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To Draw a Special Wall including Split or Overhead or Floating or internal thermal mass walls you use the new Wall button in the left toolbar or the keyboard shortcut 'N'

Special Walls needs to  begin within a zone, where they finish is not important. Regardless of where they are moved to, a special wall always remains a part of the original parent zone. 

The first click will be the left edge of the wall considering the wall facing outwards, and the second click finishes the wall. Hold shift for straight lines, or right to reset points, or enter manual wall length by typing, as per the other drawing modes.