Guidance on Radon Rough-in Layer Depth Requirements

[Mac Moonfire]

I'm seeking clarification regarding depth requirements for radon rough-in layers. My colleagues have different interpretations:
1. One's perspective suggests the layer must be directly below the air barrier for efficient gas collection. Nowhere else.
2. Another proposes using the 5" coarse clean granular material (specified in 9.14.4 for foundation drainage) in place of drainage pipes. That is 100% below the footings.

I have reviewed 9.13.4 (Soil Gas Control), CAN/CGSB-149.11, and related literature. My research points to Note A-9.13.4.3.(2)(b) and (3)(b), which emphasizes "effective depressurization" but uses performance-oriented language rather than prescriptive depth requirements. The note states that "the extraction opening should not be blocked and should be arranged such that air can be extracted from the entire space between the air barrier and the ground." My interpretation is that this cannot be achieved at the same time as draining groundwater.

In my jurisdiction, I've been failing radon rough-ins positioned at or below drainage height. This decision stems from my interpretation that water infiltration could compromise the underslab depressurization's effectiveness by occupying the interspaces. Also, my interpretation is that the layer can be placed at any height above the drain tile, provided it remains separate from the under-slab plumbing grading. Some plumbers mix the radon gravel with the sand as they grade the pipe. It completely destroys the radon rough-in layer.

Has anyone encountered specific code requirements or authoritative guidance regarding the optimal depth for radon rough-in layers?

 

[TheCommish]

2015 IRC

AF103.2 Subfloor Preparation​

A layer of gas-permeable material shall be placed under all concrete slabs and other floor systems that directly contact the ground and are within the walls of the living spaces of the building, to facilitate future installation of a subslab depressurization system, if needed. The gas-permeable layer shall consist of one of the following:

1. A uniform layer of clean aggregate, a minimum of 4 inches (102 mm) thick. The aggregate shall consist of material that will pass through a 2-inch (51 mm) sieve and be retained by a 1/4-inch (6.4 mm) sieve.
2. A uniform layer of sand (native or fill), a minimum of 4 inches (102 mm) thick, overlain by a layer or strips of geotextile drainage matting designed to allow the lateral flow of soil gases.
3. Other materials, systems or floor designs with demonstrated capability to permit depressurization across the entire subfloor area.

AF103.4.2 Soil-Gas-Retarder​

Amendment
A soil-gas-retarder shall be placed on top of the gas-permeable layer prior to casting the slab or placing the floor assembly. The soil-gas retarder shall cover the entire floor area with separate sections lapped not less than 12 inches (305 mm) and shall extend upward six inches and be sealed to the wall with an ASTM C290 class 25 or higher sealant or equal. The soil-gas-retarder shall fit closely around any pipe, wire, or other penetrations of the material. Punctures or tears in the material shall be sealed or covered. Under-slab insulation, if used, shall be placed on top of the sheeting.

 

[tmurray]

I only have the 2024 edition of the BCBC, and there appears to be significant changes to this section.

CAN/CGSB-149.11 Appears to only be referenced for the piping itself, not the air barrier assembly and associated contiguous gas-permeable layer, so it would be of no help in this discussion.

The code is unclear about this aspect of the building and how it interacts with the other adjacent required elements. The fact that the code requires foundation drain elements and a contiguous gas permeable layer, but does not require them to be distinct or separate would suggest to me as a code user that the code does not require them to be distinct from one another. If it was the intention of the code that they be separate from one another, it would explicitly state this as a requirement.

Where the code is unclear, it would be up to interpretation of the local AHJ, baring a decision by the BC appeals tribunal.

 

[jar546]

Slightly off-topic but related; I've noticed a pretty significant decrease in basement humidity with the proper installation of a radon system.

 

[tmurray]

Slightly off-topic but related; I've noticed a pretty significant decrease in basement humidity with the proper installation of a radon system.

Probably from the reduction in moisture getting to the slab by capillary force.

 

[Min&Max]

I'm seeking clarification regarding depth requirements for radon rough-in layers. My colleagues have different interpretations:
1. One's perspective suggests the layer must be directly below the air barrier for efficient gas collection. Nowhere else.
2. Another proposes using the 5" coarse clean granular material (specified in 9.14.4 for foundation drainage) in place of drainage pipes. That is 100% below the footings.

I have reviewed 9.13.4 (Soil Gas Control), CAN/CGSB-149.11, and related literature. My research points to Note A-9.13.4.3.(2)(b) and (3)(b), which emphasizes "effective depressurization" but uses performance-oriented language rather than prescriptive depth requirements. The note states that "the extraction opening should not be blocked and should be arranged such that air can be extracted from the entire space between the air barrier and the ground." My interpretation is that this cannot be achieved at the same time as draining groundwater.

In my jurisdiction, I've been failing radon rough-ins positioned at or below drainage height. This decision stems from my interpretation that water infiltration could compromise the underslab depressurization's effectiveness by occupying the interspaces. Also, my interpretation is that the layer can be placed at any height above the drain tile, provided it remains separate from the under-slab plumbing grading. Some plumbers mix the radon gravel with the sand as they grade the pipe. It completely destroys the radon rough-in layer.

Has anyone encountered specific code requirements or authoritative guidance regarding the optimal depth for radon rough-in layers?

When we were told by the state that our jurisdiction would be required to install radon systems because of our extremely high numbers there was a lot of opposition. So myself and another staff member educated ourselves on radon, radon systems and radon mitigation. The most interesting thing we found was how radon mitigation was accomplished. Cut a hole in the lowest level floor, remove enough dirt to fill a 5 gallon bucket, place a 4" pipe in the hole, run the pipe to exterior of dwelling and seal around pipe where it enters floor, test and add fan if passive system has not lowered radon level sufficiently. Costs about $1,100.00 which is much cheaper than a full blown radon system at time of new construction. So we proposed to the state that we essentially install a mitigation system at time of new construction. A minimum of 4" of fill under the lowest level floor, 10ft of 4" perforated drain tile under the floor that exits thru the roof, an outlet within 6ft of the radon pipe in the attic or garage and a sealed sump pit if there is a basement. Passive system for about $300.00 that is radon fan ready. 90% of the time this passive system is all that is needed to keep radon levels within required range.

 

[Plumb-bob]

I'm seeking clarification regarding depth requirements for radon rough-in layers. My colleagues have different interpretations:
1. One's perspective suggests the layer must be directly below the air barrier for efficient gas collection. Nowhere else.
2. Another proposes using the 5" coarse clean granular material (specified in 9.14.4 for foundation drainage) in place of drainage pipes. That is 100% below the footings.

I have reviewed 9.13.4 (Soil Gas Control), CAN/CGSB-149.11, and related literature. My research points to Note A-9.13.4.3.(2)(b) and (3)(b), which emphasizes "effective depressurization" but uses performance-oriented language rather than prescriptive depth requirements. The note states that "the extraction opening should not be blocked and should be arranged such that air can be extracted from the entire space between the air barrier and the ground." My interpretation is that this cannot be achieved at the same time as draining groundwater.

In my jurisdiction, I've been failing radon rough-ins positioned at or below drainage height. This decision stems from my interpretation that water infiltration could compromise the underslab depressurization's effectiveness by occupying the interspaces. Also, my interpretation is that the layer can be placed at any height above the drain tile, provided it remains separate from the under-slab plumbing grading. Some plumbers mix the radon gravel with the sand as they grade the pipe. It completely destroys the radon rough-in layer.

Has anyone encountered specific code requirements or authoritative guidance regarding the optimal depth for radon rough-in layers?

Sounds like an interpretation from the Buildings Safety and Standards Branch is a good idea. Dont expect the answer to come quickly though.

 

[tmurray]

When we were told by the state that our jurisdiction would be required to install radon systems because of our extremely high numbers there was a lot of opposition. So myself and another staff member educated ourselves on radon, radon systems and radon mitigation. The most interesting thing we found was how radon mitigation was accomplished. Cut a hole in the lowest level floor, remove enough dirt to fill a 5 gallon bucket, place a 4" pipe in the hole, run the pipe to exterior of dwelling and seal around pipe where it enters floor, test and add fan if passive system has not lowered radon level sufficiently. Costs about $1,100.00 which is much cheaper than a full blown radon system at time of new construction. So we proposed to the state that we essentially install a mitigation system at time of new construction. A minimum of 4" of fill under the lowest level floor, 10ft of 4" perforated drain tile under the floor that exits thru the roof, an outlet within 6ft of the radon pipe in the attic or garage and a sealed sump pit if there is a basement. Passive system for about $300.00 that is radon fan ready. 90% of the time this passive system is all that is needed to keep radon levels within required range.

This is the current requirement in British Columbia and is likely to be incorporated into the 2025 National Building Code.

 

[thomasclark102]

Place the radon rough-in layer just below the air barrier, not at drainage depth, to avoid water blocking airflow and to ensure good soil gas extraction.