Common Radon Entry Points
There are four main factors that permit radon to seep into homes. All homes have some type of radon-entry pathway:
- Uranium is present in the soil nearly everywhere in the United States.
- The soil is permeable enough to allow radon to migrate into a home through the slab, basement or crawlspace.
- There are pathways for radon to enter the basement, such as small holes, cracks, plumbing penetrations and sump pumps.
- A difference in air pressure between the basement or crawlspace and the surrounding soil draws radon into the home.
Radon enters through:
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- cracks in otherwise solid floors;
- gaps in suspended floors;
- cracks in walls;
- cavities inside walls;
- gaps around service pipes;
- construction joints; and
- the water supply.
How does air pressure affect radon entry?
The air pressure in a house is generally lower than in the surrounding air and soil, particularly at the basement and foundation levels. This difference in pressure causes a house to act like a vacuum, drawing in air containing radon, as well as other soil gases, through cracks in the foundation and other openings. Some of the replacement air comes from the underlying soil and can also contain radon.
One reason this pressure difference occurs is because exhaust fans remove air from inside the house. When this air is exhausted, outside air enters the house to replace it. Another cause of a pressure difference is that warm air rises and will leak from openings in the upper portion of the house when temperatures are higher indoors than outdoors. This condition, known as a stack effect, causes unconditioned replacement air to enter the lower portion of the house.
Does foundation type affect radon entry?
Because radon can literally be sucked into a home, any home can potentially have a radon problem. All conventional house construction types have been found to have radon levels exceeding the action level of 4 pCi/L.
Basement
Radon can enter through floor-to-wall joints, control joints, and cracks in the slab.
Crawlspace
The vacuums that exist within a home are exerted in the crawlspaces, causing radon and other gases to enter the home from the area below. Even with crawlspace vents, a slight vacuum is still exerted in the crawlspace. Measurements of homes with crawlspaces have shown elevated radon levels.
Slab-on-Grade
Radon can enter a home regardless of whether it has a basement. Slabs built on grade can have just as many openings to allow radon to enter as do basements.
Manufactured Homes
Unless these buildings are set up on piers without any skirting placed around them, interior vacuums can cause radon to enter these types of homes as well.
Can radon be kept out by sealing all of the cracks?
Sealing large cracks and openings is important when sealing a home, both in the lower portion of the home to reduce radon entry points, and in the upper portion of the home to reduce stack effect. However, field research has shown that attempting to seal all of the openings in a foundation is both impractical and ineffective as a stand-alone technique. Radon can enter through very small cracks and openings, and these can be too small to locate and effectively seal. Even if all cracks could be sealed during construction, which would be costly, building settlement may cause new cracks to occur. Therefore, sealing large cracks and openings is one of the key components of radon-resistant construction, but is not the only technique that should be employed.
1. Install a sub-slab or sub-membrane depressurization system.
The objective of these systems is to create a vacuum beneath the foundation that is greater in strength than the vacuum imposed on the soil by the house itself. The soil-gases that are collected beneath the home are piped to a safe location to be vented directly outdoors.
Usually, a 4-inch layer of clean, coarse gravel is used beneath the slab to allow the soil gas to move freely underneath the house. Other options include installing a loop of perforated pipe or a soil-gas collection mat (also known as drainage mat or soil-gas matting).
2. Use mechanical barriers to prevent soil-gas entry.
Plastic sheeting, foundation sealing, and caulking can serve as barriers to radon entry, as well as the entry of other soil gases, and of course moisture. Polyethylene sheeting should be placed on top of the gas-permeable layer to help prevent the soil gas from entering the home. The sheeting also keeps concrete from clogging the gas-permeable layer when the slab is poured.
Sealing and caulking help reduce stack effect, and thus reduce the negative pressure in lower levels of the home. Also, sealing and caulking the rest of the building envelope reduce the stack effect in the home.
3. Install air-distribution systems so that soil air is not mined.
Simply adding the vent pipe and junction box is extremely effective for reducing radon, and it’s so cost-effective that even Habitat for Humanity, which relies on donations and grants for its funding, has been adding these features in many of its homes. An electrical junction box is wired in case an electric venting fan is needed later to activate the system.
A 3- or 4-inch PVC or other gas-tight pipe (commonly used for plumbing) should be installed and run from the gas-permeable layer through the house and roof to safely vent radon and other soil gases above the house. Although some builders use 3-inch pipe, field results have indicated that passive systems tend to function better with 4-inch pipe.
Air-handling units and all ducts in basements, especially in crawlspaces, should be sealed to prevent air and radon from being drawn into the system. Seamless ducts are preferred for runs through crawlspaces and beneath slabs. Any seams and joints in ducts should be sealed.
What pulls the soil gas through the pipe?
If the pipe is routed through a warm space (such as an interior wall or the furnace flue chase, following local fire codes), the stack effect can create a natural draft in the pipe. Because this method requires no mechanical devices, it is considered a passive soil-depressurization system.
If further reduction is necessary to bring radon levels in a home below the action level of 4 pCi/L, an in-line fan can be installed in the pipe to activate the system. The system is then considered an active soil-depressurization system. The future installation of the fan can be made easier with a little planning during construction.
Radon gas is approximately seven times heavier than air. It is a “noble” gas with no chemical affinity, but it is influenced by air movements and pressure. In a house with forced-air heating and cooling, radon gas can be easily distributed throughout the entire dwelling. When radon gas is discharged via a radon mitigation system above the roof, the radon concentration depletes dramatically with distance from the point of discharge. In fact, the radon gas concentration approaches background levels at 3 to 4 feet from the discharge point. The EPA disallowed ground-level discharge of radon primarily because of the potential for re-entrainment of the gas into the house, and because of the possibility of children being exposed to high radon levels. The concentration of radon gas at the discharge point can be tens of thousands of picocuries per minute.
Daily Variations Inside a House
Indoor radon levels depend upon a number of variables and can fluctuate significantly from day to day. Short-term tests (particularly tests between two to five days) may, in some cases, reflect an unusual peak in the radon concentration, thus indicating a need for remedial action that may not actually be necessary.
Pressure and temperature differentials, weather conditions such as wind and rain, and the operation of mechanical equipment all contribute to fluctuating levels of radon inside a house. During cold-weather seasons with closed-house conditions, elevated radon levels can be found in the lowest level of the house.
Air pressure inside a home is usually lower than air pressure in the soil around the home’s foundation. Because of this difference in pressure, the house acts like a vacuum, drawing radon in through foundation cracks and other openings.
A house sucks in air like a vacuum.
Radon entry by air pressure from below grade is the main way radon enters a house. When air exits a house, air-pressure differentials between the indoors and outdoors are created.
Wind-induced pressure differentials acting on a structure’s shell may affect both radon entry into the structure, and indoor radon displacement that exits the structure, depending on the wind speed, direction, frequency, wave span, and the structure’s features. Wind blowing directly toward a side of a structure may cause an increase in pressure at the structure’s wall in order to conserve the change in momentum initiated by the change of wind velocity from the free-stream area to almost zero at the wall-side.
The most significant convective component of radon transport from the sub-structure area into the interior, and from the interior to the outdoors, is due to the pressure-driven air-flow processes. Mechanisms that generate air-pressure gradients depend on environmental and indoor operational factors. The environmental factors that induce pressure differences include temperature differences, wind, meteorological conditions, and atmospheric pressure changes. The indoor operational factors can be divided into human- and non-human-induced indoor operational factors. The non-human factors result from mechanically induced pressurization or depressurization of the indoor environment by household appliances, as well as by heating, ventilation, and air-conditioning (HVAC) systems. Human-induced indoor operational factors are characterized by effects such as the opening of windows and doors.
