I would like to insulated all the sides of my firebox except the top. I will be adding a warmer chamber on top. Do I need to enlarge the firebox to account for the added 3/4 to 1' thickness of the walls. I'm worried about the loss of cubic inches of firebox space. Maybe the insulation will account for this in less lost heat. Another thought was to insulate the top also and just have a vent on top for smoke and heat. What are your thoughts on this. I live in Michigan so heat loss can be an issue. Thanks for the inputs.