Walls with cavities consisting of two separate walls with an air gap of not less than 2 inches or more than 6 inches (51 to 152 mm) can be constructed of solid masonry or of hollow units, provided that they meet the specific requirements for columns and mooring beams set out in this section and are joined together at intervals. A cavity is not simply a measure of the distance between the sheet and the structural wall. A cavity in a masonry wall does not fulfill its function if it is filled with rigid insulation. For example, a 50 mm one (2 inches).
The code requires an air clearance of 25 mm.