Meaning
Atmospheric migration involving volatile organic compounds moving from contaminated soil or groundwater through foundation cracks into indoor spaces characterizes sub-slab vapor intrusion. This process requires a pressure differential that pulls gases upward into a structure. The occurrence stops when indoor air pressure equals or exceeds the sub-slab pressure or when concentrations beneath the foundation drop below actionable regulatory thresholds.
Pressure Dynamics
Airflow movement depends upon the pressure gradient between a lower level and the internal building environment. Sub-slab vapor intrusion creates a pathway when foundation gaps provide limited resistance to gas entry. HVAC systems often modify these gradients, either reducing risk through positive pressure or accelerating intake when balanced incorrectly.
Remediation Measures
Engineering controls prevent gas entry by altering local flow physics. Passive sub-slab depressurization systems rely on natural ventilation to divert gases away from the structure. Active methods employ mechanical fans to maintain a negative pressure zone below the slab, ensuring that air flows toward the exterior environment instead of moving toward the foundation.
Analytical Performance
Laboratory verification determines if chemical concentrations reach hazardous levels during site assessments. Practitioners test soil gas probes and indoor air quality to distinguish ambient background levels from soil-based contributions. Quantitative results verify system efficacy after installation, confirming that mechanical barriers hold the necessary pressure differential to block gaseous contaminants.
