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Transport Mechanism #4 – 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.

Figure 3-7
Neutral Pressure Plane
Source: EPA

This convective current of warm air rising and makes the house behave like a chimney.  In fact, this temperature-driven airflow is often called the stack effect, as it is powered exactly the same way as the draft on a chimney. As a chimney draws air in at the bottom, it forces the air out the top, which results in a neutral pressure plane somewhere along its length.  Airflows due to temperature differences are also called convective airflows.

Figure 3-7 shows the relationship between radon concentrations and air pressure differentials. As the pressure between the inside and the outside of the house decreases, less suction is being applied to the sub-grade soil beneath the basement. This condition causes less soil gas to be pulled into the house thereby reducing the radon concentrations. The neutral pressure plane determines the direction of interior convective airflows only. Below the neutral pressure plane (the negative side), the convective flow is towards the neutral pressure plane. Above the neutral pressure plane (the positive side), the convective flow is away from the neutral pressure plane.

Figure 3-8
Natural Stack Effect
Source: Capstone
Stack Effect [stak ih-fekt]
Stack effect is temperature-driven airflow.  This upward movement of air inside a building results when heated air rises and escapes through openings in the building envelope. This escaping air creates indoor air pressure in the lower portions of a building to be lower than the pressure in the soil beneath or surrounding the building foundation.  

When closed building conditions are disturbed, such as opened windows or doors, it is often assumed that a ventilation effect will occur and radon levels will decrease.  However, Figure 3-9 illustrates the stack effect on a home when closed building conditions are violated in a strategic location above the neutral pressure plane.  In this situation, the homeowner cracked open a second story bedroom window, approximately -2 inches each night during the measurement period for fresh air.

The homeowner assumed the marginal opening of a window located 2 stories higher than the test location would not affect the radon monitor in the basement.

Figure 3-9
Radon Spikes Caused By Increased Stack Effect

Airflow Induced by Temperature Differentials

Figure 3-10
Thermal Bypasses and Negative Pressure Sources
Source: Solaplexus/NYSEO

When outside air is colder than inside air, the warmer indoor air is lighter and tends to float out of cracks and crevices at the top of the building. Buildings have penetrations that allow inside air to flow directly into the attic or to the outdoors. (Figure 3-10) These penetrations are called thermal bypasses, which include construction features such as:

  • Recessed lighting fixtures (by code, these cannot be sealed unless designed for sealing).
  • Air gap around chimneys (no combustibles allowed within 2”).
  • Plumbing chases (especially behind baths and showers).
  • Balloon frame walls (two-story wall studs without fire stops).
Figure 3-11
Effect of Air Temperature Differentials

As depicted in Figure 3-11, warm air flows up through the thermal bypasses, the rest of the house is depressurized. When there is a 40°F temperature differential, the airflow due to stack effect is about 200 cfm for a house of typical size and construction. A vacuum of about .02 inches of water column will be created in the basement under these conditions. Sealing some of these openings and bypasses to limit depressurization is sometimes a secondary activity to support more primary mitigation techniques.

It is important to note that air conditioning causes indoor air to be cooler than outside, which causes the indoor air to sink. This reverses the pressure pattern in the house, and the lower rooms tend to have higher pressure (relative to the outdoors) than the upper floors. Since air tends to enter at the top of the building and exit at the bottom, the reversal of pressurization accounts for one reason that air-conditioned houses exhibit lower radon concentrations.

A 40°F temperature differential between  Indoor temperature and outdoor temperature can  increase Stack Effect and elevate radon levels.
Study Tip

Wind Induced Airflow

As seen in Figure 3-12, wind creates a complex pressure field around a house. This pressure field produces a positive pressure in the soil on the windward side of the building forcing soil gas entry into the building, which produces a spike in radon levels.  Wind can also have the overall effect of producing suction on a house if the openings on the leeward side and top of the building are larger than those on the windward side. Even if the leaks are the same size and uniformly distributed over the walls, wind puts an overall negative pressure on a building, because of the number of large openings found at the ceiling level. The effect is similar to wind blowing across the top of a chimney and inducing a draft, even when there is no temperature-driven stack effect.

Figure 3-12
Effect of Wind Induced Airflow on Radon Test

The effect of wind on indoor radon levels can be quite dramatic.  Figure  3-13 illustrates spiked radon levels in a home due to a microburst, a localized wind disturbance that can often go undetected even by residents in the local area.  The short-term disturbance put sufficient wind-induced pressure on the home to spike radon levels to more than 7-times their “normal” levels. The home had a passive radon mitigation system present.

Figure 3-13
Effect of Wind Induced Airflow on Radon Test
Figure 3-14
Re-test During Normal Conditions After Wind-Induced Radon Spike

Figure 3-14 depicts the graphical data from the re-test conducted at the same property 24 hours after the initial test was conducted. This home had a passive new construction system present, which worked adequately without severe weather disturbance.

Rain and Hydraulic Pressure

Spikes in radon levels can also be associated with rainfall. As shown in Figure  3-15, rainwater saturates the soil, which displaces soil gas and prevents radon from diffusing into the atmosphere. As a result, the concentration of radon in the soil may increase. The rain acts as a sealant across the soil surface preventing radon from escaping to the ambient air.  This effect extends the negative pressure field generated by the house and the increases its strength, enabling the transport of soil gas over greater distances.

Figure 3-15
Effect of Hydraulic Pressure on Foundation

The displacement of soil gas by rain may also tend to force radon into houses by hydraulic pressure. Hydraulic pressure also correlates with radon levels, probably for the same reasons that spikes occur in periods of rainfall. As water tables rise, more soil gas is forced into houses.  Other studies relating barometric pressure to indoor radon are scarce. However, since low barometric pressure is often associated with rainfall, low pressure is likely to track with higher radon levels for the same reasons as for rainfall.

Hydraulic Pressure
Pressure on the Foundation Caused by Water such as Rainwater
Study Tip

Airflow Induced by Mechanical Equipment

Mechanical devices such as kitchen range exhausts, bath fans, dryers, and heating and cooling air distribution blowers can put a suction on a house which requires makeup air to be drawn in from outside. Combustion devices such as fireplaces, woodstoves, furnaces, and boilers also exhaust air from the house. This suction increases stack effect and can elevate radon levels to abnormal levels.

Ventilation Rates and Radon Concentrations

Indoor radon concentrations depend on two factors:

  1. The amount of radon entering the building.
  2. How much dilution occurs when radon mixes with house air and ventilation air.

It is often assumed that the lower the air exchange rate in a house, the greater the radon concentration. This assumes that radon source strength does not change. Several studies have been conducted by measuring radon concentrations and air exchange rates over the same time period. Studies where ventilation rates were actually measured showed no correlation between tightness of house and elevated radon concentrations. This is because changes in the ventilation rate of a house cause many side effects: the neutral pressure plane will be moved; and the negative pressure driving the radon entry will be changed. Reducing the air exchange may even involve sealing of some of the radon entry routes, since these openings also contribute to air exchange.