3.1. Organization of radon prevention and mitigation actions

In this section, several specific points related to prevention and mitigation action in the context of an organized radon programme are discussed. General aspects of the organization of national radon programmes are outlined in detail in chapter 6.

Radon control should aim for an overall risk reduction in the population. This may not be achievable if there are only goals for mitigation in existing buildings. Therefore, prevention goals should also be established to reduce radon concentrations in new dwellings. Without such goal-setting the total number of dwellings with elevated indoor radon will increase when:

1. new dwellings with elevated indoor radon are added to the housing stock;
2. the number of new dwellings with elevated indoor radon exceed the number of existing houses that are mitigated.

The key elements for successful prevention and mitigation actions within the framework of a national radon programme are the following:

1. Radon control actions should consider a combination of building types:
   • new and existing homes, since the greatest amount of radon exposure is generally in homes;
   • buildings where the public is likely to be exposed for long periods such as schools, preschool facilities, state-owned or leased buildings, and lodging facilities.
2. Research on buildings should be used to identify the most cost-effective radon control strategies for prevention and mitigation. Structural, foundation, and ventilation systems as well as construction practices vary from region to region. Specifically, this research should be used to develop:
   • radon prevention standards and regulations such as building codes for new dwelling construction;
   • radon mitigation standards and requirements for remediation of existing dwellings (cf. Section 3.1.2).
3. The contribution of different radon sources varies between countries and even regions. The following mechanisms may be considered:
   • pressure-driven soil gas infiltration;
   • emanation of radon from building materials;
   • water transport of radon.
4. Appropriate training and certification of building professionals should be implemented to ensure the efficiency of prevention and mitigation actions.

Some of the common aspects for radon prevention and mitigation actions are discussed in the following sections.

3.1.1. Design criteria for radon control systems

Radon systems for prevention as well as mitigation require the following design criteria:

• able to reduce radon concentrations considerably below the reference level;
• safe and not creating back-drafting;
• durable and functional for the expected life of the building;
• easy monitoring of the performance;
• quiet and unobtrusive;
• low costs for installation, operation and maintenance;
• easy to install an additional fan when passive soil depression systems (PSD) are used.

Table 9 shows a comparison of different radon control systems for new construction that takes these design criteria into consideration.

Table 9

Radon control options for new construction.

3.1.2. Research-based guidelines and/or standards

Radon prevention and mitigation guidelines and/or standards should be developed or adapted to serve as a minimum requirement for good practice. The guidelines or standards should be based upon building science research. Furthermore, the guidelines and standards should be based on defined design criteria, since they cannot address every possible situation.

When developing these guidelines and standards, it is important to consult radon mitigation contractors, building researchers as well as other building and construction professionals. Flater and Spencer (1994) have shown that if these guidelines and standards become part of building codes, inspection procedures are needed to insure compliance. Countries with mitigation or prevention guidance documents or standards include Austria, Belgium, China, the Czech Republic, Finland, France, Ireland, Latvia, Norway, Russia, Sweden, Switzerland, the United Kingdom, and the United States of America (WHO 2007). Examples for some guidance documents are given in Box 2.

Box 2

Examples for radon guidance documents. China: Standard Guide for Radon Control Options for the Design and Construction of New Low-Rise Residential Buildings (GB/T 17785-1999); Indoor Air Quality Standard (GB/T 18883-2002). United Kingdom: Guide to Radon (more...)

3.2. Radon prevention strategies in new constructions

As mentioned before, the most important radon transport mechanism is pressure-driven airflow (i.e. advection) from the soil to the occupied space. Other driving forces include diffusion. Since air pressure differences between the soil and the occupied space are the primary driving force for radon entry, radon prevention strategies usually focus on reversing this pressure difference. This is commonly accomplished through the use of active (fan powered) or passive (no fan) soil depressurization. Membranes between the soil and the indoors may be used in combination with air pressure control strategies. The use of membranes as a stand-alone control technique is addressed in section 3.2.3.

3.2.3. Radon prevention strategies

Most prevention strategies address steps to limit soil gas infiltration due to air pressure differences between the soil and the indoor occupied space. Radon prevention strategies should consider the specific mix of construction practices, radon sources, and transport mechanisms in the region or country, in order to be cost-effective. Under certain conditions a combination of strategies may be necessary such as in buildings with multiple types of foundations. Several prevention strategies are summarized and listed here:

a. Active soil depressurization (ASD)

Figure 4 shows an ASD, which is simple to install and provides greater radon reduction compared to PSD systems (USEPA 1993). Thus, ASD may be a favoured option for home builders. It has a rich history, beginning with its initial experimental applications in Canada (Scott 1979, Gessall and Lowder 1980, DSMA ATCON 1982). Commonly, ASD systems include the following basic components:

Figure 4

Active soil depressurization for radon control in new constructions.

• suction point(s) located below the ground-contacted floor or slab of the home and connected to a continuous and uniform permeable aggregate layer, ground water control system, or a sump;
• a discharge point located in a manner that minimizes the opportunity for human exposure, for example above the highest roof. There is evidence that ASD discharges at ground level create a risk of radon reentering the house (Henschel and Scott 1991, Yull 1994, Henschel 1995). Therefore, even if the risk appears to be small, ASD systems should be installed in a way that minimizes this risk;
• a continuously operating inline fan is located outside and above the conditioned space of the home. An important distinction between ASD in existing homes and new construction is that, in the latter, the use of a permeable layer and sealing provide the opportunity to use smaller, more energy-efficient fans;
• a U-tube manometer may be used as a system indicator to monitor performance such as pressure differences in the vent pipe below the fan;
• systems should be labeled at every accessible level to avoid confusion with the plumbing system (similar to PSD).

b. Passive soil depressurization (PSD)

PSD (cf. Figure 5) is used in new construction. It is similar to active soil depressurization (ASD), with the following exceptions:

Figure 5

Passive soil depressurization for radon control in new constructions.

• the effectiveness of PSD depends on the thermal buoyancy of airin the vent pipe and its ability to slightly depressurize the soil under the dwelling. To make it effective the following should be considered:
  • the system must have a uniform permeable layer under all elements with direct contact to the ground (e.g. concrete slabs, crawlspace membranes);
  • the vent pipe must be routed mainly through the heated portion of the building and any sections of the vent pipe in unheated areas must be insulated;
  • the vent pipe routing must allow the easy installation of a fan if the PSD system fails to achieve sufficient radon reduction;
  • the exhaust duct must discharge above the highest roof;
  • the systems should be labeled at every accessible level to avoid confusion with the plumbing system;
• the elements of the building that are in contact with the soil must be sealed to prevent soil gas infiltration (see the sections on sealing and barriers);
• since air pressure differences are so small between the vent pipe and the occupied area, the only way to monitor system performance is via periodic or continuous radon monitoring.

In new construction, PSD appears to reduce radon by about 50% (Dewey and Nowak 1994). If the PSD system is properly designed and installed, small fans (e.g. 75 W or less) may be used to activate the system (Saum 1991, ASTM 2007). The use of a smaller fan saves energy-related operating costs.

c. Sealing of surfaces

The sealing of the surfaces which separate the indoor occupied space from the soil can improve the performance of other prevention strategies such as PSD or ASD. In these cases, sealing reduces the loss of conditioned air from indoors, which may be substantial (Henschel 1993), and increases the reversal of air pressure from the soil to the indoors.

As a stand-alone prevention strategy, sealing has limited potential for radon reduction (Brennan et al. 1990, Scott 1993), especially over time. Sealing does not address the major reason why radon moves from the soil to the indoors, i.e. pressure-driven airflow.

d. Barriers and membranes

Barriers or membranes between the soil and the indoors may be used as a stand-alone radon prevention strategy or in combination with other techniques such as passive or active soil depressurization. Membranes may also help limit moisture migration to the indoors. Consideration should be given to using barriers with independent third-party approval for characteristics such as air tightness, diffusion, strength and durability properties (SINTEF 2007).

While barriers may be useful to reduce radon transport from the soil to the indoors, opinions vary about their effectiveness:

• advocates note that there is little that can go wrong after they are installed, while acknowledging that the barrier must be air-tight. Scivyer and Noonan (2001) found in their study that there were no significant changes in radon concentrations in homes with full radon membranes over a ten-year period. However, there was no indication concerning the initial effectiveness of the membranes;
• critics of membranes note that it is very difficult to make membranes air-tight under common construction conditions. A punctured membrane would potentially act as a trap to collect soil gas and funnel it into the building through any available openings. In addition, barriers do not address air pressure differences (Scott 1993). Barriers might be more effective in moderate climates where pressure differences due to temperature are small. Examples of poor and good radon barrier installations are shown in Figure 6.

Figure 6

Examples of barrier installations.

Barriers may be used in combination with other prevention techniques such as soil depressurization. When used with soil depressurization, the barrier does not need to be continuous. For example, in Finland, when soil depressurization piping is installed, reinforced bitumen felt is installed below the floor-foundation wall.

e. Ventilation of unoccupied spaces

Ventilation of unoccupied spaces between the soil and the occupied space (e.g. vented crawlspaces) can reduce indoor radon concentrations by separating the indoors from the soil and reducing the concentration of radon below the occupied space. The effectiveness of this strategy depends upon a number of factors. These include the air-tightness of the floor system above the vented unoccupied space, and, with passive ventilation, the distribution of vents around the perimeter of unoccupied space. A variation of this approach involves the use of a fan to either pressurize or depressurize the unoccupied space. However, fan-driven depressurization of crawlspaces may pose problems such as back-drafting of combustion appliances and energy loss (ASTM 2003a). Subslab and submembrane depressurization (SSD and SMD) may be either active or passive and are recommended for radon control in buildings with crawlspace foundations. SSD and SMD offer greater radon reduction than crawlspace ventilation.

f. Ventilation of occupied spaces

For overall indoor air quality, an exchange between indoor and outdoor air is desirable. For radon prevention, ventilation has varied results and may lead to energy losses, especially in extreme climates. If the major radon source is building material, ventilation will be needed. However, it is better to avoid the use of building materials that are sources of radon in the first place (EC 1999).

g. Water treatment

Water treatment is not commonly carried out in new constructions, except in areas where high radon concentrations in water are known to be a problem. For more information about water treatment techniques to reduce indoor air concentrations, see the radon mitigation paragraph at the end of section 3.3.2.

3.3. Radon mitigation strategies in existing buildings

Some aspects of radon mitigation are similar to radon prevention, although there are subtle but important differences. The cost-effectiveness of radon mitigation varies according to the type of system installed and the quality of the installation. There is evidence that active soil depressurization most effectively reduces radon concentrations, if installed by an experienced contractor, as compared to others including householders themselves (Naismith et al. 1998).

To decide on mitigation or to determine the effectiveness of any mitigation action, radon measurements must be carried out in a manner that is consistent with recognized measurement protocols and the applicable reference level (cf. Chapters 2 and 6).

The scope and urgency of mitigation recommendations may be based upon the radon concentrations, as determined by measurements. For example, if the measurement indicates slightly elevated indoor radon and there is no time-sensitivity for radon reduction, limited or phased mitigation steps may be suggested. Then, if needed, upgrades can be carried out.

In some countries, such as the United States of America, mitigation efforts focus on more robust remediation, such as active soil depressurization. This approach maximizes radon reduction with a small incremental cost difference compared to other, more limited approaches. Furthermore, more forceful approaches give greater confidence in achieving radon reduction targets. The robust approach to mitigation is appropriate when there is time sensitivity in reducing radon, for example during the buying and selling of a house.

As discussed in chapter 2, post mitigation measurements should always be made to determine the effectiveness of the radon reduction efforts. Furthermore, mitigated homes should be periodically retested since the performance of radon mitigation systems can change (Gammage and Wilson 1993).

3.3.2. Radon mitigation strategies

Radon mitigation strategies need to be adapted to the specific mix of housing and building characteristics, climate zones, radon sources, and transport mechanisms in order to be cost-effective. A summary of radon mitigation techniques is presented in Table 10. The installation costs reflect those of experienced radon mitigation contractors. Combination techniques may be used in mitigation, as in prevention, for complicated buildings or when one approach does not produce sufficient results (BRE 1998, Henschel 1993, Pye 1993, Roserens et al. 2000, Welsh et al. 1994). In general, radon mitigation systems may be categorized as follows:

Table 10

Common radon mitigation techniques, performance and costs,.

a. Active soil depressurization

As described before, ASD is the most common form of radon mitigation in existing houses. Due to its high reliability in radon reduction in a wide variety of houses and other buildings, ASD should be one of the first approaches considered. According to a WHO survey (WHO 2007), active soil depressurization represented the majority of radon mitigation reported by the following countries: Austria, Belgium, Finland, Germany, Norway, Slovenia, Sweden, United Kingdom and the USA. The specific configurations of these systems depend on foundation characteristics (e.g. basement, slab-on-grade and crawlspace foundations).

The main difficulties of applying ASD to existing buildings compared with new construction are the following:

• the material underneath the lowest floor of the building may have very limited permeability and thus, it may be necessary to install a sump or a suction pit (to increase the sub-slab surface area upon which suction is applied) or the ASD fan may need to be resized;
• it may be difficult to seal openings between the soil and the occupied space;
• it may be difficult to route the vent piping.

b. Ventilation of occupied spaces

The ventilation of occupied spaces may be done actively by using a fan or passively by operating windows or vents manually. There is limited evidence concerning the effectiveness of passive or natural ventilation for radon control (Cavallo et al. 1991, 1996). However, in moderate climates such as in Ireland, ventilation is used as an effective radon mitigation method (Synnott 2004, 2007). Ventilation approaches to radon reduction are more common in mechanically ventilated schools and other large buildings than in small houses (WHO 2007). Fan-powered ventilation can reduce pressure differences between the soil and the occupied space, as well as dilute indoor radon after it enters. These systems are especially useful when one or more of the following factors are implicated:

• a major radon source is from building materials;
• the building is located in a non-heating or non-cooling dominated climate, thus ventilation has lower energy penalties;
• there are multiple indoor air quality problems;
• ASD is not feasible or does not sufficiently reduce radon concentrations.

Mechanical ventilation may be done in one of the three following ways, taking into account its advantages and disadvantages:

1. Exhaust ventilation, which depressurizes the indoors in relation to the soil and the outdoors, is almost never used for radon control, and especially not in heating or cooling dominated climates;
2. Supply ventilation (or positive ventilation) tends to pressurize the indoors in relation to the soil and the outdoors as well as dilute the radon after it has entered. An example with a cost-estimate is given in Box 3. Supply ventilation carries possible risks such as, in hot climates, condensation damage to the building envelope. However, small supply fans have been used successfully in the United Kingdom and Switzerland to reduce indoor radon. Critics argue that filters must be maintained by residents to be effective and that all windows and doors must be kept closed (Clarkin et al. 1992). In colder climates, the fans need to be equipped with heating elements;
3. Balanced exhaust ventilation neither pressurizes nor depressurizes the indoors in relation to the soil and the outdoors. This form of ventilation dilutes radon after it has entered the building. In heating and/or cooling climatic conditions, balanced ventilation is often done with a heat or energy recovery ventilator to reduce energy consumption.

Box 3

An example of a supply ventilation with some cost-estimates. Fans reduce radon by slightly pressurizing the indoors in relation to the soil or reducing the negative indoor air pressure. Fans with a maximum output of 52 l/s have been used in houses in (more...)

c. Sealing of surfaces

Sealing off openings in surfaces between the indoors and the soil is a controversial stand-alone mitigation technique with, at best, limited effectiveness. For example, success with sealing alone has been reported in only one out of 1500 cases and therefore sealing is not recommended (Turk et al. 1991, USEPA 1993). In Finland, sealing alone reduces indoor radon concentrations by 10 to 30% (Arvela and Hoving 1993). Norway recommends sealing as an initial step followed by, if needed, additional mitigation (SINTEF 2007). When used with active soil depressurization, sealing improves system performance. But as a stand-alone strategy, it is very difficult to seal off soil contacted surfaces enough to prevent pressure-driven radon entry.

d. Water treatment

In the relatively rare cases where significant amounts of radon are transported indoors by water from a private drilled well, radon is released into the indoor air. In such cases, water treatment may be used to reduce the indoor air concentration of radon. The health risk associated with radon in water is primarily via inhalation as opposed to ingestion. The primary strategies to reduce indoor radon from well water at the point of entry into the home are:

• aeration: in a sealed tank, air is bubbled through the water or the water is sprayed into the air or is cascaded over objects while radon is extracted from the water to the outdoors;
• filtration with granular activated carbon is generally less expensive but results in less radon reduction.

Dembek et al. (1993) and the WHO Guidelines for drinking water (WHO 2005) give further information on radon mitigation in water.

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