Diffusion Drying

Diffusion drying represents the moisture removal mechanism driven by vapor pressure gradients through building envelope materials. This process follows Fick’s law of diffusion and enables moisture redistribution from high concentration regions to low concentration regions, ultimately allowing accumulated moisture to exit building assemblies.

Vapor Diffusion Outward

Outward vapor diffusion occurs when interior vapor pressure exceeds exterior vapor pressure, driving moisture from inside to outside. The vapor flux depends on the vapor pressure difference, material permeability, and assembly thickness. This drying direction dominates during heating seasons in cold climates where interior relative humidity typically exceeds exterior dew point temperatures.

The vapor flux through homogeneous materials follows: J = -δ × (dp/dx), where J represents vapor flux (kg/m²·s), δ is material permeability (kg/m·s·Pa), and dp/dx is the vapor pressure gradient (Pa/m).

Permeability Requirements

Effective diffusion drying requires materials with sufficient vapor permeability in the drying direction. The total vapor resistance of an assembly determines diffusion drying potential. Low permeability layers (vapor retarders) must be positioned appropriately to prevent moisture accumulation while allowing seasonal drying.

Materials with permeance greater than 10 perms provide substantial diffusion drying capacity. Semi-permeable materials (1-10 perms) offer moderate drying potential. Vapor impermeable materials (<0.1 perm) severely restrict diffusion drying and require careful placement to avoid trapping moisture.

Drying To Exterior

Exterior drying paths enable moisture removal toward the outside environment through vapor-permeable exterior sheathing, water-resistive barriers, and cladding systems. Exterior insulation systems must maintain adequate vapor permeability to facilitate outward drying. Impermeable exterior finishes like vinyl wallcoverings or low-perm exterior insulation can severely restrict exterior drying.

Wall assemblies designed for exterior drying typically incorporate vapor retarders on the interior side and vapor-permeable materials on the exterior side. This configuration permits inward-driven moisture (from rain penetration or exterior moisture sources) to dry toward the inside while preventing outward moisture accumulation.

Drying To Interior

Interior drying paths allow moisture removal toward conditioned spaces through vapor-permeable interior finishes and gypsum board. This drying direction becomes critical when moisture enters assemblies from exterior sources such as rain penetration, wind-driven moisture, or construction moisture.

Latex paint on gypsum board typically maintains 5-10 perms, providing adequate interior drying capacity. Multiple coats of low-permeability paints (enamel, oil-based) or vinyl wallcoverings can reduce interior drying to unacceptable levels. Interior vapor retarders (polyethylene sheeting) completely eliminate interior drying potential.

Bidirectional Drying

Bidirectional drying assemblies permit moisture removal in both interior and exterior directions, providing maximum drying resilience. These assemblies avoid vapor impermeable layers on either side, allowing seasonal vapor drive reversal and moisture redistribution from multiple sources.

Climate-responsive design considers seasonal vapor drive direction. Mixed-humid climates experience significant vapor drive reversal between heating and cooling seasons, making bidirectional drying particularly beneficial. Vapor semi-permeable materials (1-10 perms) on both sides enable adequate bidirectional drying while providing some vapor flow control.

Engineering Considerations

Diffusion drying rates depend on vapor permeability, vapor pressure gradients, temperature, and material moisture content. Typical diffusion drying rates range from 0.01-0.1 kg/m²·day under normal conditions, significantly slower than ventilation or air leakage drying mechanisms.

Material permeability increases with temperature and relative humidity. Wood products exhibit 2-3 times higher permeability at 90% RH compared to 50% RH. This moisture-dependent permeability enhances diffusion drying when moisture content rises.

Cold-side condensation surfaces within assemblies can trap moisture if vapor retarders prevent diffusion in the appropriate direction. The temperature profile through the assembly determines condensation plane location, which varies seasonally and must be evaluated for both heating and cooling conditions.

Design Applications

Assembly design must balance vapor flow control with drying potential. Vapor retarders control moisture entry but can trap moisture from other sources. Smart vapor retarders with variable permeability provide vapor flow control when assemblies are dry while enabling enhanced drying when moisture content rises.

Roof assemblies require careful permeability design due to upward vapor drive from interior sources and downward solar-driven vapor drive. Conventional wisdom placing vapor retarders at the warm side must be modified for climate-specific conditions and assembly configuration.

Foundation walls and below-grade assemblies face continuous inward vapor drive from ground moisture. Interior vapor retarders would trap this moisture, requiring vapor-permeable interior finishes for interior drying. Exterior insulation must maintain sufficient permeability for outward drying during drier periods.