Vapor diffusion is the movement of moisture vapor through building materials driven by differences in vapor pressure.

Vapor diffusion is the process by which water vapor moves through building materials due to differences in vapor pressure between indoor and outdoor environments. Unlike air leakage, vapor diffusion occurs even in airtight assemblies, as moisture migrates molecule by molecule through permeable materials such as drywall, insulation, and wood.

In construction, vapor diffusion is a key consideration in wall, roof, and floor assembly design. Materials are rated by vapor permeability, which indicates how easily moisture can pass through them. Assemblies are often designed with varying permeability layers to allow drying in one direction while limiting moisture accumulation that could lead to condensation, mold, or material degradation.

From a building science and code standpoint, controlling vapor diffusion helps improve durability, indoor air quality, and energy efficiency. Climate zone, occupancy conditions, and interior humidity levels all influence vapor diffusion strategies. Proper coordination of vapor retarders, insulation, and ventilation systems ensures moisture is managed effectively throughout the building envelope.

Even solid materials like concrete and brick can slowly transmit water vapor, which is why vapor diffusion must be considered in both masonry and framed construction.

• Poly Vapor Barrier
• Vapor Barrier
• Vapor Retarder