Page: Last modified: 2024-01-30
Code Reference(s):
NBC20 Div.B (first printing)
NBC20 Div.B (first printing)
Radon and Soil Gas Mitigation
Sealed Overlapping Seams for Air Barriers on the Ground
This proposed change requires that air barriers in contact with the ground have overlapping seams that are sealed.
This change could potentially affect the following topic areas:


Currently, the National Building Code of Canada (NBC) does not require the seams of air barriers to be sealed when installed under a concrete slab. The previous assumption was that a mechanical clamping action between the concrete slab and the ground would provide continuous clamping along the length of the joint to seal the seam. However, the current requirements for granular fill to allow the movement of soil gases below the air barrier, as well as the presence of expansive/shrinking soils in certain areas of Canada, invalidate this original assumption.

An overlapping seam that is not properly sealed can cause soil gases, such as radon, to leak into the occupied space of the building. These gases affect indoor air quality, and radon exposure can cause an increase in the probability of lung cancer.


This proposed change requires that all overlapping seams of air barrier systems in contact with the ground be sealed prior to ballast, whether granular fill or a concrete slab is applied. 

This action reduces the probability of soil gas leakage at the overlapping seams, which reduces the probability of soil gases, such as radon, leaking into the occupied space of the building where the health of the occupants could be impacted.


[] Cover in Heated Crawl Spaces

[1] 1)Where a crawl space is heated, a ground cover consisting of not less than 0.15 mm polyethylene sheet conforming to CAN/CGSB-51.34-M, "Vapour Barrier, Polyethylene Sheet for Use in Building Construction", shall be installed as part of an air barrier system in accordance with Subsection 9.25.3.
[2] 2)The ground cover required in Sentence (1) shall have its joints lapped not less than 3100 mm, be sealed, and
[a] a)be sealed and evenly weighted down, or
[b] b)be covered with concrete not less than 50 mm thick.
[3] 3)The perimeter of the ground cover required in Sentence (1) shall be sealed to the foundation wall. (See Notes A-9.13.4.PROPOSED CHANGE A-9.13.4., A- and CHANGE A- and, and A- and (3).)
[4] 4)All penetrations of the ground cover required in Sentence (1) shall be sealed against air leakage. (See Subsection 9.25.3.)

[] Barrier Systems in Floors-on-ground

(See Note A- and CHANGE A- and
[1] 1)Materials used to provide a barrier to the ingress of air through floors-on-ground shall conform to CAN/CGSB-51.34-M, "Vapour Barrier, Polyethylene Sheet for Use in Building Construction".
[2] 2)Where the floor-on-ground is a concrete slab, the air barrier shall be
[a] a)installed below the slab (see Note A-, or
[b] b)applied to the top of the slab, provided a separate floor is installed over the slab.
[3] 3)Where the air barrier installed below a floor-on-ground is flexible sheet material, joints in the air barrier shall be lapped not less than 300 mm. (See Note A- and (3)PROPOSED CHANGE A- and (3).)
[a] --)have its joints lapped not less than 100 mm,
[b] --)be sealed
[i] --)at all joints, penetrations and junctions with foundation walls, footings and adjacent air and soil gas barrier systems, and
[ii] --)with flexible sealant conforming to Article
[4] 4)Where installed in conjunction with a framed floor-on-ground or above a floor-on-ground, the air barrier shall be installed in accordance with Article
[5] 5)A floor-on-ground shall be sealed around its perimeter to the inner surfaces of adjacent walls using flexible sealant.
[6] 6)All penetrations of a floor-on-ground that are required to drain water from the floor surface shall be sealed in a manner that prevents the upward flow of air without preventing the downward flow of liquid water.

Note A- and (3) Polyethylene Air Barriers under Floors-on-Ground.

Floors-on-ground separating conditioned space from the ground must be constructed to reduce the potential for the entry of air, radon or other soil gases. In most cases, this will be accomplished by placing  0.15 mm polyethylene under the floor.
Finishing a concrete slab placed directly on polyethylene can, in many cases, cause problems for the inexperienced finisher. A rule of finishing, whether concrete is placed on polyethylene or not, is to never finish or “work” the surface of the slab while bleed water is present or before all the bleed water has risen to the surface and evaporated. If finishing operations are performed before all the bleed water has risen and evaporated, surface defects such as blisters, crazing, scaling and dusting can result. In the case of slabs placed directly on polyethylene, the amount of bleed water that may rise to the surface and the time required for it to do so are increased compared to a slab placed on a compacted granular base. Because of the polyethylene, the excess water in the mix from the bottom portion of the slab cannot bleed downward and out of the slab and be absorbed into the granular material below. Therefore, all bleed water, including that from the bottom of the slab, must now rise through the slab to the surface. Quite often in such cases, finishing operations are begun too soon and surface defects result.
One solution that is often suggested is to place a layer of sand between the polyethylene and the concrete. However, this is not an acceptable solution for the following reason: it is unlikely that the polyethylene will survive the slab pouring process entirely intact. Nevertheless, the polyethylene will still be effective in retarding the flow of soil gas if it is in intimate contact with the concrete; soil gas will only be able to penetrate where a break in the polyethylene coincides with a crack in the concrete. The majority of concrete cracks will probably be underlain by intact polyethylene. On the other hand, if there is an intervening layer of a porous medium, such as sand, soil gas will be able to travel laterally from a break in the polyethylene to the nearest crack in the concrete and the total system will be much less resistant to soil gas penetration.
To reduce and/or control the cracking of concrete slabs, it is necessary to understand the nature and causes of volume changes of concrete and in particular those relating to drying shrinkage. The total amount of water in a mix is by far the largest contributor to the amount of drying shrinkage and resulting potential cracking that may be expected from a given concrete. The less total amount of water in the mix, the less volume change (due to evaporation of water), which means the less drying shrinkage that will occur. To lessen the volume change and potential cracking due to drying shrinkage, a mix with the lowest total amount of water that is practicable should always be used. To lower the water content of a mix, superplasticizers are often added to provide the needed workability of the concrete during the placing operation. Concretes with a high water-to-cementing-materials ratio usually have high water content mixes. They should be avoided to minimize drying shrinkage and cracking of the slab. The water-to-cementing-materials ratio for slabs-on-ground should be no higher than 0.55.

Impact analysis

This proposed change only pertains to overlapping seams under a concrete slab. The NBC currently requires all exterior seams and penetrations to be sealed, as well as where ballast is placed on ground cover instead of a concrete slab. The cost of material and labour to seal the overlapping seams is estimated to range between $55 and $60.

There is an expected benefit of an increase in airtightness. This increased airtightness would reduce radon entry into the building, which could cause the adverse health effect of lung cancer. Increased airtightness could also reduce moisture entry where excessive moisture could cause mould issues. However, there is no research at this time to quantify this impact.

Enforcement implications

There are no expected enforcement implications as the inspection of air barriers is already required and the sealed seams would be addressed during these inspections.

Who is affected

Occupants would benefit from the reduced risk of soil gases and moisture leaking into the building, causing adverse health effects.

Contractors would be affected by the labour and material required to seal overlapping seams where a concrete slab is to be poured.


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