Chimney Sweating In Attic

Normal amounts of humidity and moisture in the hot air condense inside your chimney as the air cools leaving water which can be as acidic as bleach.

Chimney sweating in attic.

The new decking was mostly dry except for droplets of moisture apparently condensate not leakage from the roof on a few nails. For example your roof might have a leak through the attic vent or roof shingle at the top. If there is excessive moisture accumulation in your attic it will condense on cold areas mainly on the underside of your roof deck. If the air inside the home is warm and humid and the walls of the chimney are cold this can result in sweating.

The pattern of wetness is relatively uniform on all the sides of the chimney exposed in the attic. The smaller amounts of heat which do go into the chimney are cooled by the surplus cold air and simply don t have the energy to make it to the top of the stack. During the winter months the recommended relative humidity inside a house is anywhere between 30 50 percent. Staining on the exposed chimney in the attic often reveals which side of the chimney water begins its travel to other areas.

When it s cold outside and the warm humid air inside the home contacts the cold walls of the chimney the warm air will condense on the cold surface and result in dripping. The wetness on the outside faces of the chimney occurs when the condensate migrates through the mortar. A flue that is too large allows too much air to circulate making it difficult for the warm gas and vapors exiting the furnace to create a strong updraft up and out of the vent. The sweating in this case is normal and will cease once a fire is built and the chimney heats up.

In early january we cut an access to observe the chimney and the underside of the roof deck. However the chimney was a different story. Look for stains on the rafters close to the chimney as well as insulation with water damage on the attic floor. When the warm air of the house comes into contact with the cold wall that separates the living space from the chimney water from the air will condense onto the wall s surface.

The same thing can happen with your chimney. Knowing which side water is coming in on is powerful information.