I have an LP gas fireplace that I believe is direct vent that I'd like to "seal in". It is an insert that is living in my old fireplace spot in the chimney. I want to prevent all outside air coming in from coming down the chimney. The company that installed it just stuffed a bunch of un-faced pink insulation up there around the vent pipes. That looks like a bad idea to me and isn't air tight. I'd like to use rigid foam and fire-blocking spray foam if possible. What's the best way to do this?