One thing is for sure, buildings are never constructed exactly as they are designed. Tolerances, differences in workmanship, and unrealistic expectations are all responsible for variations that exist between the actual design and completed construction. No masonry standards specifically regulate how much and which types of variation from design are allowed within a cavity wall. But recognizing which variations are acceptable is the key to providing a cavity that functions properly. A cavity commonly consists of two components:insulation board and an airspace. It is essential to install rigid board insulation in many types of structures, such as industrial buildings and public institutions, where an exposed interior CMU wall is desired. The current edition of the Masonry Standard Joint Committee (MSJC)'s Building Code Requirements for Masonry Structures limits the maximum width of a cavity to 4 1/2 inches. Two conditions ultimately will determine the makeup of a cavity--the size of the airspace (drainage space) and the required R-value of the wall. The airspace should be 1 1/2 to 2 inches wide--large enough that masons can keep it mostly free of mortar. An airspace less than 11/2 inches wide is difficult to keep clear of mortar droppings, and an airspace less than 1 inch wide is almost impossible to keep clean. In addition to mortar droppings, the airspace can be affected by the misalignment of rigid board insulation. Some misalignment always occurs, but it can be minimized by specifying that the board be mechanically fastened or adhered to the CMU backup. There are three ways in which a constructed cavity will vary from the design detail: (1) Some decrease in cavity size is acceptable due to allowable construction tolerances, such as the outer wythe of masonry being out-of-plumb. (2) Another variation is that mortar droppings and extrusions inevitably will occur in the airspace regardless of its width. Mortar in an airspace is considered excessive when it impedes the flow of water down to the wall's flashing and weep-hole system. (3) The vertical misalignment of rigid board insulation is another common variation from the cavity design. Some variation is inevitable due to construction and material tolerances. But it is not acceptable for the board insulation to be so misaligned that it exposes the cavity to water bridging.