Message From the President: CAN/CGSB 51.34-M86 for underslab? Really?

By David Boyle, CTR
The ‘asbestos of the 2010s,’ radon is a radioactive, colourless, odourless, tasteless gas that occurs naturally in soil. Everyone’s concern is how to vent off this dangerous gas from new and existing buildings. Unfortunately, there is confusion over which standard to use. Seminars are being conducted for municipal and government employees who are now on a quest to ensure their jurisdictions are safe for us and our future generations to come and safety features are being put into place. Many municipalities are now asking for any new residential or commercial structures to have a radon mitigation system that includes granular material, gas barrier, and vent stack. This is a great idea except most people only think of CAN/CGSB 51.34-M86, Vapour Barrier, Polyethylene Sheet for Use in Building Construction—a fine standard, but one intended for polyethylene vapour barrier in wall assemblies rather than underslab applications.

I have contacted various architectural firms and municipal employees who look after the construction and codes to educate them that they should instead be using ASTM E1745, Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs. I cannot tell if they are being patriotic by wanting to use a “Canadian” source, but this ASTM standard is international—and applies here.

In Scope, Section 1.2, CAN/CGSB-51.34-M86 explicitly says the standard is not intended to apply to vapour barrier materials used under concrete slabs (i.e. below-grade radon barriers), stating “Material to this standard is primarily intended for use in above-grade building construction and on the interior of below-grade building construction. This standard is not intended to apply to vapour barrier materials for use under concrete slabs or as ground cover in crawl spaces.”

Further, while some polyethylenes meet the vapour permeance requirements of CAN/CGSB-51.34-M86, they may fall short when it comes to punctures, throwing the permeance out the window. In ASTM E1745, each class has the same vapour permeance requirement, but it is differentiated based on puncture resistance and tensile strength. The CGSB standard also refers to ASTM testing like D882 (tensile strength and elongation), D1709 (impact strength), and E96 (water vapour permeance)—all already part of the requirements for ASTM E1745.

In the next issue of Construction Canada, I’ll have more to say on this important topic.

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