Hi All,
During the winter we discovered that our fireplace was leaking cold air into the house! I poked around and figured out the the flue would not close all the way (pretty rusted).

I proceeded to get one of those "chimney balloons" to try see up the leak (or at least eliminate 95% of the leak.) No dice! Apparently, I have a "ash box" of some sorts also. (small "box/ditch" in the fireplace that looks to be used to collect ashes; there's a small door on the outside of the chimney that lets you collect the ashes from the outside.) The ash box lets air in like no other.

When we remodeled the house, I had the contractors run a gas line to the fireplace. I cannot see us every burning wood in the fireplace (unless there is no power/gas for days and desperate times calls for desperate measures). Heck, we may never even burn gas in the fireplace.

So my questions are these:
-Can I seal up the ash box? If so, what' the recommended way to do this? Buy some concrete and seal it up like that?
-Since the flue won't seal all the way, I was thinking about buying a chimney cap and sealing it from the top. Any pros/cons/recommendations in regards to this?

Thanks!