between 12 inches and 24 inches thick hallowblocks ceiling
For ceiling the 24 inch thick is slightly better, but not for roof as more afternoon heat is stored in the increased mass of the 24 inch blocks. If the ceiling hollows are filled with fiberglass or rockwool, a 2% reduction in electric consumption is possible as the convection currents are reduced for both 12 inch and 24 inch hollow blocks. Neil
