Liz, there is a screened cap on top of the chimney, but there is always a collection of spiders, spider ball thingies, pill bugs and occasional snails in there. I doubt anything I could achieve with insulation would be remotely airtight, but I'd like to keep the draft out of the house and the moisture down in that area. This is just a thought - the fireplace is rather attractive as I have it arranged now, with a mirror propped on the back of the log support and a plank in the middle of it that supports a multi-candle stand. I light that occasionally when I have company and it has the beauty of a fire without the puffs of smoke when the fire is going out. But walling it off entirely could involve taking off the glass doors and inserting a shallow cabinet built to fit it and look like a couple of shelves in the stone wall when in place. Or leaving the doors and putting the television in front of it.

Here's a photo of it - this really is an ugly fireplace. I had moved a speaker off (visible on the far left) that usually sits in the corner, and the various dog-walk related items that sit on the edge near the door. It is just a place to set stuff, it isn't an attractive feature. But I need to think about "staging" it to see if it would look better. I put lovely things on the mantle and distract from the wall behind it.

SRS