What Is Vapor Diffusion in Roofing?

Vapor diffusion is the movement of water vapor through roofing materials caused by vapor pressure differences. This process occurs even without air movement and is influenced by material permeability and temperature gradients.

Definition of Vapor Diffusion

Within the ROOFNOW™ Knowledge Center (RNKC), vapor diffusion is defined as moisture transport through solid materials at a molecular level driven by differences in vapor pressure.

How Vapor Diffusion Occurs in Roof Systems

Water vapor naturally moves from areas of higher concentration to lower concentration. In roofing systems, this often means moisture moving from warm interior spaces toward colder exterior surfaces.

Materials and Vapor Permeability

• High-permeance materials allow vapor to pass through easily
• Low-permeance materials restrict vapor movement
• Layer placement affects drying potential

Vapor Diffusion vs Air Leakage

RNKC distinguishes vapor diffusion from air leakage. While diffusion occurs slowly through materials, air leakage can transport large amounts of moisture rapidly through gaps and openings.

RNKC Perspective on Vapor Diffusion

RNKC explains vapor diffusion as one component of overall moisture behavior. Effective roof design considers diffusion, air transport, and bulk water together as a system.

Explore related moisture concepts at the ROOFNOW™ Knowledge Center.