Attic of a 100 year old wood frame house in Kleinburg

The Problem
In the first photograph, you can see the wooden rafters, a masonry chimney, attic ceiling insulation and the south facing wall made from 2x4 pine studs, covered by horizontal pine sheathing. In the second picture, you see the pine rafters and sheathing that forms the roof. Why is this pine, which is relatively resistant to decay, rotting away? Why has it survived 100 years and only recently started to decay?

The Causes
This is a "classic" attic moisture problem. For decay to occur, the wood must be sustained at a moisture content in excess of 20%, and at a temperature above 5 degrees Celsius. The roof sheathing has been exposed to conditions that allow for decay. As you can see from the effloresence, some moisture is entering the attic from the chimney. Rain leakage is a possibility but the shingles are in good condition and the sheathing decay is widespread. Ordinarily, when small roof leaks occur and remain undetected, decay is localised to the leaky area. Moisture could be diffusing through the second floor ceiling, but this is unlikely due to the many layers of oil-based paints that have been applied over the years. This leaves the mechanism of gross air movement. During the winter, the air inside of the heated part of the house is warmer, it contains more moisture, and it is at a higher pressure than the air in the attic. This pressure difference leads to the movement of air from the house to the attic. The air travelsthrough the inevitable leaks around the chimney, vents, attic hatches, partition wall intersections and electrical ceiling fixtures. The moisture comes from the warm moist air condensing on the relatively cold roof sheathing.

The question remains, why has this roof survived for so long and only recently started to decay? The relatively recent installation of the thermal insulation is responsible for keeping the attic cooler, which makes it less able to hold moisture. This creates pressure and temperature gradients between the attic and the rest of the house.

Solution
Simply replacing the sheathing would be treating the symptom and not the disease. Adding vents to the attic may help, but there is a chance that they might reduce the air pressure in the attic, thereby increasing the pressure gradient across the ceiling, making the problem worse. The solution lies in temporarily removing the insulation and properly sealing the leaks in the ceiling. Had the ceiling been sealed before the installation of the insulation, this whole problem would not have occurred.