I see all these exercise ball dome builds using a perlite/vermiculite layer

 Wouldn't it be better to have a non insulating inner layer?

And how about regular used red ceramic brick with no fire brick?

Is spalling ever an issue.? 

Thank everyone. I'm all excited about this. I used to work at Pizza Hut and never got tired of pizza. Now I'm working outside Chicago and am getting some Lou Malnati's Deep Dish.