If I make sure to purchase very high PSI blocks and use a lot of rebar is it really necessary to fill the cores of the blocks? It seems to me that the heat transfer would be much less with an air chamber in the walls. If the air provides natural and free insulation it seems illogical to fill this cavity if the blocks themselves are very strong. Also, If the blocks are strong and the walls are constructed very well is it really necessary to cover the blocks with mortar? Why not just pay to have the blocks laid really good and then just paint them directly?