I will be starting the construction of my oven in a few days and I have a question regarding the installation of loose perlite that I will use to fill the area on the sides and top of my oven. I will frame around the oven with hardybacker and metal studs and fill the void as I have seen and read in the plans but I can't find any clear pictures or instructions on how to deal with the area surrounding the flu. I will be using 8" double wall stainless for the flu which of course will be inside the hardybacker with the oven itself. Do I just fill the entire void with perlite and let the loose insulation fill in around and contact the flu itself, or is it best to section off the area around the flu with another piece of hardybacker and leave open air space around the stainless pipe? Thanks for any help you can give me.