Soil Gas Barrier and Radon Requirements for the 2024 OBC

Date of Publication:  November 21, 2025 [2024.09.V-01.C]

Subject:  Soil Gas Barrier and Radon Requirements for the 2024 OBC

2024 Ontario Building Code Reference(s):

Subsection 5.4.1. of Div. B.

Article 9.1.1.7. of Div. B.

Subsection 9.13.4. of Div. B. and Appendix Notes

Subsection 9.25.3. of Div B.

Supplementary Bulletin SB-9

Incoming Request: 

What has changed in the 2024 OBC for radon and soil gas control?

Executive Summary:

Soil gas barriers are specifically required for all assemblies in contact with the ground. Joints, penetrations and perimeters must be sealed to provide an effective soil gas barrier. In a typical residential basement this means a separate membrane air barrier, as a typical concrete slab is not considered to provide an air barrier. A sub-slab depressurization system rough-in is also required for all Part 9 residential uses.

Discussion & Considerations:

The 2012 OBC contained a number of measures meant to reduce soil gas ingress, but the 2024 OBC has reorganized and expanded these requirements, with the intent to reduce radon and other soil gas levels inside enclosed and occupied buildings as much as possible. See A-9.13.4. for general background on the risks of radon. The main changes are summarized below:

• Part 9 air barrier requirements have been expanded to clarify that an air barrier is required to “minimize the ingress of soil gas”. See Sub-clause 9.25.3.1.(1)(b)(iii).
• Part 5 air barrier requirements have been expanded to specifically call out exclusion of soil gas. See Clause 5.4.1.1.(1)(e), Sentence 5.4.1.2.(4). and Appendix Note A-5.4.1.2. 
• Subsection 9.13.4. (Soil Gas Control) has been expanded and reorganized to include scope and application, protection from gas ingress and the installation of a rough-in for a future sub-slab depressurization system in buildings with a residential use. See Articles 9.13.4.1., 9.13.4.2.., and 9.13.4.3., and the new Appendix Notes A-9.13.4.(2) and (3).
• Per Sentence 9.13.4.2.(1)., required protection from soil gas ingress can be provided either in accordance with SB-9 (which requires compliance with dampproofing Article 9.13.2.6, and air barrier Articles 9.25.3.2 and 9.25.3.3.) or in accordance with Subsection 9.25.3.. If polyethylene sheeting is used, it must conform to CAN/CGSB‑51.34‑M., be lapped a minimum of 100 mm and sealed.
• 2012 OBC Article 9.25.3.3.(9), which spoke to the use of a basement slab as an air barrier, has been removed.
• In addition to the clarified soil gas barrier requirements, Part 9 residential buildings (or residential parts thereof) require a rough-in for a future sub-slab depressurization system. See Sentence 9.13.4.2.(2)., Article 9.13.4.3., and Notes A-9.13.4.3. and A-9.13.4.3.(2)(b) and (3)(b)(i). for details.
• Part 9 buildings of non-residential occupancies are required to have a means “to address high radon concentrations in the future” (see Article 9.13.4.2.(3)).
• Part 3 residential buildings are not subject to 9.13.4.2.(2) or 9.13.4.3., however they are subject to Article 5.4.1.1. (air barriers) and Article 6.2.1.1. (good engineering practice). 
• Notably, the 2012 OBC requirement for radon testing results to be provided to the AHJ has been removed. However Article 9.13.4.1.(3). still references the limit of 200 bq/m3 in “areas where radon gases are known to be a problem” (see Article 9.1.1.7.), above which a system must be installed to reduce the radon level.
• Appendix Note A-9.13.4.2.(3). discusses exemptions for buildings that are infrequently occupied.

Risk/Benefit Analysis:

The 2024 Code has made it clear that a sealed soil gas barrier is required, in an effort to exclude soil gas from entering the building through assemblies (typically walls or floors) in contact with the ground. For typical low-rise residential basement, a common method to achieve this barrier will be the installation of 6 mil CGSB polyethylene sheeting under the basement slab with 300mm laps, sealed at all edges and penetrations. Where other means are proposed (including, but not limited to under-slab rigid panel-type insulation or spray foam insulation), the designer must demonstrate that the system, including penetrations, joints and perimeters, meets the air barrier requirements of Subsection 9.25.3.. 

In addition, all Part 9 residential buildings require a sub-slab depressurization system rough-in that can be put into service in the event that the soil gas barrier is not sufficient to keep radon levels below 200 bq/m³. These systems are detailed in the Appendix Notes for subsection 9.13.4.3. The Code does not delve deeply into the design of these systems, but for them to be effective the following should be considered:

• Underslab piping - both solid pipe and perforated pipe have been shown to be effective.
• Will the location of the extraction point be effective, i.e. an extraction point directly adjacent to a walk-out condition will not effectively extract from the entire slab.
• Size of the sub-slab area – are multiple extraction points necessary?
• Multiple slab levels will require multiple extraction points.
• High water table – will the extraction pipes become silted or submerged?
• Ensure the sub-slab material is not compacted and is gas-permeable.
• Will the location of the extraction point impede future finishing of the basement space?

It should be noted that the 2024 OBC does not directly link a Part 3 house to the Part 9 radon requirements, specifically the sub-slab depressurization system rough-in. However, Subsection 5.4.1. (Air Barrier Systems) requires air barrier assemblies to “minimize the ingress of airborne radon and other soil gases from the ground with an aim to controlling the indoor concentrations of these gases to an acceptable level”, and Article 6.2.1.1. references EPA 625/R-92/016, “Radon Prevention in the Design and Construction of Schools and Other Large Buildings”. 

Further information:

The Home Construction Regulatory Authority (HCRA) has produced a radon guide for licensed builders in Ontario. Health Canada has a detailed guide for radon risks and mitigation in houses. Finally, CAN/CGSB-149.11 and CAN/CGSB-149.12 contain information on radon reduction in new and existing buildings respectively. 

Final Recommendation:

To avoid problems arising during construction, submitted plans should include air barrier (soil gas barrier) details where assemblies are in contact with the ground, and details of the depressurization system rough-in, including compatibility of sealants and air barrier (soil gas barrier) materials. As this may be a new building practice for some, proactive builder education may be required.   

Recommendation to Ministry of Municipal Affairs and Housing:

None

Referenced Documents:

2024 Ontario Building Code:

Disclaimer:

This guidance document is intended to assist building officials by gathering relevant information to interpret the OBC Act and the prescriptive requirements of the Ontario Building Code, and is intended to be a best practice aid for building officials.

The views expressed within this guidance document should not be considered as the official interpretation of legislated requirements based on the Ontario Building Code, as the final responsibility for interpretation rests with the local Authority Having Jurisdiction. 

The views of this advisory committee should not be construed as legal advice.