Pressure Driven Airflow

Radon entry by pressure driven airflow from below grade is the dominant entry mechanism in the majority of buildings with elevated radon levels. Pressure driven airflow drives soil gas from areas of higher to lower pressure.  These pressure differentials drive soil gases that are produced by temperature differences, wind pressures, barometric pressure, and the displacement of soil gas by rainwater. The atmosphere, soils, fissures, caverns, discontinuities in soils, and building interiors comprise a network of zones that are connected to each other by air passages.

Pressure driven airflows can significantly increase stack effect, which can result in radon spiking to abnormally high levels in the home. 

Factors that contribute to increased pressure driven airflows are:

  1. Air Pressure Differentials          4.   Mechanical Equipment
  2. Temperature Differentials        5.   Hydraulic Pressure
  3. Wind Induced Airflows

Air Pressure Differentials

Airflow between these zones is determined by the pressure differential between them and the gas flow resistance of the material comprising each zone. For example, soil gas flow is poor in clay soils that have a great deal of resistance, better in sandy and gravelly soils, and best in open passages. As air in the soil flows past radium, it carries radon away from its source and forms an underground gas stream of elevated radon concentration. This soil gas follows the paths of least resistance from higher to lower gas pressures.

When air escapes from a house (exfiltration), air pressure differentials between the inside and outside are created. This results in air being pulled into the house (infiltration) to replace the air that has left. This convective current puts a suction effect on the lower part of the building, drawing in air. The air may be drawn out of the house by a mechanical ventilation device such as a fan, clothes dryer, or combustion appliance. It may also escape because of the tendency of the warm air in the house to rise.

Neutral Pressure Plane

When warm air rises, the upper floors of the house are slightly pressurized, while the lower floors are depressurized. As illustrated in Figure 3-7, the place where these zones of pressurization and depressurization meet is called the neutral pressure plane, which determines the direction of interior convective airflows only. The location of the neutral pressure plane is determined by the temperature of the house relative to the outdoors, the amount of air leakage at the top and bottom of the structure, and the presence of mechanical ventilation devices

When the lower part of a house is depressurized in this way, air enters through cracks and holes located over the building envelope below the neutral pressure plane. In this situation, the house substructure is under negative pressure and creates suction on the surrounding soil.  It is estimated that between 5 and 20 percent of the infiltrating air enters from below ground level and can carry radon in with it.

Soil Permeability Affects Airflow

  • Poor in clay soils (great deal of resistance)
  • Better in sandy and gravelly soils
  • Best in open passages
  • Air in the soil flows past radium
    • carries radon away from its source
    • forms an underground gas stream of elevated radon concentration
  • Soil gas follows the paths of least resistance from higher to lower gas pressures.