Sheathing a Vaulted Ceiling

Framing and Sheathing

• To provide for proper ventilation, a cathedral ceiling must have an air space between the exterior roof sheathing and the interior ceiling, and the need for this space has to be addressed during the framing stage of construction. The use of scissor trusses, in which the interior ceiling is formed by the bottom chords of the truss rather than the roof rafters, provides a small attic space that will allow airflow under the sheathing. If there is no attic space, adequately large roof rafters should be used so that there is enough room for an air space between the ceiling insulation and the roof sheathing.

Insulation

• The goal of insulation in a vaulted ceiling is not just to keep the interior of the building warm but to keep warm air from producing condensation above the ceiling and causing the roof sheathing to rot. One technique for achieving this goal is to fill the spaces between rafters with batt insulation then sheath the top of the rafters with rigid foam insulation. Two-by-four strapping is then nailed to the top of the rafters, and plywood sheathing is attached to this strapping, creating a small air space between the sheathing and the rigid insulation. This combination of extra insulation and a ventilation space helps keep the roof cool and limits the chance of condensation under the sheathing.

Vapor Barriers

• Other methods of insulating vaulted ceilings attempt to stop moisture from entering the ceiling and, therefore, eliminate the need for roof ventilation entirely. These methods use vapor barriers between the interior space and the rafters to prevent moisture from entering the ceiling and dense insulation between the rafters to stop moisture migration from the ceiling to the roof sheathing. Advocates of these methods claim that the moisture barriers make ventilation space unnecessary and allow roof sheathing to be attached directly to the rafters.

Ventilation

• If ventilation is required, it can be installed by adding soffit vents under the eaves and a continuous ridge vent at the peak of the roof. These vents will create a path for air to flow in at the edge of the roof and out at the peak, creating air movement under the sheathing and removing moisture that could cause the sheathing to decay.