Radon Mitigation - Methods of Mitigation

Methods of Mitigation

Because high levels of radon have been found in every state of the United States, testing for radon and installing radon mitigation systems has become a specialized industry in the last two decades. Many states have implemented programs that affect home buying and awareness in the real estate community, however radon testing and mitigation systems are not generally mandatory unless specified by the local jurisdiction.

According to the EPA's "A Citizen's Guide to Radon", the method to reduce radon "primarily used is a vent pipe system and fan, which pulls radon from beneath the house and vents it to the outside", which is also called sub-slab depressurization, active soil depressurization, or soil suction. Generally indoor radon can be mitigated by sub-slab depressurization and exhausting such radon-laden air to the outdoors, away from windows and other building openings. "EPA generally recommends methods which prevent the entry of radon. Soil suction, for example, prevents radon from entering your home by drawing the radon from below the home and venting it through a pipe, or pipes, to the air above the home where it is quickly diluted" and "EPA does not recommend the use of sealing alone to reduce radon because, by itself, sealing has not been shown to lower radon levels significantly or consistently" according to the EPA's "Consumer's Guide to Radon Reduction: How to fix your home". Ventilation systems can utilize a heat exchanger or energy recovery ventilator to recover part of the energy otherwise lost in the process of exchanging air with the outside. For crawlspaces, the EPA states, "An effective method to reduce radon levels in crawlspace homes involves covering the earth floor with a high-density plastic sheet. A vent pipe and fan are used to draw the radon from under the sheet and vent it to the outdoors. This form of soil suction is called submembrane suction, and when properly applied is the most effective way to reduce radon levels in crawlspace homes."

The most common approaches are active soil depressurization (ASD) and mechanical ventilation (MV). Experience has shown that ASD is applicable to most buildings since radon usually enters from the soil and rock underneath and MV is used when the indoor radon is emitted from the building materials. A less common approach works efficiently by reducing air pressures within cavities of exterior and demising walls where radon emitting from building materials, most often concrete blocks, collects.

• Above slab air pressure differential barrier technology (ASAPDB) requires that the interior pressure envelope, most often drywall, as well as all ductwork for air conditioning systems, be made as airtight as possible. A small blower, often no more than 15 cubic feet per minute (0.7 l/s) may then extract the radon-laden air from these cavities and exhaust it to the out of doors. With well-sealed HVAC ducts, very small negative pressures, perhaps as little as 0.5 pascal (0.00007 psi), will prevent the entry of highly radon-laden wall cavity air from entering into the breathing zone. Such ASAPDB technology is often the best radon mitigation choice for high-rise condominiums as it does not increase indoor humidity loads in hot humid climates, and it can also work well to prevent mold growth in exterior walls in heating climates.
• In hot, humid climates, heat recovery ventilators (HRV) as well as energy recovery ventilators (ERV) have a record of increasing indoor relative humidity and dehumidification demands on air conditioning systems. It is very clear that serious mold problems have originated in homes that have been radon mitigated with HRV and ERV installations in hot, humid climates. HRVs and ERVs have an excellent record in heating climates.
• A recent technology is based on building science. It includes a variable rate mechanical ventilation system that prevents indoor relative humidity from rising above a preset level such as 50% which is currently suggested by the U.S. Environmental Protection Agency and others as an upper limit for the prevention of mold. It has proven to be especially effective in hot, humid climates. It controls the air delivery rate so that the air conditioner is never overloaded with more moisture than it can effectively remove from the indoor air.
• It is generally assumed that air conditioner operation will remove excess moisture from the air in the breathing zone, but it is important to note that just because the air conditioner cools does not mean that it is also dehumdifying. If delta t is 14 degrees or less, it may not dehumidify at all even though it is cooling.
• Factors that are likely to aggravate indoor humidity problems from mechanical ventilation–based radon installations are as follows and an expert radon mitigator/building scientist will check for and correct any and all of the following when he or she performs radon mitigation procedures:

1. Air conditioner duct leaks located outside the breathing zone, such as in the attic.
2. Excessive exhaust fan operation
3. Oversize or over-capacity air conditioners
4. AC air handler fans that do not stop running when the air conditioner compressor stops running.
5. Delta t (Δt), which is the amount that the air is cooled as it is passed through the air conditioner's cooling coils. A good delta t performance figure for home air conditioners is about 20 °F (11 °C). In comparison, automobile air conditioners deliver Δt performance of 32 to 38 °F (18 to 21 °C). A delta t of 14 °F (8 °C) will dehumidify poorly if at all.

In South Florida, most all radon mitigation is performed by use of fixed rate mechanical ventilation. Radon mitigation training in Florida does not include any segment addressing mechanical ventilation or of problems associated with mechanical ventilation systems such as high indoor humidity, mold, moldy odors, property damage or health consequences of human occupation in high humidity of moldy environments. As a result, most Florida radon mitigators are unaware of and do not incorporate existing building science moisture management technology into mechanical ventilation radon installations.

Home inspectors are generally unaware of the mold risks associated with radon mitigation by mechanical ventilation even though mold may make occupants sick, destroy their home and property, and/or be very expensive to clean up and then make the home difficult to sell. In Florida the lawsuits are just beginning.

It appears that in thousands of Florida condominiums and apartments, radon mitigation mechanical ventilation systems were installed in a concealed fashion, escaping even the recognition of professional home inspectors and real estate professionals.