That's a good thing if you implement this design, because firebrick and refractory mortar are still going to let a lot of heat get past. Your design is a poor idea and will never meet code, no offense intended.
There's a lot of code giving the minimum requirements for masonry fireplaces. R1003.5 requires 2" of firebrick and 8" of solid masonry at the firebox walls or 10" of solid masonry alone, for instance. The smoke chamber must be 6" thick solid masonry. Etc..........................
