Staining on the Interior & Exterior Faces of the Chimney
What do we have here? The staining on the siding at the chimney at first appears to be a case of roof leakage due to poor chimney flashing (See featured photograph.) However, the moisture staining is relatively uniform with a white and brown coloring. The uniformity indicates that it is not emanating from the roof, but coming through the siding. The white coloring is characteristic of efflorescence from moisture that is passing through the chimney mortar (Figure 1).
Further examination on the interior (Figures 2 and 3) reveals again a relatively uniform pattern of white and brown moisture staining o the surface of the chimney. The plaster also exhibits deterioration that is more severe than expected for exposure to moisture from the infiltration of roof runoff (Figure 3).
The relatively uniform pattern of moisture staining, that is, not more severe at the top and extending downward and the white color on the moisture staining indicate that the source of moisture is the condensation of moisture from the exhaust gasses from the furnace and the water heater inside the chimney and the migration of the condensation through the chimney walls and the siding.
A qualified furnace installer will be familiar with this condition and probably recommend that the chimney be lined with a stainless steel liner to protect the masonry chimney. They may also need to make other adjustments as necessary to the furnace to reduce the output of moisture.