How Radon Mitigation Works and Best Practices
ASD – Active Soil Depressurization
Radon mitigation and radon system design can seem like an obscure concept but understanding a few details can make the haze clear. Online research will lead you on what seems like a wild goose chase with website after website introducing unfamiliar words for techniques to reduce radon. Are they all the same? Is one mitigation type more beneficial than another?
While there are a few approaches to radon mitigation and system design, the best approach (in most cases) is to install a system which prevents any gas from entering the home, thereby, depressurizing the area directly underneath the residence. ASD mitigation, on average, will lower levels of radon far more significantly and with a reduced operational cost and often, a lower installation cost. For now we’ll dive into the basics of understanding a ASD mitigation system.
For more info on other approaches such as the use of HRV’s, check out this post on our website
Numerous names for the same approach. You may encounter ASD mitigation terms described as:
- ASD (Active Soil Depressurization) – Often used to describe how mitigation system operate.
- SSD (Sub-Slab Depressurization) – Achieving an ASD mitigation system by drawing air from underneath the slab of a building.
- SPD (Sump Pit Depressurization) – Achieving an ASD mitigation system by drawing air from within the sump pit.
- SMD (Sub-Membrane Depressurization) – Achieving ASD mitigation system by drawing air from underneath a membrane. This is most common when there is a dirt or open aggregate crawl space below the building.
How Radon Mitigation via ASD Works
Put simply, radon, in addition to other soil gasses, builds up (ie. becomes pressurized) beneath your home and has nowhere else to go. Since the area under your floor is surrounded by foundation walls with footings the radon is forced to move along the only path it can: into your living space.
Radon mitigation ASD is done by removing this pressure and by changing the path of least resistance for this gas to migrate to. Instead accumulating into the home it is collected and drawn outside and away from the home. The gas migration is similar to the ventilation of a chimney channel.
In radon system design it is vital to size the system appropriately. Too big or too small can be ineffective: which will not lower radon, inefficient: increase heating, cooling, and electrical costs, and in some cases even create dangerous conditions. Next up PFD and how we customize the system to directly suit the needs of your home.

Pressure Field Diagnostics (PFD): Designed for your Home
The most crucial part of the designing a radon mitigation system is customization and you don’t need to take our word alone for it, check out Health Canada’s Radon Reduction Guide for Canadians.
Oversized vs. Undersized
An Undersized System
Since there is no “safe” level of radon, we aim to reduce the radon levels as much as possible and in correlation your associated health risk. A 90% reduction is good, but 98% reduction is even better. An undersized mitigation system will not adequately reduce radon levels as significantly as a properly designed and sized mitigation system.
In addition to the risk of elevated radon levels above that which is achievable it is important to consider the fan selection. Each radon mitigation fan is designed to operate to within a maximum pressure value. If a fan is working harder than it has been designed to operate, the motor is likely to fail prematurely. Just as we wouldn’t recommend towing a large tractor trailer with a Mini Cooper – radon systems should be installed in accordance to their intended design point.
An Oversized System
- Noise – ASD systems run 24 hours per day, 365 days a year. Even if the radon levels drop with an oversized system the fan will sound as if it is a jet engine – one that few homeowners will enjoy spending anytime around.
- Heating / Cooling / Electrical Cost – Any air which is pulled in by the fan has to originate from somewhere. As an example, in the case where an ASD fan pulls 100 CFM when only 20 CFM are available from beneath a home it is most likely that additional air will be drawn from the basement interior resulting in heated and/or conditioned air displaced outdoors to compensate. Air removed from the home must be replaced and will add to the draw of your HVAC requirements. During our Canadian winters when the temperature outside reaches -30 degrees C and the home interior is heated to a toasty +20 degrees C the heating and ventilation costs increase proportionally.
- Back Drafting – Your home is designed to maintain a consistent pressure. Mid-efficient appliances and fireplaces can draft (exhaust) correctly as they were designed only when this design balance is maintained. If too much air is pulled from the home, back drafting occurs, a process by which exhaust gasses (most notably – and of significant risk -carbon monoxide) are drawn within the home. At RadonCare, in addition to properly sizing our systems, we always perform a backdraft test with our mitigation systems to eliminate this risk.
- Other Plumbing Issues – While uncommon, there have been reports of other issues from improperly designed mitigation systems including frozen water pipes. If the additional air a fan draws from outside the home during cold weather temperature drops, freezing air can circulate under the home and contact underground plumbing and water lines. With the proper installation process, we ensure this too will not be a problem. Radon mitigation should not be achieved at the cost of a frozen water main.
How it Works – Sizing it Right
Achieving the perfect Goldilocks-sized radon mitigation system requires a few important steps and considerations. First, the concrete slab must be sealed as much as possible. While it is of course not possible to achieve and maintain a perfectly airtight concrete slab, our technicians treat as many areas as possible to reduce air loss. For more on this step, check out our blog.
The radon mitigation system design is then determined through pressure field diagnostics, the process of measuring the pressure field under the concrete floor to determine the amount of suction required for the system. In brief, the pressure field diagnostics is completed by:
- Suction. A suction is created under the slab. This can be through a sump pit, a radon rough in, or a hole drilled through the floor.
- Test holes of ¼” are drilled around the perimeter of the slab, ideally at locations on the opposite side of the home from the suction point (don’t worry these holes are hidden in inconspicuous areas and sealed following system completion).
- Micro-manometer measurements are completed. A micro-manometer tube is inserted into the drilled test holes measuring to within a 0.1 Pa pressure differential. Measurements are collected to determine the required amount of suction to create a negative pressure around the slab. These measurements are vital as furnaces, HRV’s, zoned heating systems, and even outdoor weather can have a substantial impact on this calculation.
- Adjust as required. If the suction point is capable of creating a negative pressure under the entirety of the home, we will have achieved what we call 100% “communication”. In this case we determine the design pressure, or required pressure at the farthest point, and can size the system appropriately and create the ventilation to direct radon outside the home. If 100% communication can not be achieved at the test holes regardless of the strength of suction applied at the design point, additional test holes are added to determine the maximum possible pressure field from the design point. A map is then drawn to visualize areas where the system is achieving communication and the areas where it is not. Dependent on the home and the requirements, next steps are determined – see below.
Design Pressure: System Variance Consideration
One of the biggest reasons radon levels fluctuate in a home is due to pressure fluctuations. As pressure changes, radon levels follow suit. In consideration of these changing pressures, it is important to determine which factors will affect the efficiency of the mitigation system when designing the system to ensure a full reduction in levels to be maintained throughout seasons and during varying operation of the home’s HVAC systems. Included below are a few examples of why these considerations are important.
HRV/ERV - HRV (heat recovery ventilation) and ERV (energy recovery ventilation) is a ventilation system designed to pull fresh air from outside the home while exhausting stale air from inside the home out. These systems must be balanced to maintain proper airflows to ensure the amount of air entering the home is equivalent to the amount of air exiting the home. Improperly sized, the system can either pressurize or depressurize the home causing the ASD system to operate improperly.
Zoned Heating Systems - A zoned forced air system is capable of adjusting the amount of supply air to varying areas of the home via zone dampers on supply air ducts. As an example, say we have a bungalow residence. If the main floor temperature is set to 19 degrees C and the basement temperature 21 degrees C the zoned system will only supply air to the main floor to even out the temperature variance according to preset parameters. Importantly for the specifically balanced mitigation system is that pressure design be weighted appropriately. While supply air lines are often adjusted, the return air is often not. In these cases while the furnace of this bungalow residence is only supplying air to the main floor, it is still pulling in return air from the basement which in turn may cause an excess negative pressure in the basement - a pressure difference which will directly impact radon system operation. If careful consideration to the required pressures on each of these floors is not made, radon levels can spike each time these zones are activated.
Unbalanced Forced Air - In theory all rooms of a home should have the same amount of supply air as return air. However anything from poorly considered layouts to closing off a particularly troublesome vent and even moving a couch in front of a return air can alter this delicate balance. Pressure changes occurring from HVAC operation should always be measured to ensure they will not have an adverse effect on the operation of the ASD system.
What If A Radon System Doesn’t Work?
We hear you. You’re looking to correct the problem entirely not make some adjustments which may improve your radon levels. The good news is there is always a way to achieve an effective radon solution. We’ve included a couple of the techniques commonly used in trickier mitigation systems. By no means is this a comprehensive list of available solutions but common techniques we can employ if a system is struggling to achieve full communication include:
Better Fans – We are proud to use the RadonAway line of radon mitigation fans. RadonAway’s commitment to the radon industry sets them far apart from the competition offering a wide range of fans superior to those available from the competition. While most manufacturers provide 3-4 fan sizes for mitigation system design, RadonAway boasts over 20 fans to meet varying conditions of a mitigation solution. Instead of just small, medium, and large options, RadonAway offers fans designed for a variety of unique conditions. To learn more about RadonAway fan selections and varieties click here.
Suction Pit – If the point the system is pulling from is not getting enough air we have several ways of opening up “veins” or air pathways beneath the home to assist the system and draw from all directions, improving air flow and system coverage.
Additional Suction Points – At times additional locations will be required to achieve full coverage. Just as a sprinkler in your lawn may not water all the grass in one location, another suction (or sprinkler) location can increase the range of coverage.

Gophering – A process in which multiple access holes are drilled in a line with cavity holes excavated directly below to open up additional air pathways sub-slab. After completion access holes are sealed which can extend communication and reach developed areas of the basement without requiring any additional equipment outside the mechanical room.
Auxiliary System – In some cases a second mitigation system