My house was built in 1967 and has a real working fireplace. The problem is, it allows a ton of cold air into the house. There is a chimney cap (and I climbed up on the roof to verify it still works), as well as glass doors on the front of the fireplace which were clearly made for safety and not sealing the room from cold air. Since cold air was still pouring in I added a chimney balloon.
However on cold mornings I use an IR thermometer and I can see the difference in temperature around the fireplace - it's still letting tons of cold air in. At this point I no longer care if I can use the fireplace for fires (I got an outdoor firepit for when we want to have bonfires and/or make s'mores). What is the best way to stop cold air coming in? Do fireplace inserts effectively seal them off from the outside? I keep searching for info but everything I see assumes I'll be using the fireplace insert to generate heat by burning things (which I don't really plan on doing). I don't want to heat my home with the fireplace, I just want to stop cold air from pouring in.