Moved into a new home and I noticed something about my fireplace. I have a large brick enclosure that surrounds my wood burning fireplace and when I was up in the attic I noticed that the space surrounding the fireplace between the walls is wide open to my attic. I could fit down in the space. I was spraying cellulose insulation and was about to fill in the gaps but stopped. Should I seal these cavities in or is there a need for the gaps?