Understanding the New Radon Requirements (PDF)

Radon may be present in any home in Canada,  and because radon levels can only be confirmed when a home is occupied, building codes aim to minimize radon entry from the ground and reduce potential exposure for homeowners – at the time of construction, making it critical that builders understand how to minimize the entry of radon to give homeowners the best chance of achieving the lowest possible radon levels.

How Building Codes Deal with Radon

Historically, protection from soil gas ingress into homes was achieved through basic passive resistance of the home’s components such as below-ground air barriers, dampproofing products, the quality of concrete for foundation walls and basement slabs. As well, mechanical ventilation – as an active measure – helped to dilute any indoor levels of radon.

The 1995 National Building Code (NBC) included a major update to introduce soil gas protection. It did however not require soil gas protection in every home. The code exempted protective measures where it could be shown that soil gas did “not constitute a hazard”. In other words, only where a problem was thought to be likely (such as in radon hot spots like Elliott Lake, ON where levels were known to likely be above 800 Bq/m³). For example, the code included a requirement to seal the slab perimeter to the foundation wall and sealing of all penetrations (mostly pipes) and sealing the perimeter of polyethylene ground cover in a crawlspace to the foundation wall.

The code also included a requirement to test for radon levels if an active subfloor depressurization system was actually installed. In practice, this meant that most homes were built without any radon mitigation measures.

In response to Health Canada lowering the acceptable level of radon from 800 to 200 Bq/ m³, the 2010 NBC introduced performance and prescriptive requirements for a pipe stub rough-in connected to the space below the slab, intended to facilitate future mitigation (should a test by the homeowner indicate that mitigation is necessary). The 2010 code also added a requirement to seal the sump pit cover. These requirements remain in the 2025 NBC.

The 2025 NBC now requires all homes across Canada to comply with a new prescriptive requirement for a radon pipe stack extending from the pipe stub rough-in at the basement slab up to and through the roof (this is also known as a ‘passive stack’). The new code also requires all joints in the air barrier material below the basement or crawlspace slab to be sealed. The goal of the 2025 code is to reduce radon levels in all homes across Canada, thereby reducing the instances of lung cancer. The new 2025 requirements for a passive stack are based on requirements that were first adopted in British Columbia and have been shown to lower radon levels by 40-90% through research carried out by the National Research Council.

By mid 2027, the 2025 Harmonized National Construction Codes are expected to be adopted by all provinces.

New Requirements in the 2025 NBC

Application

The radon requirements apply to all conditioned spaces in contact with the ground for all buildings falling under Part 9.

What’s New? The 2025 code now clarifies that the radon requirements apply to all conditioned spaces in dwelling units and home-type care occupancies, but only when a significant portion of the living space is on the ground level, and not if less than 10% of the occupied area is a floor on ground, such as a stairwell in a walk‑up unit, the requirements do not apply. This avoids having to install a passive stack where it would be difficult to do so.

Compliance Methods

There are two compliance methods for the radon requirements. A prescriptive path and a performance path.

The prescriptive path includes specific technical specifications for the fill under the slab, the soil gas barrier, the pipe- rough-in connecting the sub-slab radon collecting space with the above-slab space and the specific details of the stack pipe connecting the rough-in to the exterior above the roof.

The performance path – unchanged in the 2025 code – still provides more flexibility but only applies to fill under the slab, the soil gas barrier, and the pipe rough-in.

Prescriptive Requirements

Reducing the likelihood of radon entry

To reduce radon entry into the home, the code already requires a number of basic air/soil gas  barrier details, such as installing a 0.15 mm (6 mil) thick polyethylene sheet conforming to CAN/CGSB-51.34 or another radon/soil gas barrier that has been tested and proven to perform at least as well as 6 mil polyethylene under the basement slab. For heated crawlspaces, the code requires the same radon/soil gas barrier but requires that it is weighed down or covered with 50 mm concrete.

The code also requires the lids of sump pits and any other penetration through the floor or the foundation wall to be sealed (which would also include back water valve basins to be sealed, which are reportedly overlooked, despite the fact that off-the-shelf solutions are available).

What’s New? The 2025 NBC now requires that all joints in the radon/soil gas barrier are sealed with a flexible sealant, and – in the case of a heated crawl space the code now clarifies that the membrane has to be weighed down with 50 mm of clean granular fill. This means that in addition to sealing the radon/soil gas barrier at the foundation, all overlapping joints will also need to be sealed with a “flexible sealant” designed for this purpose.

Preparing a radon extraction system – creating an air space below the slab

The code already requires a layer of clean granular fill (usually crushed stone or gravel) under the slab to create a connected air space that allows radon to move freely toward the open pipe end of the rough-in, reducing the chance that it enters the home. No changes were made in 2025.

Preparing a radon extraction system – the pipe

To extract radon from under a basement or crawlspace slab, the NBC already requires a “pipe stub  rough-in” installed to create a connection to the space under the slab which will “facilitate future radon mitigation”.  The above-the-slab end of the pipe rough-in needs to be capped and labelled.

What’s New? If using rigid pipe under the slab to collect the radon, the 2025 NBC requires the intake end of that pipe to be protected with stainless steel mesh to prevent the pipe opening from getting blocked with gravel. The rough-in is now being used for connecting a passive stack while the requirement to cap and seal it remains, if the rough-in is not being connected to a passive stack.

ALL NEW! – Completing the radon extraction system to vent above-roof level (Passive Stack)

A “Passive Stack” is technical shorthand for a pipe extending from the rough‑in to above the roof. The system is described as “passive” because it relies on natural air movement through the pipe rather than a fan or other mechanical equipment. It is described as a “stack” because the vertical pipe functions like a chimney, using the natural tendency of warm air to rise to draw soil gas upward and vent it above the roof.

What’s New? The stack has to be installed conforming to three specific clauses in CAN/CGSB-149.11-19, Radon control options for new construction in low-rise residential buildings, which require: 1) installing the passive stack inside habitable space within walls that are surrounded by “conditioned space”; 2) testing of the stack for leaks by one of two methods outlined in the standard; 3)  locating the stack penetration at the roof level away from roof valleys or locations “where snow or ice are likely to accumulate”. The standard also sets out acceptable pipe materials, fittings, and installation practices including use of collars to maintain fire resistance though fire rated walls or ceiling, and use of schedule 40 pipe with an inside diameter of 100 mm. The standard is available on-line at no cost.

What’s New? Continuous vertical pipe with limited offsets – The radon stack must form a continuous pipe from the soil gas layer beneath the slab to a termination above the roof. The pipe must be routed as vertically as practicable, with limits on horizontal runs and bends, to support natural upward air movement and effective stack action. If horizontal offsets in the passive stack are unavoidable, they need to be limited to 3.6 metres on each storey with a 1-in-50 slope back to the stack for drainage.

What’s New? Damage protection – Much like a plumbing stack, where it runs through conditioned space, a radon stack needs to be protected against puncture with metal plates at all intersections of joists, studs, plates and other framing members.

What’s New? Insulation in unheated spaces – Where the radon stack passes through unheated spaces, including attics, it must be insulated to prescribed levels to reduce condensation and help maintain the stack effect, and any penetration must be sealed where the stack passes through the air barrier system. In some regions, insulation is also required above the roof depending on stack height and January temperature.

What’s New? Roof termination requirements – The radon stack must terminate above the roof, in a location that minimizes the potential for exhausted soil gas to re‑enter the building, such as openings, for which the code provides minimum clearances. The pipe termination must be protected against blockage, debris and vermin with stainless steel mesh or another material with equivalent air flow performance.

What’s New? Future activation capability – The installation must allow for future conversion to an active system, meaning the pipe size, routing, and access to the stack must accommodate the later addition of a fan if post‑occupancy radon testing by the homeowner shows elevated levels. Allowance needs to be made to have enough space in an unconditioned attic to install a radon mitigation fan in the future.  Where this is not possible, say in the presence of a cathedral ceiling, a space can be identified in a conditioned area of the house including the basement.  (Note: While an electrical outlet is not required by these new code provisions, it is certainly good practice and would make it much easier to install a fan should one be required in the future.)

Performance Requirements

Unlike the prescriptive provisions, which specify materials, dimensions, and installation details, the performance language focuses on achieving effective sub‑slab depressurization and allows flexibility in how that outcome is met. It uses generic language to provide that flexibility, so that proprietary radon protection systems that serve the same purpose as the gravel and air barrier can meet the code, for example proprietary insulation products installed over compacted fill that allow for air movement under the slab and offer a central collector point for the rough-in.

The performance path requires:

• A gas‑permeable layer between the air barrier and the ground that allows soil gas to move freely so the space can be depressurized.
• The system must include one or more inlets arranged so the gas‑permeable layer can be effectively depressurized, not just locally but across the sub‑slab area.
• An outlet must be provided in the conditioned space that allows future connection to radon depressurization equipment.
• The outlet must be sealed to maintain the continuity and integrity of the air barrier system where it passes through the slab or floor assembly.
• The rough‑in outlet must be clearly identified as a radon rough‑in, intended only for the removal of radon from below the floor‑on‑ground.
• The rough‑in must be arranged so it can be converted to an active system in the future, if post‑occupancy radon testing shows elevated levels.

Alternative Solutions

If a builder chooses or needs to install an active (with fan) mitigation system instead of a passive stack, the Code language includes an explanatory note to enable builders to apply for an alternative solution. The CAN/CGSB-149.11 standard also contains detailed guidance on how to install an active system. It is considered best practice to consult with a local certified radon mitigation contractor during the design and construction stages. The Canadian National Radon Proficiency Program maintains an up-to-date list of trained and certified radon professionals.

Design Advice from Radon Mitigators

The following issues are often encountered by radon mitigation contractors. They are worth taking into consideration when developing project specifications and monitoring work on-site:

Use “Schedule 40” PVC pipe for below-ground and above-ground applications 

Radon mitigation contractors have arrived at homes to install a radon mitigation system and discovered the pipe that was originally installed beneath the slab and protruding out of the slab was not the correct type of pipe for above ground use and could not be used  (e.g. ABS piped rated as schedule 40 regarding wall thickness, inside and outside diameters and pressure ratings). For the portion below ground, some contractors will use a loop of perforated drainage tile instead of solid pipe, however the pipe above the slab needs to be solid.

Do not cap the intake end of the soil gas collector pipe under the slab

Because of an error in a code illustration, some soil gas collector pipes were capped at the end that is under the slab making the entire assembly under the slab unusable because it does not allow airflow from the sub-slab space through the pipe.  In this case, radon mitigation contractors had to abandon the existing pipe and start over by coring a new hole in the concrete slab and creating a new “suction pit”. The incorrect diagrams that created confusion in the industry were corrected in the 2020 NBC to help resolve this misunderstanding.

Always cap, seal and label the rough-in pipe stub

Mitigators occasionally find rough-in pipes that are not capped or sealed – which can increase the radon level. It’s important to cap and seal the rough-in pipes and to label them as required by Code so that they can be easily identified (C-NRPP labels for radon control systems can be found here.)

Use a high‑movement elastomeric sealant at the slab perimeter

The joint between the slab and the foundation wall is a common radon entry point, as the joint often widens after the concrete has fully cured. Standard sealants may not be able to accommodate this movement and can crack or lose adhesion over time. It is therefore recommended to use a high‑performance elastomeric sealant for concrete joints that are capable of accommodating significant joint movement. Where joints are wider, a backer rod should be used to support the sealant and maintain long‑term flexibility.

Insulate Pipes in Attics and add sufficient pipe supports

A lack of insulation of pipes in attics can cause freezing inside a radon stack, which can lead to the pipe being entirely blocked. The additional weight of the pipe with the ice and insufficient pipe supports have caused pipes to fall on mitigation contractors when cutting the pipe to add a fan for an active system. It is worth spending time on the sourcing of the proper insulation products meeting the prescribed RSI values for unconditioned areas of a home, as well as above the roof.

Plan for a chase or wider framing members to accommodate passive stack

It’s important to determine where the radon stack will be installed during the design stage. Either a chase or wider framing members would be required to accommodate a radon stack that meets all of the prescribed requirements, all of which is easier to be addressed at the design stage. 

Further reading

CGSB 149.12-2024 Radon mitigation options for existing buildings

For additional guidance on installing an active radon mitigation system you may wish to refer to CGSB 149.12-2024 Radon mitigation options for existing buildings, which can be downloaded at no cost.

CGSB-149.11-2024 Radon control options for new construction in low-rise residential buildings

The 2025 code requirements reference the 2019 version of CAN/CGSB-149.11 standard rather than its latest version published in 2024. Note that Building officials may allow compliance with the updated standard.  However, strictly speaking, what the National Code says will govern in case of a conflict or inconsistency, which, in this case, is the 2019 edition of the standard. 

Health Canada’s Technical information on radon includes

• the ‘Radon Guideline’ for new and existing homes and other helpful resources such as the ‘Guide for radon measurements in homes’

• The Canadian National Radon Proficiency Program (C-NRPP) trains and certifies Radon Measurement & Mitigation Professionals and hosts a list of recommended consumer and professional measurement test devices. C-NRPP also offer courses aimed at builders and trades involved in new home construction.

Our research on radon - National Research Council Canada describes ongoing radon field and lab studies being conducted to help inform the work of committees developing codes and standards.