All masonry chimney construction materials will suffer accelerated deterioration as a result of prolonged contact with water. Masonry materials deteriorate quickly when moisture that has penetrated the materials freezes and expands causing extreme stress. Water in the chimney also causes rust in steel and cast iron, weakening or destroying the metal parts.
Most masonry materials are porous and will absorb large amounts of water. Common brick is like a sponge, absorbing water and wicking moisture to the chimney interior.
Several products have been developed specifically for use as waterproofing agents on masonry chimneys. These formulas are 100% vapor permeable, which means that they allow the chimney to breathe. Therefore, water that has penetrated and the vapors produced when the chimney dries out or the water vapors produced during use are allowed to escape, while the waterproofing agent prevents water from entering from the outside. Paint or clear sealers should never be used as a waterproofing agent because they will trap water vapors and moisture inside the chimney causing further deterioration. When damage or deterioration (gaps, voids, cracks, missing mortar, etc.) already exists in a masonry structure, the chimney should be repaired before the waterproofing agent is applied.