In the world of building science and environmental engineering, understanding how substances move through materials is crucial for preventing moisture damage and controlling the spread of residual contaminants. Most importantly, however, this knowledge is needed to design effective systems to prevent water and airborne contaminants from entering a building.
Advection, diffusion, and permeability
Advection, diffusion, and permeability – also known as transport mechanisms - each describe a unique way that fluids, vapors, or molecules move through materials:
Advection is the large-scale movement of substances, via the flow of fluids or through the air. In building science, this manifests as:
- Water penetration through wall assemblies during wind-driven rain
- Air leakage, which can carry moisture through gaps in air barriers
- Contaminants moving with bulk water flow
Prevention strategies include:
- Proper flashing details
- Comprehensive air barrier systems
- Positive drainage planes
- Capillary breaks
- Protective barrier membranes
Diffusion is a molecular-level process where substances move from areas of high concentration to low concentration.
- The movement of particles is driven by concentration differences, not pressure
- This is naturally occurring without external force
- Can be accelerated by heat
In building science, this might look like the diffusion of a contaminant like PCE (perchloroethylene) up through a concrete basement floor from underground.
Permeability describes a material's ability to allow fluid or vapor passage. Unlike diffusion, it is:
- Pressure dependent
- Influenced by material properties like porosity
- Follows pressure gradients
To illustrate permeability, think of how water drips through the porous material of a coffee filter. That’s basically how it works!
Practical Implications
These transport mechanisms are critical in three key areas of environmental and building protection:
Vapor Intrusion Mitigation (VIM)
In sites with contaminated soils, understanding transport mechanisms is crucial for protecting indoor air quality. Contaminants like PCE can naturally migrate through concrete slabs, moving from areas of high concentration in soil to lower concentration, which are the open spaces inside buildings.
Above-Grade Air Barrier Assemblies
Advection, diffusion, and permeability play critical roles in controlling moisture and air movement across the building envelope. The interplay of these transport mechanisms determines how effectively a building can resist wind-driven rain, reduce energy loss, and maintain indoor comfort. Air barrier systems must prevent bulk air movement while managing vapor transmission.
Below-Grade Waterproofing
Underground structures face constant challenges from water movement. Permeability and advection determine how water and moisture interact with foundation walls and underground membranes. Effective waterproofing strategies must account for both pressure-driven bulk water movement and the molecular-level diffusion of water vapor through materials.
EPRO: Making buildings dry and safe
Understanding transport mechanisms is more than an academic exercise. It's a critical approach to creating healthier, more durable buildings. By recognizing how moisture, contaminants, and vapors move through different materials, architects and engineers can design more effective barriers, predict potential problem areas, and ultimately create environments that are safer, more comfortable, and more resilient to environmental challenges.
At EPRO, we have seen it all – well, almost! – and we can help find the ideal solution for any job site, in any location. Drop us a line to discuss your project’s unique needs.
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