Mitigation methods to lower Radon levels.
Active Soil Depressurization
Active Sub-Slab Depressurization
Sub-slab depressurization (also called active soil depressurization) is the most effective and reliable radon reduction technique. It is also the most common method used by C-NRPP certified professionals.
Examples of venting options for active sub-slab depressurization
This method involves installing a pipe through the foundation floor slab and attaching a fan that runs continuously to draw the radon gas from below the home and release it into the outdoors where it is quickly diluted. This system also reverses the air pressure difference between the house and soil, reducing the amount of radon that is drawn into the home through the foundation. One, or sometimes multiple, suction points are inserted through the floor slab into the crushed rock or soil underneath to effectively reduce the radon level in the home.
The sub-slab depressurization pipe can be vented at either the roof level or ground level of the home. The fan can be placed in the basement or an area outside of the living space such as in a garage or attic. If the fan is placed inside the living space of the home, it is usually vented sideways through the rim joist at ground level, with the fan close to the exhaust location. When the fan is placed outside of the living space (e.g. attic or garage) then it is typically vented upwards above the roof.
In many Canadian climates, a fan and pipe located outside the living space (garage or attic) will cool during the colder months of the year, leading to condensation and possibly ice, which can damage the fan and affect the effectiveness of the radon-reduction system.
Condensation problems can be reduced if the fan is placed indoors and the exhaust is discharged from a shorter pipe near ground level at right angles to the wall, much like the power vented exhausts from combustion appliances such as natural gas-fired water heaters.
If the fan is placed inside the home, it is important to confirm with your contractor that it is air tight and that all pipes and plumbing joints have been sealed. Properly installed fans and pipes will not leak radon into the building and are usually installed in the basement. When the fan and pipe are placed inside the home and combined with a ground level discharge, almost the entire system is located indoors, which helps to avoid problems that can arise from cold climates.
Current field test studies of indoor mounted fans with near ground level discharges show this is an effective technique. Further field testing of this system in urban environments where houses are built in close proximity to each other is necessary. To verify continued performance of any radon-reduction system, an initial long-term measurement should be made within two years of the system activation and at five-year intervals afterwards.
When any active depressurization system is installed, it is recommended to make sure that its operation does not cause backdrafting of combustion appliances such as a furnace, water heater, fireplace, or wood stove in the home. Backdrafting can happen when a room with a combustion appliance is depressurized so much that smoke and combustion gases spill into the home instead of venting outdoors. Backdraft testing may be done by a trained radon-reduction specialist or a heating contractor.
A variation of active sub-slab depressurization is active sump-hole depressurization. Often, when a home with a basement has a sump pump to remove unwanted water, the sump can be capped and sealed so that it can continue to drain water and also serve as the location for a radon suction pipe. If a basement floor drain is connected to the sump, a mechanical trap seal device or water trap should be installed to prevent house air from entering the sump via the drain.
Drainage System Depressurization
Some homes have drain tiles or perforated pipe to direct water away from the foundation of the home. Suction on these tiles or pipes can be effective in reducing radon levels, especially for a block wall foundation. This method, called drainage system depressurization, is worth considering if it is certain the tile forms a complete loop around the foundation. This type of system will be less effective if only a small area of the basement perimeter is covered.
Homes with Crawlspaces: Active Sub-Membrane Depressurization
The soil in a crawlspace can be vented using a similar technique called active sub-membrane depressurization. It involves laying a thick plastic sheet (often a polyethylene membrane) over the soil, sealing the air-tight membrane to the foundation walls and placing a pipe with fan through it to draw the radon from under the plastic sheet and vent it to the outdoors. For this method to be effective, special attention is needed to seal around the pipe where it penetrates the plastic sheet.
Sub-slab or sub-membrane depressurization systems range in cost from about $2,000 to $3,000 including material and labour. There is also a small operating cost for electricity for the fan, approximately $50 to $75 a year, depending on size of fan and energy rates.
When large radon reductions (50 per cent or more) are desired, active soil depressurization is almost always the recommended approach. If smaller reductions are sufficient, the other radon reduction methods described below may be reasonable alternatives. A certified radon professional can help you determine the best solution for your home.
Other Radon Reduction Methods
Sealing Major Entry Routes for Radon
Sealing off openings in a home where radon could be entering may help to lower radon levels in your home. However, because it is difficult to identify, access and permanently seal all openings it is not a standalone technique for reducing radon levels.
Major openings that can be important to seal include:
Floor drains – Trap for floor drains that allow water to drain but prevent radon from entering the basement
Floor Wall Joint
Figure 3 – Sealing foundation wall and basement floor joint
Voids in concrete block walls
Figure 4 – Sealing voids in the top of concrete block walls
After closing major openings, a further reduction in radon levels can sometimes be achieved by sealing minor entry routes that are visible or accessible. Minor cracks in foundation walls and floors can be sealed. Larger cracks require special techniques; consult your building material supplier or a contractor. The gap around utility penetrations (e.g., water, sewer, electrical, natural gas, fuel oil) in walls and floors can also be sealed.
Figure 5 – Sealing foundation wall and floor cracks
Sealing cracks and other openings in the foundation is a basic part of most approaches to radon reduction and can help increase their effectiveness. Proper preparation of the surface area to be sealed is extremely important to create an effective and long lasting seal.
The cost of sealing entry routes is highly variable. It can range from a few hundred dollars to $2,000 or more. Although the material cost is relatively low, it is very labour-intensive to do a comprehensive job. As the house ages and settles, the seals can deteriorate, and new cracks or entry routes can appear. As a result, there will be an ongoing cost to maintain the seals.
Increasing Mechanical Ventilation of the Home
A heat recovery ventilator (HRV) or energy recovery ventilator (ERV) can be installed to increase ventilation, which will help reduce the radon levels in your home. An HRV increases ventilation by introducing outdoor air as it uses the heated or cooled air being exhausted to warm or cool the incoming air. It is important to ensure that this type of system has balanced intake and exhaust air flows so that the house is not depressurized, which can draw in more radon.
The effectiveness of ventilation for radon reduction is limited and only appropriate for situations where only modest reductions are needed. In general, increased ventilation methods for radon reduction will be most successful in houses that are more airtight and have low natural ventilation rates (are not ‘drafty’). It is also important that the HRVs are properly balanced and maintained (i.e. check filters). In most homes, an HRV might reduce radon levels by 25 to 50 per cent.
An HRV will cost between $1,500 and $3,500 (material and labour). There is also an operating cost for electricity for the HRV’s fans as well as an increase in heating and cooling costs due to greater ventilation of the home.
|Foundation Type||Poured Concrete||Slab-on-grade|
|Foundation Floor||Exposed soil/ pavers||Concrete slab||Exposed soil (building extension)||Concrete slab|
Table 1 footnotes
Caution – Back-drafting of combustion appliances possible. e.g. Wood stove, oil/gas furnace, oil/gas water heater.
|Close large openings to soil in any accessible parts of foundation walls/floor||Yes||Yes||No||Yes|
|Trap floor drains that lead to soil||Yes||Yes||No||Yes|
|Cover soil water drain sump and exhaust it to outside||Yes||Yes||No||Yes|
|Isolate foundation area from living area. Exhaust foundation area air to outside||Yes||Yes||Yes||No|
|Isolate foundation area from living area. Install Heat Recovery Ventilator to supply fresh air to living area, and exhaust foundation area air to outside||Yes||Yes||Yes||No|
|Install Heat Recovery Ventilator to supply fresh air to living area, and exhaust from bathroom or furnace area to outside||No||No||No||Yes|
|Cover accessible area of exposed soil/pavers with plastic membrane, exhaust from beneath to outside||Yes||No||Yes||No|
|Exhaust from beneath concrete slab to outside||No||Yes||No||Yes|
Follow-Up Radon Testing
When a radon reduction system is first activated, the contractor should make sure seals and joints are working effectively and correct any faults or defects found. The contractor should place a label on the system listing when it was activated, and the suggested re-test intervals. The suction and flow in the piping should be measured and noted on the label for comparison when the system fan is serviced.
It is recommended that a certified C-NRPP professional carry out a short-term test after a system is activated to demonstrate that it is working effectively. The test should be started at least 24 hours after the fan is turned on. The radon test should ideally be in the same location where the measurements were originally made.
The homeowner should also ensure that a long-term three-month test is performed the following fall /winter season to confirm that the annual average radon level has been reduced to below the Canadian guideline. To avoid conflict of interest, the test should not be performed by the company that installed the radon mitigation system.
Change in Radon Concentration after Mitigation
Preventing Radon Problems in New Homes
It is not possible to predict before construction whether or not a new home will have high radon levels. Fortunately, preventive measures can be taken by your builder during the design and construction process to reduce the amount of radon that gets into the home and make it easier to install a radon reduction system, if required.
The 2010 National Building Code (NBC) includes requirements that address radon. Parts five and six of the code require that engineers and designers consider radon protection in their designs and ensure control of air leakage and soil gas entry to minimize the level of radon entering a home through the foundation.
Part nine of the code includes consolidating air barrier requirements such as a sealed plastic membrane under the foundation slab, and requiring that every building have granular fill under the slab and a rough-in for a future radon reduction system, should the need for radon reduction later arise.
Many provinces have adopted or are in the process of adopting these 2010 National Building Codes. Homeowners should ask their builders if they include building practices that help reduce radon entry (sealed membrane) and make it easier for radon removal (rough-in for a radon reduction system) if necessary.
Builders can minimize radon entry into the home by:
- Installing a sealed plastic (polyethylene) membrane under the foundation floor slab or on top of exposed soil in crawl spaces. The membrane should be a minimum of 6 mil thick with taped seams. Research in other countries, such as the United Kingdom and United States, indicate that thicker membranes can be more effective at reducing radon entry.
- Sealing the basement floor/foundation wall expansion joint. There are several options for sealing this potential radon entry point Note that proper preparation of surfaces to be caulked is critical to obtain an effective, long-lasting seal. (see figure 3, page 21)
- Sealing around all objects that penetrate foundation walls and basement floors, including utility lines for water, sewer, electrical, natural gas, or fuel oil. The centre of hollow objects that penetrate the walls or floors (e.g., metal support posts or masonry for fireplaces) should also be sealed or blocked.
- Providing proper curing conditions. Moistening the slab or coating it with a special compound during curing will result in stronger, more durable concrete. If the weather is hot and dry or below freezing, your contractor must take appropriate precautions to ensure the cement is cured properly.
- Using control joints in the concrete floor slab. While some cracks in the basement slab may be unavoidable, your contractor can direct cracks into controlled locations where they can be sealed.
- Installing special traps in floor drains that allow water to drain but prevent radon from entering the basement (see floor drain image on page 20).
- Using a sealed lid on the sump. Your builder may either purchase a sealed unit or field fabricate a sealed lid (See open sump image on page 20).
The actions mentioned on the previous page can help reduce the amount of radon that enters a home but do not guarantee that annual average radon levels will be below the Canadian guideline of 200 Bq/m³. Therefore, in addition, your builder should install a rough-in for a radon sub-slab depressurization system. It is more practical and less expensive to install the pipe through the foundation slab while your new home is being constructed.
Figure 6 – Example of a rough-in installation for a radon sub-slab depressurization system
The steps that should be followed to properly install this rough-in and maximize its effectiveness include:
- Before pouring the slab, ensuring that the entire sub-slab area is filled with at least 100 mm (4 in.) of coarse gravel to allow for good air flow/movement under the slab.
- Installing a plastic membrane (polyethylene) air barrier under the foundation slab. The membrane should be a minimum of 6 mil in thickness with taped seams.
- Ensure that all penetrations through the slab (plumbing, electrical, teleposts) are well sealed.
- Casting a short length of PVC pipe of at least 100 mm (4 in.) diameter vertically through the floor slab. If the top end of the pipe is placed away from the centre of the foundation slab, a longer run of PVC pipe will need to be placed horizontally in the coarse gravel and all joints in the pipes should be sealed. The pipe protruding from the foundation slab should be capped and sealed properly to avoid radon entry from the sub-slab area. It should also labelled clearly that it is intended for a radon reduction system.
Maintaining Your Radon Reduction System
As with a furnace or air conditioner, radon reduction systems need occasional maintenance. If you have a fan-powered depressurization system, you should look at your system performance-indicating device, usually a manometer, on a regular basis to make sure the system is working correctly. A U-tube manometer is used as an indicator that the mitigation system is working. The manometer is filled with a liquid and indicates pressure or flow.
Figure 7 – U-tube manometer
This image shows two U-tube manometers. One U-tube manometer shows the level on both sides of the U-tube is the same, indicating that the mitigation system is not working properly. The other U-tube manometer shows a higher level on one side of the U-tube, indicating a pressure difference, which confirms that the system is working properly. Minor variations in the level can be expected. The level in the manometer should be checked from time to time. Ask your certified radon reduction expert to show you how to check if the fan is working properly and follow the instructions that are supplied with the U-tube manometer.
Remember, the fan should NEVER be turned off; it must run continuously for the system to work properly. The lifespan of a fan can vary between five to ten or more years. Replacement cost ranges between $200 and $300.
The filter in a Heat Recovery Ventilator (HRV) requires periodic cleaning and should be changed twice a year. Replacement filters for an HRV are easy to change and reasonably priced. Ventilation systems should be checked annually by a heating, ventilating and air conditioning professional to make sure the air flow remains properly balanced. HRVs used for radon control should run all the time. Also, vents that bring outdoor air into the home must be inspected regularly for leaves and other debris.
Remodelling Your Home after Radon Levels Have Been Lowered
If you decide to make major structural changes to your home, including, for example, converting an unfinished basement area into living space or a creating a new foundation for an addition after you have had a radon reduction system installed, ask your radon contractor what should be done to help ensure that radon levels throughout the home continue to be reduced. After you remodel, retest in the least lived-in area to make sure the construction did not reduce the effectiveness of the radon reduction system.