If there is moisture present at the gable end of a brick or stone building and the roof system is working and the flashing system is working around any chimney’s that extend past the roof line, the moisture may be due to condensation. A column of air that exists, in used or unused and capped-off tight chimneys, moves the expanded air through a natural draw upward and the moisture content in the air may be condensed by change in pressure, (dew point), or thermal shock, (at the line where the air in the chimney goes through the roof). All masonry, wood and building materials have an “R” value which means “restriction” to airflow. So a cap that seals a chimney does restrict airflow but does not eliminate it. It takes approximately 11 inches of masonry to equal the “R” value in 1 inch of wood to demonstrate approximate value differential. Air which tries to move up and out but condenses near the top gets quickly absorbed into an unlined and capillary filled brick masonry units of the chimney. The plaster, the lime bedding mortar and soft salmon brick are highly absorptive.
The moisture problem is compounded when a high efficiency gas or oil heating system pumps out an exhaust, into an uncapped/unlined chimney, that is practically nothing but hot air full of moisture and some unburned impurities. Even in the case of a lined chimney, the liner itself should be insulated on the outside of the liner within the walls of the chimney. The “salmon” un-vitrified center of the brick exfoliates on to the floor in the attic as salt crystals, (efflorescence), build up under the “fired skin” of the brick. Salt build-up jacks out the face of the brick and even the continually building up “salt skins” to finally hollow out the brick faces. The moisture wants to move toward the inner warmer attic instead of the colder temperatures outside of the building. Unused chimneys should be filled with free-flowing vermiculite masonry fill insulation and capped off at the top. Liners should be cast-in-place lightweight concrete which is creosote/acids resistant and insulative, or lined with stainless steel with an insulative blanket or vermiculite placed around the outside of the liner with in the walls of the chimney chase. Unfortunately even non-rusting stainless steel liners may deteriorate in time by acids in creosote eating holes in the liner. The stainless option is less expensive at the front-end of installation. The poured-in-place type is an excellent long-term value if installed properly.