Radon and Soil Gas in the Ontario Building Code

Soil gas protection: the barrier

Every wall, roof and floor assembly in contact with the ground must be built to resist soil gas leakage into the building, either through the air barrier rules of Subsection 9.25.3. or through Supplementary Standard SB-9, "Requirements for Soil Gas Control" (9.13.4.2.(1)OBC Div. B 9.13.4.2. Protection from Soil Gas Ingress(1) Except as provided in Sentence (1.1), all wall, roof and floor assemblies in contact with the ground shall be constructed to resist the leakage of soil gas from the ground into the building in accordance with Subsection 9.25.3. or MMAH Supplementary Standard SB-9, "Requirements for Soil Gas Control." (1.1)…). The air barrier system must be continuous and extend throughout the basement, and one of its stated jobs is to minimize the ingress of soil gas (9.25.3.1.OBC Div. B 9.25.3.1. Required Barrier to Air Leakage(1) Wall, ceiling and floor assemblies that separate conditioned space from unconditioned space or from the ground shall be constructed so as to include an air barrier system that will provide a continuous barrier to air leakage (a) from the interior of the building into wall, floor, attic or roof spaces sufficient to…).

Where polyethylene is the soil gas barrier under a floor-on-ground, it must meet the CGSB polyethylene standard named in the code. SB-9 adds detail: a sheet barrier under a slab has joints lapped at least 300 mm, and one laid on top of the slab has sealed joints and a separate floor over it (SB-9, soil gas barriers in floorsOBC SB-9 3.1. Soil Gas Barriers in Floors(1) Except as required in this Section, where the floor-on-ground is a concrete slab, the soil gas barrier shall conform to Articles 9.13.2.6., 9.25.3.2. and 9.25.3.3. of Division B of the Building Code and be (a) installed below the slab, or (b) applied to the top of the slab, provided a separate floor is installed…). Perimeter joints and penetrations through the slab are sealed too.

Garages and unenclosed portions of buildings, such as open porches, do not need this protection (9.13.4.2.(1.1)OBC Div. B 9.13.4.2. Protection from Soil Gas Ingress(1) Except as provided in Sentence (1.1), all wall, roof and floor assemblies in contact with the ground shall be constructed to resist the leakage of soil gas from the ground into the building in accordance with Subsection 9.25.3. or MMAH Supplementary Standard SB-9, "Requirements for Soil Gas Control." (1.1)…).

The rough-in for a radon system

Dwelling units and buildings containing residential occupancies must have the rough-in, unless the space between the air barrier and the ground is designed to be accessible for a future system (9.13.4.2.(2)OBC Div. B 9.13.4.2. Protection from Soil Gas Ingress(1) Except as provided in Sentence (1.1), all wall, roof and floor assemblies in contact with the ground shall be constructed to resist the leakage of soil gas from the ground into the building in accordance with Subsection 9.25.3. or MMAH Supplementary Standard SB-9, "Requirements for Soil Gas Control." (1.1)…). For other occupancies, the builder can use the same rough-in or follow Parts 5 and 6.

A floor-on-ground needs one of two rough-ins (9.13.4.3.OBC Div. B 9.13.4.3. Providing for the Rough-in for a Subfloor Depressurization System(1) Floors-on-ground shall be provided with a rough-in for subfloor depressurization consisting of (a) a gas-permeable layer, an inlet and an outlet as described in Sentence (2), or (b) clean granular material and a pipe as described in Sentence (3). (2) The rough-in referred to in Clause (1)(a) shall include (a) a…):

1. Gas-permeable layer. A layer between the air barrier and the ground that can be depressurized, an inlet that allows effective depressurization of that layer, and an outlet in the conditioned space that can connect to a fan, is sealed into the air barrier, and is clearly labelled as only for removing radon from below the floor.
2. Granular fill and a pipe. Clean granular material under the slab (the same layer required by 9.16.2.1.OBC Div. B 9.16.2.1. Required Installation of Granular Fill(1) Except as provided in Sentence (2), not less than 100 mm of coarse clean granular material containing not more than 10% of material that will pass a 4 mm sieve shall be placed beneath floors-on-ground. (See also Subsection 9.13.4. and Note A-9.13.4.) (2) Granular material need not be installed under (a) slabs in…, at least 100 mm thick), and a pipe of at least 100 mm diameter through the floor. Its bottom end opens into the granular layer at or near the centre of the floor, with at least 100 mm of granular material beyond the pipe end. Its top end can connect to a fan and has an airtight cap. The pipe is labelled near the cap and, where applicable, every 1.8 m and at every change in direction.

What the code does not require

For a typical new home, the code requires the barrier and the rough-in, not an operating radon fan. The appendix note to SB-9 says that if testing shows an average annual radon concentration above 200 Bq/m³ in the normal occupancy area, measures such as converting the rough-in to an active subfloor depressurization system should be taken, with makeup air and the effect on foundation soil temperature considered (SB-9 note on radon testingOBC SB-9 Radon Testing and Active Depressurization Systems.Where radon concentration testing is carried out, it should be done in accordance with HC Pub. 4171, "Guide for Radon Measurements in Residential Dwellings (Homes), 2008", to determine the radon concentration in the building. The sampling should include basement concentration measurements. If the average annual radon…). Health Canada's guideline uses the same 200 Bq/m³ level and recommends testing every home.

A separate rule applies in three designated areas: the City of Elliot Lake, the Township of Faraday and the geographic Township of Hyman. Buildings there must be designed and built so the annual average radon concentration inside does not exceed 200 Bq/m³ (9.1.1.7.OBC Div. B 9.1.1.7. Radon(1) In addition to all other requirements, a building in the following designated areas shall be designed and constructed so that the annual average concentration of radon 222 does not exceed 200 Bq/m³ of air and the annual average concentration of the short lived daughters of radon 222 does not exceed 0.02 working…). Part 3 buildings have a matching rule (3.1.1.5.OBC Div. B 3.1.1.5. Radon(1) In addition to all other requirements, a building in the following designated areas shall be designed and constructed so that the annual average concentration of radon 222 does not exceed 200 Bq/m² of air and the annual average concentration of the short lived daughters of radon 222 does not exceed 0.02 working…).

How to look it up yourself

Open Volume 1 of the 2024 Compendium, Division B, Part 9, Section 9.13 "Dampproofing, Waterproofing and Soil Gas Control" (Section 9.13 in our viewer). Search terms that appear in the code text: "Protection from Soil Gas Ingress", "subfloor depressurization", "radon", "gas-permeable layer" and "Requirements for Soil Gas Control" (the SB-9 title, in Volume 2).

The BCIN exam angle

Soil gas and the radon rough-in are part of the House and Small Buildings syllabi. Read the free chapter on excavation, dampproofing, footings and foundations for the House exam, or foundations, framing and structure for the Small Buildings exam. Exam questions here are usually lookups: the pipe size, where the pipe ends, labelling, or which spaces are exempt. The Calculation Lab covers the numeric methods that sit beside these topics, such as footing sizing.

Related pages

• Ontario Building Code guide
• Basement insulation and the Ontario Building Code
• 2024 Ontario Building Code changes
• Part 9 overview