Which factor most directly affects joint sealing between brick and CMU masonry?

• A. Changes in moisture content over time in brick and CMU.
• B. The thickness of the brick shelf cast in the foundation wall.
• C. Changes in the direction of the exterior cavity wall construction.
• D. Quantity of rigid insulation in the exterior cavity wall construction.

Answer: (A)

Moisture-driven dimensional changes in masonry units drive how joints between brick and CMU perform. Brick and CMU absorb and release moisture over time, causing them to swell when wet and shrink when dry. This ongoing moisture cycling produces movement at the interface between the two materials. Sealants must accommodate this movement without losing adhesion or cracking. If moisture fluctuations are not accounted for in joint design and the sealant’s flexibility and adhesion, leaks and failure can occur.

The other factors don’t directly control joint sealing behavior at the brick–CMU interface: the thickness of a foundation shelf mainly affects water shedding there, the direction of exterior cavity wall construction changes detailing but not the fundamental joint movement, and the amount of rigid insulation influences thermal performance rather than the direct moisture-related movement that governs joint sealing.