Applying foam atop the roof's sheathing was the least expensive way to add needed insulation to this home's vaulted ceilings.
Framed with a combination of standard and raised-heel scissors trusses, the conditioned attic was insulated with low-density open-cell foam sprayed on the underside of the roof sheathing.
The foam installation had some voids that could have allowed warm, moist air to reach the underside of the cold roof sheathing and condense; the void shown here corresponded with an area of rotted sheathing near the skylight.
Temperature and humidity data showed abnormally high wintertime relative-humidity levels — almost 50 percent in the home's conditioned attic and even higher in the living space.
While the initial problem appeared to be a leaking skylight, there were other areas on the roof that had deteriorating OSB sheathing and corroding plywood clips and fasteners.
The north-facing roof sheathing over the vaulted living space suffered the most damage, while the sheathing over the unconditioned garage attic (shown here at the far end of the roof) showed virtually no damage at all.