Cathedral ceiling assemblies and unvented conditioned attics often suffer from problems with damp roof sheathing. For years, building scientists have been proposing a variety of solutions to these damp sheathing problems. One possible solution (at least in hot climates) is to abandon the practice of installing ventilation baffles under roof sheathing, and instead to promote a new kind of ridge vent called a vapor diffusion port.
A vapor diffusion port — also known as a vapor diffusion vent, a diffusion vent, or a vapor vent — is located at the ridge of a gable roof or at the hips of a hipped roof. In some ways, a vapor diffusion port resembles a ridge vent. Like a ridge vent, a vapor diffusion port requires the roof sheathing to be cut back for several inches on both sides of the ridge. Unlike a ridge vent, however, a vapor diffusion port is airtight.
After creating open slots near the ridge — by either stopping the roof sheathing short of the ridge, or by cutting back the roof sheathing where necessary — workers cover the open slots with a vapor-permeable material like vapor-permeable roofing underlayment or gypsum-based sheathing. This vapor-permeable material is taped to the OSB sheathing on all sides, ensuring an airtight installation. Finally, the vapor diffusion port is protected by conventional ridge vent flashing.
Attics and cathedral ceiling assemblies with vapor diffusion ports don’t need soffit vents. In fact, if you are retrofitting vapor diffusion ports into an older house, you have to seal the soffit vents carefully. The aim is to create a relatively airtight attic.
Here’s the theory behind vapor diffusion ports: moisture in attics or cathedral ceiling rafter bays tends to concentrate near the ridge. Whenever the outdoor air is less damp than the air at the top of the attic, a…