I have a 6.75" x 12", inside dimensions, clay lined chimney. Will a 6" liner and perlite around it be enough? My insert needs a 6" liner. I would insulate but it will not fit. It's an exterior chimney in Pittsburgh, PA, one story house, 15' chimney. I want to do it right but wrapping the pipe is not possible. Any help would be great... local companies want 1.5k to 2k to install a liner.