Flashing is metal such as copper or aluminum that is applied in long strips on a roof where different roofing materials meet. It is especially important around the masonry of the chimney to keep this area free from leaks. Moisture that creeps down the exterior sides of a chimney forms a condensation that mixes with brick dust and other debris. This acidic mixture stains interior walls. The stained wallpaper or plaster must be removed and the walls left to dry before the moisture damage can be repaired.
The damper on your chimney acts as a barrier to keep out animals and debris when the chimney is not in use. However, no damper is fully waterproof and rain can get into a chimney even when the damper is completely closed. Many masonry chimneys are lined with clay tiles that can break down over time. The condensation can run down the interior of the chimney to find its way into unprotected bricks and mortar, causing damage.
A chimney with adequate ventilation at the top and bottom will maintain a uniform temperature and be free of condensation. However, it can be difficult to maintain a steady temperature throughout the chimney whether the chimney is in use or not. Cooler temperatures at the top will cause condensation that mixes with the byproducts of combustion in the chimney. The resulting creosote is flammable and can start a fire, which may jump from the chimney to the interior walls and roofs, destroying the house.
Prevent chimney and interior wall damage from condensation by inspecting the exterior chimney flashing and replacing it if necessary. Install a chimney cap to keep rain from running down the interior walls of your chimney and forming acids that will eat away at the masonry. A chimney liner with insulation will protect the interior masonry of your chimney. It will keep the chimney at a constant temperature and will not allow harmful condensation from rain or uneven temperatures to form. A regular inspection of your interior and exterior chimney masonry may also help prevent further condensation damage.