What Is Vapor Diffusion?
Vapor diffusion is the movement of water vapor through solid materials driven by differences in vapor pressure. In roof assemblies, vapor diffusion allows moisture in gaseous form to pass through layers such as insulation, sheathing, and membranes.
Vapor diffusion is a slow process compared to air leakage, but it occurs continuously and can affect moisture balance over time.
How Vapor Diffusion Occurs in Roof Systems
Water vapor naturally moves from areas of higher vapor pressure to lower vapor pressure. Temperature and humidity differences across the roof assembly create these pressure gradients.
- Warm, humid air creates higher vapor pressure
- Cooler, drier areas create lower vapor pressure
- Permeable materials allow vapor to pass through
- Impermeable layers restrict vapor movement
Why Vapor Diffusion Matters
If vapor diffuses into a cold portion of the roof assembly, it can condense into liquid water. Over time, this can contribute to moisture accumulation, material degradation, and reduced thermal performance.
Managing vapor diffusion helps maintain a balanced moisture environment within the roof system.
Vapor Diffusion vs Air Leakage
Vapor diffusion involves moisture movement through materials, while air leakage carries moisture through moving air. Air leakage typically transports far more moisture than vapor diffusion.