Omar Khalid
Concrete slabs are prone to issues caused by moisture vapor movement, which can be prevented by using a vapor barrier. The thickness of the vapor barrier is critical to its effectiveness, and while 6-mil plastic was once commonly used, it is now considered insufficient due to its low puncture resistance and water vapor permeability. Newer vapor barriers are thicker, ranging from 10 to 20 mils, and are made of high-quality polyolefin resins that provide superior protection against moisture and gas migration. For residential construction, a 10-mil vapor barrier is often sufficient, while commercial applications typically require the increased durability and puncture resistance of a 15-mil barrier.
| Characteristics | Values |
|---|---|
| Purpose | Protects the building structure from damage that can occur underneath the concrete foundation |
| Common issues prevented | Ground moisture migration, harmful gas migration, energy inefficiency, non-compliance with building codes and safety regulations |
| Minimum thickness | 10 mils |
| Thickness for commercial applications | 15 mils |
| Thickness for residential applications | 10 mils |
| Thickness for laser screeds or heavy placing equipment | 15 mils |
| Thickness for thinner plastic | 6 mils |
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What You'll Learn
Vapor barriers are better than plastic sheets
Concrete slabs require a barrier to prevent moisture vapour movement, which can cause concrete to deteriorate over time, leading to foundation and building failures. While a layer of plastic sheeting is often used, a vapor barrier is a more effective solution.
Vapor barriers are classified as either Class A, B, or C, based on their level of tensile strength and puncture resistance. The minimum thickness of an effective vapor barrier is 10 mils, which is sufficient for residential construction. However, thinner plastic can be prone to punctures and tears during construction, which can allow water vapour to penetrate the slab. For commercial applications or projects involving heavy equipment, a thicker 15 mil vapor barrier is recommended to provide better puncture resistance and durability.
Vapor barriers made of high-quality polyolefin resins and additives offer superior performance compared to plastic sheets. They provide a homogenous membrane with consistent permeance and puncture resistance throughout the entire sheet. In contrast, plastic sheets, especially those made of recycled materials, can break down over time, allowing moisture to pass through.
Additionally, vapor barriers can block gases such as methane or radon, which is the second leading cause of lung cancer in the United States. They also protect against environmental damage caused by moisture, pests, and extreme temperatures, ensuring the longevity of the building and the safety of its occupants.
By choosing a vapor barrier that meets ASTM E-1745 standards and has a water vapour permeance of less than 0.3 perms, you can ensure that your concrete slab gets dry and stays dry, preventing moisture-related issues.
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Thickness recommendations
The thickness of the plastic sheet to be used under a concrete slab depends on the specific requirements and characteristics of the construction project. The main purpose of the plastic sheeting, often referred to as a vapour barrier, is to protect the concrete slab from moisture damage and related issues, such as the growth of pests and mould.
The recommended thickness of the plastic sheet varies, with some sources recommending a thickness of 10 mils for residential construction. This thickness is specified in the ACI 302.1, Guide for Concrete Floor and Slab Construction, and is supported by field studies. However, it is important to note that 10-mil barriers may not completely seal the slab from ground moisture and are more susceptible to punctures during construction.
For commercial applications or projects requiring higher puncture resistance, a thicker plastic sheet of 15 mils is recommended. This thickness is more durable and provides better protection against water vapour penetration. It is also important to consider the quality of the plastic material used, as older, thinner barriers were made with recycled materials that break down over time, allowing moisture to pass through.
To ensure the effectiveness of the vapour barrier, proper installation is crucial. The plastic sheeting should cover the entire area under the slab, extending through the edges, and any seams should be overlapped and sealed with tape to prevent gaps and moisture infiltration. Additionally, care should be taken to avoid punctures or tears during the installation process, as this can compromise the barrier's ability to block moisture.
In summary, the thickness of the plastic sheet for a concrete slab depends on factors such as the project's specific requirements, local regulations, and the level of protection needed. While 10-mil plastic sheets are commonly used in residential construction, thicker options of 15 mils or greater provide enhanced durability and moisture protection, making them more suitable for commercial applications or projects with higher puncture risks.
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Puncture resistance
The minimum thickness of an effective vapor barrier is 10 mils, as specified by ACI 302.1, Guide for Concrete Floor and Slab Construction. This thickness provides sufficient puncture resistance for residential construction. However, thinner plastic sheeting, such as the commonly used 6-mil Visqueen (polyethylene plastic), is more susceptible to punctures and tears during construction, compromising its effectiveness.
To enhance puncture resistance, newer vapor barriers are thicker, typically ranging from 10 to 15 mils. These barriers are made of high-quality polyolefin resins and additives that cross-link to form a homogeneous membrane. The increased thickness significantly improves puncture resistance, making the barrier more durable and less prone to damage.
For commercial applications, a 15-mil vapor barrier is recommended. This thickness provides superior puncture resistance and is more cost-effective, as it reduces the need for repairs during concrete pouring. The added thickness ensures that the barrier can withstand job-site traffic and heavy equipment without tearing or puncturing.
It is important to note that puncture resistance is not solely dependent on thickness. The material's tensile strength and ability to withstand sharp objects or rapid impacts also contribute to its puncture resistance. Therefore, it is crucial to select a vapor barrier that meets the specific requirements of the construction project, adhering to standards such as ASTM E-1745 and IRC guidelines, which classify barriers based on their puncture resistance and tensile strength.
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Radon blocking
Radon is a radioactive gas that can seep into homes from the ground. It is a significant health concern as it is the second leading cause of lung cancer in the US, according to the American Lung Association. It is colorless, odorless, and tasteless, making it difficult to detect without special equipment.
To prevent radon from entering a home, it is essential to seal any openings, cracks, or crevices in the concrete foundation floor and walls. This can be done using polyurethane caulk, which will create a barrier to block the entry of radon and other soil gases. In addition to sealing, it is recommended to install an electrical junction box in the attic, which can be used for a vent fan if needed. These radon-resistant construction techniques are important for new homes to reduce the risk of lung cancer for occupants.
Vapor barriers are also an effective method for blocking radon. These barriers are made of plastic sheeting and are placed under concrete slabs to prevent vapor transmission. The recommended thickness for vapor barriers is now 10 mils or 15 mils, made with high-quality resins that provide superior puncture resistance. Thicker barriers, such as the 15 mil option, are more suitable for commercial applications or when heavy equipment will be used on the barrier.
One specific product designed for radon blocking is Radon Block, a high-performance barrier membrane evaluated by NRC CCMC. This product is an alternative solution for radon mitigation and can be used in conjunction with vapor barriers to provide a comprehensive radon blocking system.
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IRC guidelines
The IRC guidelines refer to the International Residential Code, which was updated in 2021 to enhance moisture protection in residential construction. The new guidelines specify that vapor barriers used under concrete slabs should be Class A barriers with a minimum thickness of 10 mil. This is an increase from the previous recommendation of 6 mil.
Class A vapor barriers are defined as having a water vapor permeance of 0.1 perms or less, a tensile strength of 45 lbs. per square inch, and a puncture resistance of 2200 grams. The higher puncture resistance is particularly important in construction as it means the plastic sheeting can withstand the heavy-duty activity on a building site.
The 2021 IRC guidelines also reference ASTM E1745, which is the Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill Under Concrete Slabs. This standard requires products to undergo conditioning testing to simulate in-service conditions and helps to ensure the performance and durability of vapor retarders for residential applications.
To install a vapor barrier, the sub-base material should be levelled and then the barrier is rolled out over the sub-base with the longest dimension parallel to the direction of the slab pour. It is important not to puncture or drive stakes through the barrier, and any seams should be extended and sealed with vapor barrier tape.
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Frequently asked questions
A vapor barrier under a concrete slab is necessary to stop vapor transmission and the related problems it causes.
The thickness of the plastic depends on the application. For residential construction, a 10-mil vapor barrier is usually sufficient. For commercial applications, a 15-mil vapor barrier is recommended as it is more durable and offers better value for money.
A mil is a unit of measurement equal to one-thousandth of an inch.
VaporBlock® Plus™ 20 Mil Vapor Retarder/Gas Barrier is one of the most effective under-slab gas barriers in the industry. It exceeds ASTM E-1745 Class A, B, and C requirements.
