The moisture in the home, however, was escaping through the open damper. As the moisture cooled inside the flue liner, it condensed and migrated through the cracks in the flue to the brick. Wet or damp basements and crawl spaces are also good sources of moisture that can cause efflorescence, especially around foundation vents. Efflorescence around windows and doors may be a sign of poor drainage or improper flashing in those areas. This should be checked by a qualified home inspector or engineer.
Water trapped behind the brick can cause wood rot and serious structural damage.
To prevent water buildup, brick should be installed with a waterproof membrane fastened to the wood structure behind the brick. A portion of the membrane is laid in the mortar joint of the brick, and holes are left in the joints between some of the brick to act as weep holes. Without the weep holes, the air pressure on the back side of the brick would be less than the pressure on the outside. Tests have shown that water from a gentle spray can penetrate a brick wall that does not have weep holes in as little as two minutes. I have seen wood rot caused by a buried sprinkler system that sprayed against a brick wall.