In the 2021 edition of the International Residential Code (IRC), the guidelines were revised to recommend a minimum 10 mil vapor barrier for use under concrete slabs in residential buildings instead of a 6 mil.
The American Concrete Institute (ACI) also recommends a minimum of 10 mil polyethylene or thicker for vapor retarders under concrete. According to its Guide to Concrete Floor and Slab Construction ACI 302.1R-15, thicker poly provides a better barrier and is also more puncture resistant. ASTM E-1745 also advocates for thicker poly or other membrane material to be used to form more of a barrier against vapor.
Benefits of using a Vapor Barrier Under Concrete Slabs
Protection against Degradation
Excess exposure to ground moisture can cause concrete slabs to degrade faster and rebar corrosion. A vapor barrier protects the concrete from failure and prevents damage to flooring.
Protection from Toxic Gasses
Gasses like radon and methane naturally occur in soil, but increased exposure can lead to health complications and lung cancer. Vapor barriers offer increased insulation from toxic gasses, making the air healthier for residents.
How to Choose a High-Quality Vapor Barrier
The new IRC guidelines have specified that vapor barriers used under concrete slabs should be Class A barriers and a minimum of 10 mils thick. Vapor barriers are classified as either class A, B, or C according to their level of tensile strength and puncture resistance. This new standard ensures the barrier upholds its integrity during installation.
Americover recommends these vapor barriers for use under concrete slabs:
How to Install a Vapor Barrier Under Concrete Slabs
Builders should first level the sub-base material, which could involve pouring a thin layer of sand over a gravel sub-base to reduce punctures. The vapor barrier should then be rolled over the sub-base with the longest dimension parallel with the direction of the slab pour. Do not puncture or drive stakes through the barrier. Extend the vapor barrier over seams and seal them with vapor barrier tape.
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