"Why do bricks need to breathe?" - Because bricks and mortar aren't waterproof. "So what happens if my bricks don't breathe?" - There is a greater chance for you to have moisture problems with your home or building. "What kind of problems?"
The importance of drainage and ventilation is widely acknowledged throughout the Masonry industry. The typical brick or stone wall (masonry veneer) on a building is made of very porous materials which take on a tremendous amount of water during rain