used to cover it. My new home has similar set up and when I checked there were gaps up to half inch around a poorly cut piece of scrap siding. The hearth is tile and very cold all the time. My plan is to pull the siding panel off that off and use a layer of fiberglass. Then reinstall new (better cut) siding, pack edges foam wheatherstriping, and then seal with silicone.