Why is My Chimney Sweating?

Posted by in Chimneys, Masonry, Roof, Winter Concerns | April 07, 2014

When the chimney and/or the framing, plaster, or drywall around it are wet, most assume that the wetness is due to a roof leak.  And, although roof leaks at chimneys are not uncommon, sometimes the wetness is due to another cause.

Interior finish on the chimney exhibits moisture damage, staining, and mineral deposits.

Roof leaks generally exhibit a pattern of wetness or water damage, such as wetness, staining or decay, that emanate from a specific location.  If the pattern of wetness or decay does not emanate from one or few specific locations and some type of breach in the roof system cannot be found, the wetness and subsequent damage are probably not due to a roof leak.

On occasion we have identified the wetness around a chimney to be due to the condensation of the airborne moisture in the exhaust gases discharged by the furnace on the cold chimney walls.  The wetness on the outside faces of the chimney occurs when the condensate migrates through the mortar.  Condensation of exhaust gases are evidenced or accompanied by one or more of the following:

  • The pattern of wetness is relatively uniform on all the sides of the chimney exposed in the attic.
  • The wetness is more severe at the basement and/or the first floor than the second floor or the roof.
  • White and tan mineral deposits on the surface of the chimney, plaster, or drywall.
  • Mortar, plaster, drywall, etc. deterioration from exposure to the acidic condensate.
  • Missing, cracked, or separated flue liner.
  • Exhaust gases in the chimney are from a higher efficiency, non-condensing furnace (usually 85% plus).
  • Excessive corrosion exhibited on metal imbedded in or in contact with the chimney.
  • Higher than normal relative humidity in the dwelling, especially during winter months.
  • The chimney only “leaks” during the cold weather seasons.

Chimney exhibits moisture staining and mineral deposits on all sides.

When a number of these characteristics are found, consider that the wetness and subsequent damage on and around the chimney may be due to the condensation of the exhaust gases and the migration of the condensate through the mortar.

Lower portions of chimney exhibit moisture staining and mineral deposits.

This issue may be addressed by replacing the furnace with one that exhausts through a wall, thus, not introducing exhaust gases into the chimney or by the installation of a non-corrosive flue liner.  Contact a qualified and experienced heating contractor for more ideas.

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