Bricking/Insulating new arch questions in regards to 1/2" ceramic insulation

[claystroup]

I just took delivery of my new 2 x 5 XL Mason arch with flat pans and am going to start soon on insulating and bricking it. I plan on using the 1/4" ceramic arch paper Bill supplied along with full size fire bricks in the fire chamber area. My question really concerns the rear raised flat area that is approx. 4' long x 24" wide and 8" deep. From what I have read it seems like firebrick aren't really necessary in that area. I have a 75' roll of 1/2" x 24" wide 1900 degree ceramic insulation that I bought for my previous evaporator left over that I would like to use in this rear area of the arch. I can add multiple layers to build up to as thick as I need. At what point (or thickness) does it make sense to go to before I am throwing money out the window? I am thinking 3 layers would be 1-1/2" total would be about right. Thoughts? One other question-do you insulate under the layer of bricks that makes the floor in the fire chamber or just sit that row directly on the steel bottom?
Thanks, Clay

[psparr]

Three layers should be fine. You'll only want between 1-2" under the back pan. I'd fill that void with vermiculite then lay the blanket on top. The arch was made for a drop flue which is good for future expansion.
If it were me, I'd put insulation under the floor brick. Without auf it can get pretty hot.