Glossary term
Infiltration
The process by which water enters soil, porous media, or building envelopes from the surface or surrounding environment.
Definition
processInfiltration is the entry of water, air, or another fluid into a porous medium, soil, structure, or enclosure.
In hydrology, infiltration is the movement of rainfall or surface water into soil. It controls runoff, groundwater recharge, erosion, slope stability, irrigation performance, and stormwater design. In buildings, infiltration often refers to uncontrolled air leakage through the envelope, affecting heating and cooling load, moisture transport, indoor air quality, and comfort. The exact meaning must be read from context.
Infiltration describes fluid entering a medium or enclosure. In environmental and civil engineering, it most often means water entering soil from rainfall, irrigation, snowmelt, or ponded surface water. In building physics, it often means outdoor air entering through cracks, joints, penetrations, doors, windows, and envelope defects. Both meanings involve flow through openings or porous pathways, but the governing details differ.
Soil and hydrologic infiltration
In soil, infiltration begins at the surface and proceeds downward under capillary suction, gravity, and pressure gradients. The rate depends on soil texture, structure, compaction, moisture content, vegetation, crusting, slope, rainfall intensity, surface storage, and hydraulic conductivity. Dry soil can initially absorb water quickly because capillary suction is high. As the soil wets, infiltration may decrease toward a more stable rate.
If rainfall intensity exceeds infiltration capacity, excess water becomes surface runoff. This is central to stormwater design, flood modelling, erosion control, irrigation, groundwater recharge, and slope stability. Infiltration can also be undesirable when it carries contaminants into groundwater or weakens soil around foundations, retaining walls, or embankments.
Building infiltration
In buildings, air infiltration is uncontrolled leakage across the envelope caused by wind pressure, stack effect, and mechanical ventilation imbalance. It can increase heating and cooling load, carry moisture into assemblies, reduce comfort, introduce pollutants, and make ventilation control less predictable. Airtightness testing, air barriers, sealing, pressure diagnostics, and commissioning are used to control it.
Water infiltration into buildings is a durability problem. Rain penetration, groundwater entry, capillary rise, failed flashing, roof leaks, and poorly sealed penetrations can damage insulation, finishes, structural elements, electrical systems, and indoor air quality.
Measurement and modelling
Hydrologic infiltration may be measured with infiltrometers, rainfall simulators, soil cores, or field water-balance methods. Building air infiltration may be measured with blower-door testing and tracer-gas methods. Water leakage is often assessed through visual inspection, spray testing, moisture measurement, and pressure testing.
Common mistakes
A common mistake is using a single infiltration rate as if it were constant. Soil infiltration changes with time, moisture, compaction, vegetation, and surface condition. Building infiltration changes with wind, temperature difference, ventilation operation, occupant behaviour, and construction quality. Good documentation states the medium, driving pressure, measurement method, moisture condition, units, and whether the value is peak, average, design, or measured performance.