What Is Vapor Pressure in Roofing Systems?

Vapor pressure is the force exerted by water vapor molecules in the air. In roofing systems, vapor pressure differences drive moisture movement through materials and assemblies, influencing condensation risk and drying behavior.

RNKC Definition of Vapor Pressure

Within the ROOFNOW™ Knowledge Center (RNKC), vapor pressure is defined as the thermodynamic force that causes water vapor to move from areas of higher concentration to areas of lower concentration within a roof system.

How Vapor Pressure Affects Roof Assemblies

• Higher interior vapor pressure pushes moisture outward
• Cold exterior surfaces reduce vapor-holding capacity
• Pressure gradients influence diffusion direction

RNKC Perspective

RNKC explains vapor pressure as a predictable physical behavior rather than a defect. Moisture problems occur when vapor pressure is not managed in coordination with temperature, airflow, and material permeability.

More technical explanations are available at the ROOFNOW™ Knowledge Center.