Sofia Martinez
When laying concrete, it is important to consider the use of plastic sheeting underneath to prevent moisture damage. This is especially crucial for indoor slabs, as moisture from the soil can move upwards and cause problems for the concrete and any materials above it, such as flooring. The thickness of plastic sheeting is typically measured in mils, which represent thousandths of an inch. The recommended thickness for polyethylene plastic sheeting under concrete is 15 mils (15 thousandths of an inch), although some sources suggest that 10 mils is sufficient for residential construction. In certain regions, the use of a vapour barrier may not be necessary, especially in areas with dry ground conditions. However, in locations with high humidity or radon risk, a thicker vapour barrier made of low permeability materials may be preferred to completely eliminate moisture migration from the ground.
| Characteristics | Values |
|---|---|
| Thickness of plastic sheeting | Can range from 1 mil to over 10 mils |
| Thickness for light-duty applications | 1 to 3 mil |
| Thickness for industrial and heavy-duty applications | 8 mil or higher |
| Thickness for vapor barrier under concrete slabs | 10 mil or 15 mil or greater |
| Thickness for vapor retarder under concrete slabs | 6 mil |
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What You'll Learn
Vapor barriers are more effective than plastic sheets
Concrete slabs are prone to moisture-related issues, such as water vapour transmission from the ground beneath. While thin plastic sheets, also known as vapor retarders, have been traditionally used to address this problem, they are not very effective. These sheets merely slow down vapour movement without truly stopping it.
Vapor barriers, on the other hand, provide a more comprehensive solution. They are designed to completely seal the bottom of the concrete slab, eliminating any moisture migration from the ground. This not only allows the slab to dry out faster but also ensures that it stays dry over time.
One of the key advantages of vapor barriers over plastic sheets is their superior puncture and tear resistance. Thinner plastic sheets are prone to damage during construction, often sustaining tears and punctures that compromise their effectiveness. Vapor barriers, however, are engineered to meet specific standards for tensile strength and puncture resistance, making them much more durable. For instance, ASTM E-1745 sets minimum requirements for these characteristics, with Class A offering the highest level of protection.
Vapor barriers also offer greater flexibility in terms of thickness. While a minimum thickness of 10 mils is generally recommended for concrete installations, thicker options, such as 15 mils or 20 mils, are available for enhanced protection. This adaptability allows for a more tailored approach based on the specific requirements of the project, such as the need for outdoor installations or the use of heavy equipment.
In summary, vapor barriers offer a more effective and reliable solution for managing moisture-related issues in concrete slabs. Their ability to completely stop vapour transmission, coupled with their durability and flexibility, makes them a superior choice over traditional plastic sheets. By investing in vapor barriers, builders can ensure the long-term dryness and integrity of concrete structures.
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Thicker plastic is less susceptible to tears and punctures
When it comes to concrete slabs, a vapor barrier is essential to prevent moisture-related issues. While thin plastic or vapor retarders can slow down vapor transmission, they don't completely stop it. This can lead to moisture buildup and potential damage to the slab over time.
Thicker plastic is advantageous in this context because it is less susceptible to tears and punctures. This is crucial for the long-term durability and integrity of the concrete. A tear or puncture in the plastic can compromise its effectiveness as a vapor barrier, allowing moisture to seep through and affect the concrete.
The thickness of plastic sheeting plays a significant role in its tear resistance. Thicker plastic has higher tensile strength, making it more resistant to punctures and tears. This is especially important in construction applications, where the plastic needs to withstand the abuse of construction activities without failing. ASTM E-1745 specifies minimum values for tensile strength and puncture resistance, with Class A offering the highest level of protection.
Additionally, thicker plastic provides better protection against sharp objects that may be encountered during construction, such as gravel or reinforcement materials. It is also less likely to be damaged during the placement of concrete, ensuring the integrity of the vapor barrier.
However, it's important to strike a balance between thickness and functionality. Thicker plastic can be stiffer and less manageable, making it less suitable for applications that require frequent folding or movement. The specific requirements of a project should be considered when selecting the thickness of plastic sheeting. Other factors, such as UV resistance, can also be important to prevent sun damage and extend the lifespan of the plastic.
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Plastic thickness is measured in mils
In construction, the thickness of plastic sheeting is crucial. For instance, a vapor barrier beneath a concrete slab must be at least 6 mils thick. This thickness is specified by ASTM E-1745, which classifies vapor retarders into three classes (Class A, B, and C) based on their ability to resist puncture and tensile strength. However, traditional 6-mil plastic often fails to effectively prevent water vapour transmission and is susceptible to damage during construction.
To address these issues, newer very low permeability barriers with thicknesses of 15 mils or greater are available. These thicker materials offer superior tear and puncture resistance and significantly lower permeance. They effectively eliminate moisture migration from the ground, allowing the concrete slab to dry out and remain dry.
The use of plastic sheeting in construction is not limited to vapour barriers. For example, 10-mil poly (10 thousandths of an inch) is commonly used in commercial construction underneath concrete slabs. It helps to retain moisture in the concrete and protect against radon. However, the use of poly under slabs has been controversial, with some regions mandating its use while others have discontinued the practice due to moisture-related issues.
The thickness of plastic sheeting can impact its durability and weight. While thicker plastic sheeting is generally associated with increased durability, other factors such as reinforcement and density also contribute to overall performance. For instance, high-density polyethylene (HDPE) sheeting offers superior heat resistance but is more prone to cracking during installation compared to medium or low-density polyethylene.
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Thicker plastic is not always better
Firstly, it is essential to select the appropriate type of plastic for a concrete vapour barrier. Polyethylene plastic, commonly known as poly, is frequently used. However, thin poly, such as 6-mil Visqueen polyethylene plastic, has been shown to be ineffective in preventing moisture vapour transmission. While thicker poly, such as 10-mil or 15-mil, can provide better protection, it is not the only factor to consider. The quality and characteristics of the plastic are crucial, as specified by ASTM E-1745, which outlines the standards for water vapour retarders used with concrete slabs.
Secondly, the decision to use thicker plastic should be informed by the specific climate and site conditions. In areas with high groundwater levels or moist ground conditions, thicker plastic may be more effective at preventing moisture from seeping up into the concrete. However, in regions with dry ground or low radon risk, a vapour barrier may not be necessary at all. Additionally, thicker plastic can affect the curing process of concrete, and in some cases, excess water may rise to the top, creating a "heavy cream" that prolongs the finishing process.
Moreover, the intended use of the concrete structure should be taken into account. For example, if flooring will be installed on top of a concrete slab, a moisture barrier is typically required to protect the flooring from potential water damage. On the other hand, for unheated structures like carports and outbuildings, a vapour barrier is usually not required, and thicker plastic may be unnecessary.
Thicker plastic can also present challenges during construction. Thinner plastics may be more susceptible to tears and punctures during the placement of reinforcement and concrete. This can compromise their effectiveness as a vapour barrier. Therefore, it is crucial to consider the specific construction techniques and equipment used to determine the appropriate thickness and type of plastic.
In conclusion, while thicker plastic can offer advantages in certain situations, it is not always the best solution for concrete. A comprehensive assessment of the site conditions, climate, intended use, and construction methods should inform the decision to use thicker plastic. Additionally, the quality and characteristics of the plastic, as outlined by industry standards, play a significant role in ensuring effective moisture control in concrete structures.
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Vapor barriers are common under concrete
Vapor barriers are essential in construction to prevent ground moisture from migrating into and damaging the concrete slab. They are also used to prevent water vapour from reaching and entering the concrete slab, which can cause a lot of damage. Vapor barriers are also used to protect the flooring above the slab.
Vapor retarders, commonly made of thin plastic, are often used as vapor barriers. However, they only slow vapour down and do not completely stop it. They are also susceptible to tears and punctures during construction, which can lead to holes that allow water vapour to enter the slab. A vapor barrier made of special plastic is a better option as it completely blocks vapour transmission.
The thickness of plastic used for vapor barriers is typically 10 mils (one-thousandth of an inch), which is the minimum thickness recommended by the American Concrete Institute. A 10-mil vapor barrier is sufficient for residential construction in terms of puncture resistance, although it may not completely isolate the slab from ground moisture. For laser screeds or heavy placing equipment, a thicker barrier of 15 mils is recommended.
Vapor barriers are becoming increasingly common in construction, especially in regions with high moisture content. They are also used to protect against radon, the second leading cause of lung cancer in the United States.
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Frequently asked questions
Plastic is placed under concrete slabs to prevent moisture damage. Concrete is porous and permeable, allowing moisture from the soil to penetrate the slab and cause issues such as cracking. The plastic acts as a vapour barrier, stopping vapour transmission and related problems.
The recommended thickness of plastic sheeting for use under concrete slabs is 15 millimetres or 15 mils (15 thousandths of an inch). This thickness is specified as it is much less susceptible to tears or punctures and has lower permeance.
Yes, for outdoor slabs, a vapour barrier is not always essential. Moisture on the surface of exterior concrete slabs in unheated structures is not an issue. However, if an impermeable membrane or layer is applied to the surface, a vapour barrier is recommended to prevent trapped moisture, which can cause the concrete to swell, expand, cup, and degrade.
Yes, newer very low permeability barriers are available from various manufacturers, including W. R. MEADOWS, Fortifiber, Interwrap, Raven, Reef, Polyguard, Stego, and Grace Construction Products. These materials have permeance values as low as 0.01, completely eliminating moisture migration from the ground.
