Radon is a naturally occurring radioactive gas that forms when uranium in soil and rock breaks down. It’s colourless, odourless, and tasteless. Which means you won’t know it’s there without testing or knowing there’s a mitigation system designed to move it out. It’s also the leading cause of lung cancer in non-smokers in Canada, according to Health Canada.
Here’s what most homeowners don’t realize. Radon isn’t a problem you discover and then solve; it’s a problem you plan for, ideally before you break ground.
When a builder treats radon control as part of the design process rather than an afterthought, the result is a system that costs less, performs better, and doesn’t require tearing apart a finished basement to install.
This is usually the first question homeowners ask, and the most common answer, “it depends on your area,” is technically true but not particularly useful.
Radon concentrations vary dramatically from one property to the next, even on the same street.
Two homes with identical footprints, built in the same year, can have meaningfully different radon levels depending on microsite soil conditions, foundation type, and how the home is ventilated. A high-end custom build in a “low-risk” postal code is not automatically safe. A modest home in a “high-risk” area isn’t automatically problematic.
The only way to know what’s happening in a specific home is to test it or build a system designed to manage soil gases regardless.
That said, a few things genuinely increase the likelihood of elevated radon levels:
Foundation type and condition play a significant role. Any home with a basement, crawl space, or slab-on-grade foundation has a direct interface with the soil, and that’s where radon originates. Cracks in the slab, gaps around service penetrations, sump pits, floor drains, and hollow-core block walls can all act as entry points.
Lowest occupiable level matters too. Radon concentrations are highest closest to the soil. A basement bedroom or home office is a higher-exposure space than a second-floor living room, even in the same building.
Occupancy patterns and airtightness affect how radon accumulates. A well-sealed, energy-efficient home, the kind built for comfort and performance, can actually concentrate radon faster than a leaky older home, simply because there’s less dilution from outside air. This doesn’t mean airtight homes are riskier; it just means proper radon planning matters more when you’re building to a high-performance standard.
Uranium is present in varying concentrations in virtually all soils and rock. As it decays, it produces radium, which in turn produces radon gas. That gas moves through soil and can enter any structure that sits on or in the ground. The mechanism is straightforward: radon follows the path of least resistance, and buildings, even well-built ones, can create enough pressure differential with the surrounding soil to pull soil gases inward.
Buildings tend to operate at slightly lower pressure than the soil around them. Warm indoor air rises and escapes through the upper portions of the structure (the stack effect), exhaust fans pull air out, and HVAC systems create localized pressure zones. The result is a subtle but consistent negative pressure at the lowest level of the home, exactly where it meets the soil. That pressure difference is what can draw radon in through any available gap or opening.
This is also why simply sealing cracks, while helpful, isn’t a complete solution on its own. As long as the pressure differential exists, radon can find a path. Mitigation systems address the pressure relationship directly, rather than just plugging individual entry points.
The most typical radon entry points in residential construction include: cracks or cold joints in concrete slabs and foundation walls, the gap between the slab and the foundation wall, construction joints, sump pits and floor drains, and penetrations for plumbing, electrical, or HVAC services. In homes with block foundations, the hollow cores of the blocks themselves can act as conduits.
A radon mitigation system is any combination of construction methods and mechanical systems designed to prevent radon from accumulating inside a home at harmful concentrations.
In new construction, the goal is prevention, designing the building so radon entry is minimized and any gas that does enter is routed out before it reaches occupied spaces. In an existing home, mitigation typically means retrofitting a system that depressurizes the soil beneath the slab and vents soil gases to the exterior.
The terms radon abatement and radon mitigation are often used interchangeably, particularly when referring to corrective work in existing homes.
The fundamental goal is the same in either case: disrupt the pressure relationship between the soil and the interior of the home and provide a controlled pathway for soil gases to escape safely.
A radon venting system is the physical infrastructure that channels soil gases out of the building. It typically consists of a suction point below the slab (a collection pit filled with aggregate), a vertical pipe (the vent stack) that runs from below the slab to above the roofline, and, in active systems, an inline fan that creates continuous negative pressure in the sub-slab zone.
A passive system relies entirely on the natural pressure differential between the sub-slab zone and the exterior atmosphere. The vent stack creates a pathway for gas to escape, but there’s no mechanical assistance. Passive systems are less expensive to install, have no operating costs, and require little to no maintenance. However, their effectiveness depends on the pressure dynamics of a given site and building.
In some homes and climates, passive venting is sufficient. In others, it’s a starting point that will need to be upgraded.
An active system adds a continuously running fan to the vent stack, which creates reliable negative pressure in the sub-slab aggregate layer regardless of seasonal or building-related pressure changes.
Active systems are substantially more consistent performers. The fan draws soil gases out from under the slab and exhausts them through the stack, well above head height and away from windows and air intakes. They do consume a small amount of electricity, but modern radon fans are quiet and energy-efficient.
In a well-designed radon mitigation system installation, the visible components are minimal. The vent stack typically exits through the building, usually in the mechanical room or utility space, runs up through the conditioned envelope (or an insulated chase, if exterior routing is required), and terminates above the roofline.
In an active system, the fan is usually mounted in a location that is accessible but unobtrusive, such as the attic, garage, or home exterior. The above-roof termination is the most visible exterior element, and in a thoughtfully detailed build, it’s no more noticeable than a plumbing vent.
New construction is where radon planning pays the biggest dividends. The cost of roughing in a radon system during construction is a fraction of the cost of retrofitting one later, and the quality of the result is typically better, because the builder has full access to the sub-slab zone and can integrate the system into the building’s layout from the start.
The foundation of any effective radon system is a continuous layer of permeable material that allows communication under the slab. This aggregate layer is what allows the sub-slab zone to be depressurized effectively.
The suction point of the under-slab is typically positioned near the centre of the slab and is protected by a cage to ensure the suction end can not be blocked. This is where the vent stack will connect.
Air sealing at the slab level is the next critical layer. Every plumbing, electrical, and HVAC penetration is sealed at the slab. The joint between the slab and the foundation wall is also addressed.
This isn’t unique to radon control; it’s consistent with a high-performance building approach generally. Reducing the pathways for soil gases to bypass the mitigation system makes the system more effective and reduces the load on the fan if one is added later.
Ideally, the vent stack runs within the conditioned building envelope — through a mechanical room, a closet, or a dedicated chase. Interior routing keeps the stack warmer, which helps passive convection in passive systems and protects the fan in active ones. It also makes for a cleaner exterior appearance.
In our BC climate, a passive stack must be installed in the conditioned space. Should it not be practical to run a passive stack or any stack due to architectural or engineering constraints, a side-wall-vented pipe with a fan can be used.
The only time you can perform a radon test is when the home is complete. Proper tests last 60-90 days, and the house needs to be sealed. Winter is therefore the best time to test.
The critical thing is that the infrastructure supports a future upgrade: the aggregate and suction pit are already in place, the stack is already routed, and adding an active fan at any point in the future is a straightforward connection at the stack. Leaving just a few hours of work to upgrade rather than a major renovation.
At RDC Fine Homes, radon rough-in planning is a standard part of our pre-construction process. By the time we’re ready to pour, the system is designed, the sub-slab depressurization system is specified, and the stack route is confirmed. It adds minimal time and cost at that stage and ensures we’re never caught flat-footed by a post-occupancy test result.
Retrofitting a radon mitigation system into an existing home is entirely achievable, but it involves more variables and typically more cost than planning it from the start.
The most common and effective approach is sub-slab depressurization: a contractor cores through the slab, excavates a small suction pit in the aggregate (if present) or removes material to create one, installs a pipe through the penetration, seals around it, and connects it to a vent stack routed to the exterior. An inline fan is added to create active negative pressure. Done correctly, this approach can bring elevated radon levels down to acceptable concentrations in most homes.
In homes with crawl spaces, a sealed vapour barrier and sub-membrane depressurization is the standard approach. Similar in principle to sub-slab work but adapted for the crawl space geometry.
Sealing alone, filling cracks and penetrations without adding a depressurization system, is sometimes used as a supplementary measure but is not as effective as a standalone solution. Radon will find new pathways as the building moves and settles over time.
The main complications in a retrofit are access, routing, and existing finishes. A fully finished basement limits where the contractor can core the slab, reduces access to the foundation wall, and makes routing the vent stack through the interior challenging without disturbing ceilings, walls, or mechanical systems.
Homes without sub-slab aggregate require additional excavation to create a functional suction pit, and the depressurization field may be limited if the pit is small or isolated.
This is why discovering a radon problem after a basement is finished and furnished is a substantially more disruptive (and expensive) situation than addressing it pre-construction.
After any mitigation system is installed, new build or retrofit, a long-term test is the standard way to confirm the system is performing as expected. This typically means a 90-day test in the lowest occupied level of the home, ideally during heating season when windows are closed, and conditions are most likely to produce elevated readings.
A well-designed and properly installed system should bring levels well below Health Canada’s action guideline of 200 Bq/m³.
There is no safe limit to radon levels. Health Canada’s current guideline establishes a reference level of 200 Bq/m³ (becquerels per cubic metre). While other countries suggest an even lower reference of 100 Bq/m³. At or below this level, no action is required. Above it, mitigation is recommended.
The important thing to understand is that these are action thresholds, not sharp lines between “safe” and “dangerous.” Radon exposure risk accumulates over time, and lower is always better. If a test result comes back elevated, mitigation and venting is the practical, effective solution path.
Yes. Sub-slab depressurization is used in slab-on-grade construction; the approach is the same, though aggregate prep and suction pit placement need to be planned at the time of the pour.
This is actually one reason radon rough-in planning matters in all foundation types, not just basements.
No. There is no test data that shows that air filters or purifiers reduce radon levels. They do not address radon gas itself and should not be relied upon as a mitigation strategy. A properly designed venting or depressurization system is the only effective approach.
Yes, meaningfully. Radon concentrations in Canadian homes tend to be highest during the heating season, fall through early spring, when windows and doors are kept closed, and the building operates at greater negative pressure relative to the exterior.
This is why Health Canada recommends long-term tests be conducted over at least 90 days, ideally spanning the winter months, to get an accurate picture of actual exposure.
The best time to ask these questions is before construction begins. A well-prepared builder should have answers ready for all of them.
If a builder hasn’t thought through these questions before you ask them, that tells you something.
Radon is a very manageable problem, but it’s most manageable when it’s treated as a design consideration rather than a remediation project.
In new construction, the cost of roughing in a complete, upgradeable system is modest, the disruption is zero, and the long-term result is a home that performs well regardless of what the soil beneath it is doing.
In renovations, solutions exist for virtually every situation, but the constraints are real, and the costs are higher.
Either way, the answer is the same: mitigation and venting, designed well and installed correctly, is the practical path to a home where radon simply isn’t a concern.
If you’re planning a new build or renovation and want to understand how radon planning fits into the process, we’re happy to walk through the specifics with you.
The fastest way to get rid of radon would be to hire a radon mitigation company for an assessment on how to remove the radon from under your house.
So it’s important that everybody understands that there is no safe level of radon per se.
The Canadian government and Health Canada have deemed that a radon level over 200 Bq/m³ requires mitigation.
Yes, there’s always a solution to mitigate radon in a Radon mitigation.
An active radon mitigation system would modestly increase energy use in a house; it doesn’t increase energy use much, but it would certainly increase your usage.
A national code change under consideration would require a passive mitigation system in every new home. In certain municipalities in British Columbia, mandatory radon mitigation is currently required for new construction.
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