Mitigation techniques are divided mostly into three groups: 1) those that address reduction of radon gas; 2) those that address the reduction of SLRDs; and 3) those that address the DP.
The average 19 radon gas reduction, as of 1989, from mitigation techniques is 70%. However, it is more practical to speak of absolute reductions rather than per cent reductions. For example, it is easy to get a 98% reduction when reducing the radon in a building from 500 pCi/l to 10 pCi/l but it is extremely difficult to get a 20% reduction when attempting to reduce the radon from 5 pCi/l to 4 pCi/l.
The life time effectiveness of the mitigation techniques is still under review.
Ventilation as a radon reduction technique usually addresses reduction of the radon gas rather than the DP, because usually when a contractor is referring to ventilation, they are referring to ventilation of a crawl space, not a living area. When this is the case, the radon contractor usually refers to “isolation and ventilation”. The Building Official’s Code Agency recommends 1 square foot (865 cm2) of passive vent per every 150 square feet (14 m2) of floor space with vents within 6 feet (1.8 m) of each corner.
Passive ventilation of a working or living area is typically considered to be an inappropriate technique for buildings in temperate climates because of the difficulty of maintaining comfort zone temperatures during the winter months. An additional problem with passive ventilation is that one does not have good control of the ventilation. A window may be open one minute until someone else feels cold and closes the window.
Additionally, it has been shown 20 that if other than the very lowest level of the building is passively ventilated (say by opening a window) then winds blowing through the building can create a venture effect and actually increase the radon concentration. As discussed earlier, there is no correlation between air changes per hour and radon concentration, but there is a strong correlation between DP and radon concentration.
Passive ventilation of some heating cellars where the pipes are insulated and the room contains a sump may be a viable option. Ventilation systems designed for radon reduction are often fitted with heat recovery devices to help reduce the loss of heated air to the outside. Passive ventilation is obviously cheap and easy but it has met with rather checkered results. It is most appropriate for small buildings with very low levels of radon.
Active ventilation, on the other hand, is usually in the form of HVAC systems which are not specifically designed as radon reduction systems. Nonetheless, because the HVAC systems are designed to maintain the building at slightly positive pressure, they address the DP issue. By maintaining a slight positive pressure, HVAC systems overcome the negative pressures of the stack effect and prevent radon from entering a building. The system should be capable of maintaining a positive pressure of at least 0.02 inches of water column (5 Pa) above the ambient pressure.
Filtration devices address the SLRDs without addressing the radon problem or the DP problem. Filtration devices circulate the room air through a filter which scrubs out the SLRDs.
On the surface, this type of technique appears to be an excellent solution, however, the filters will also remove the airborne particulates (ultrafine particles, dust, pollen, etc.) thus increasing the ratio of unattached daughters and actually increasing the bronchial radiological dose21. As mentioned earlier, the unattached daughters have a much higher probability of adhering to the lining of the lung wall. Therefore, the sole use of filtration devices is not considered to be an appropriate mitigation technique.
Air Movement Device: Ceiling Fans
This type of a system addresses neither the DP problem nor the radon entry problem, but rather the SLRDs themselves.
Unlike a filtration device, a ceiling fan does not remove the desirable airborne particulates but rather encourages the plate-out of the SLRDs. Since this type of technique can be installed by the homeowner (as a rather attractive addition to a living room or dining room), radon contractors do not have an incentive to disclose this technique to the general public.
Remarkably good reductions (as high as 95%) of SLRDs have been achieved 22 by simply placing a “Casablanca” type ceiling fan in a room. The fan should be capable of complete air movement within the room. Where the desired reduction is on the order of 50%, the ceiling fan alone can correct most of the problems.
Where a reduction of 80% or better is needed, a ceiling fan in conjunction with a positive-ion generator may correct the problem. The ceiling fan/positive-ion generator combination has been tested in the U.S., Denmark, Finland and Canada 22 with similarly excellent results. The reduction in SLRDs has been consistently as high as 95% and where bronchial doses have been measured 22, the reduction in bronchial dose has been as high as 87%.
The positive ion generator should not be confused with an electrostatic precipitator (ESP). Using an ESP could result in the removal of airborne particulates and an increase in unattached daughters. Also, negative ion generators have been shown to be less effective than the positive ion generators 22. While the fan speed is not critical, the fan should be placed in the center of the room and be large enough to effectively move the air in the room.
A disadvantage to this type of reduction technique is that post-mitigation monitoring would have to involve a continuous working level monitor, instead of the charcoal canisters. Nonetheless, the savings achieved by the technique over some other mitigation methods would nearly off-set the cost of purchasing such an instrument (not to mention renting one).
Another disadvantage to this technique is that it can be readily turned off if not properly installed. The fan and the positive ion generator should be wired such that it cannot be deactivated by unauthorized personnel. The system should be labeled as a “radon reduction” system and allowed to run continuously.
Sealing Floor and Foundation Wall Cracks
Since some 90% of the radon 23 comes from sub-slab infiltration, one of the earliest mitigation techniques involved simply sealing floor and foundation wall cracks to prevent entry. The advantages of this method are its relative ease and low cost.
The disadvantages of the technique include its poor record of success, its limitation to only unfinished basements and the fact that it does not address DP, or SLRDs.
It has been shown that where high levels of radon are present, sealing alone is a very poor mitigation technique. However, sealing of floor and foundation wall cracks is often a necessary supplement to sub-slab depressurization (this will be discussed below).
When such sealing is required, the crack needs to be properly routed out first and then sealed back in with an appropriate material, such as backer-rod and foam.
Where high levels of radon are present, floor sealing technique is not recommended as a sole corrective action.
In some mitigation cases, the technique was to positively pressurize the basement. This technique has a poor record of success because it involves upsetting the normal use of the basement. It has a potential ability to blow out pilot lights and can be noisy. It is no longer generally considered to be an acceptable mitigation technique.
Sub-slab Depressurization (SSD)
Approximately 90% of the reduction techniques used in the U.S. today are SSD.
The idea of SSD is to address the driving force of the radon entry; the DP between the slab and soil gas. Instead of increasing the pressures within the building, SSD reduces the soil gas pressure below the slab. This author has measured in-house/sub-slab pressure differentials of as high as 89 Pa.
The SSD technique involves penetrating the slab with a 7cm to 20cm inside diameter PVC pipe and running the pipe up through the structure and exhausting to above the roof line. A centrifugal fan capable of developing high static pressure is mounted at the exhaust (outside the shell of the structure) to depressurize the slab. The fan should be capable of maintaining a pressure of at least 5Pa below the highest DP recorded or expected.
In some cases, a passive turbine has been used with encouraging results. The driving force for the depressurization is the stack effect and a wind driven turbine at the exhaust.
SSD has a proven track record of achieving 80% to 90% reduction in radon gas levels in favorable structures. SSD works best when the soil type is a sandy or a loam. Additionally, the slab should be in good condition; slab cracks and expansion joints will limit the extent of the pressure field. If the slab is damaged or the soil has high clay content, then SSD can still be used by inserting more and more collection pipes in the slab to extend the pressure field.
Prior to SSD, soil communication tests should be performed. Pilot holes are drilled into the slab and a vacuum cleaner is used to create a negative pressure field below grade. The DP is measured at each of the pilot holes. If the DP at each of the holes is acceptable (5 Pa or greater) then only one hole is needed. If the DP at any one of the pilot holes is less than 5 Pa, then that hole should be enlarged and incorporated as a collection point. Typically, one collection point is needed for every 600 to 1000 ft2.
SSD works well for recessed floating slabs, slab-on-grade and floating slab-on-grade structures.
There are several important variations on the SSD theme. The first is perimeter drain depressurization whereby the pressure field is created using the existing exterior perimeter footing drain.
Block wall depressurization is used when the foundation wall consists of hollow block construction on a poured concrete footer. The foundation wall is penetrated with PVC piping and suction is applied to the wall. Prior to block wall depressurization, a block wall communication test should be performed to ensure uniformity in the depressurization. The block wall communication test is similar to the soil communication test. Doors, fire walls and other anomalies will disrupt the pressure field within the wall and require additional collection points. The block wall may be depressurized from the interior of the building or the exterior of the building.
In addition to the block wall depressurization technique, the same concept may be used for stem wall depressurization. Baseboard depressurization may be appropriate where there is a French drain present. Sumps may be used to depressurize the sub-slab soil.
Another important variation is the membrane suction technique used in crawl spaces. Since the earthen floor in the crawl space is incapable of allowing for an extended pressure field to develop, an impermeable membrane is placed over the entire floor of the crawl space. In some studies and case histories, the membrane is anchored to the floor using furring strips, and in other cases, the membrane is simply allowed to rest on the earthen floor.
Once the membrane is in place, a suction point is cut into the membrane roughly in the central portion and the soil gas is evacuated in the normal fashion.
One of the disadvantages of the SSD type systems is the cost. The initial cost of the installation is higher than most other techniques. The operating costs and the maintenance costs are also higher. The system can become noisy, prompting complaints from the building occupants and even prompting the occupants to deactivate the system.
The radon levels at the exhaust can be quite high and care must be taken to ensure that the radon does not reenter back into the building shell.
Some contractors have experienced water vapor build-up from improperly installed systems. As the water vapor is extracted from the soil gas beneath the slab, it can condense within the pipes of the system. When this happens, the fan may be incapable of overcoming the back pressure and the pressure field below grade is disrupted. In some cases, the water vapor has condensed in the fan housing causing fan failure. The system must be designed to ensure that water build-up can safety be drained back into the slab, or to the out-of-doors.
The systems need to be installed with elaborate control panels which indicate the total pressure on both sides of the fan. Alarms are recommended to alert the building occupant in the event of fan failure, unacceptably high static pressure in the up-stream side of the fan or other problems which may develop.
If the SSD type systems are installed improperly, they can greatly increase the overall radon concentration in the building. Common faults include:
- Placing the fan in the shell of the structure; if the fan leaks the radon is exhausted into the building.
- Placing the fan in such a position that the radon is pushed along the exhaust pipe rather than pulled through the exhaust pipe. That is to say, the exhaust is under positive pressure with regard to the ambient pressure of the structure, if the pipe has a leak, the radon will enter the building.
- Placing the exhaust too close to the plane of neutral pressure. During the stack effect, the lower portion of the building is under greater negative pressure than the top of the building; at a certain point, the pressure within the shell of the structure will equal the pressure outside and the DP will be zero. This point is called the plane of neutral pressure and is typically located 5cm to 8cm below the top most ceiling in the structure. If the exhaust of the SSD system is located at or below the plane of neutral pressure, the radon can be reintrained into the building.
Furthermore, improperly installed systems can result in exposing passers-by to the exhausting radon. The following criteria should be met:
- The discharge point must be at least 3.5 meters above ground level.
- The discharge point must be at least 3.5 meters (line-of-site) from any door, window or other structure openings that are less than 0.75m below the discharge point.
- The discharge point must be at least 3.5 meters away from any private or public access.
- The discharge point must be at least 3.5 meters away from any opening into an adjacent building.
The SSD systems have also been associated with back drafting problems whereby the exhaust from other sources of combustion (fireplaces, gas fired heaters and water heaters, etc.) within the building are disrupted. Therefore, following the installation of any depressurization system a test must be performed on any building which contains combustion appliances.