Alexei Volkov
Concrete is a porous material that easily allows moisture to pass through it. This can cause damage to the concrete and the flooring on top of it. To prevent this, a vapour barrier is often placed under a concrete slab. This barrier is usually made of polyethylene plastic sheeting, which comes in various thicknesses, typically measured in mils (thousandths of an inch). The thicker the plastic, the more effective it is at keeping out moisture. The vapour barrier not only protects the concrete from moisture damage but also strengthens it by reducing the amount of water that the wet concrete absorbs from the ground.
| Characteristics | Values |
|---|---|
| Purpose | Prevent moisture from seeping up through the porous nature of concrete and into flooring or anything else in contact with it. |
| Type of plastic | Polyethylene sheeting is the most commonly used type of plastic due to its effectiveness in blocking moisture, affordability, and availability. |
| Thickness | Polyethylene sheeting comes in various thicknesses, typically measured in mils (thousandths of an inch). For residential applications, a thickness of 6 mils is commonly used, but in areas with higher moisture levels or for commercial projects, thicker sheeting (10-15 mils) might be preferred. |
| Durability | Higher thickness generally correlates with greater resistance to punctures and tears during installation and over the lifespan of the building. |
| Colour | While often found in clear or black, choosing a black polyethylene sheet can help reduce light penetration, which may discourage the growth of mold or mildew under the slab. |
| Installation | The sheeting should cover the entire area under the slab and extend through the edges. Seams between sheets should overlap by at least 6 inches and be sealed with tape to prevent any gaps where moisture could enter. |
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What You'll Learn
Why use plastic under concrete?
Concrete is a porous material, which means that moisture can easily enter a concrete slab from the soil underneath it. This can cause damage to the flooring on top of the concrete slab, any frames, and the concrete itself. To prevent this, a layer of plastic is often placed underneath the slab as a vapour barrier.
A vapour barrier is a plastic lining that goes under a concrete slab to prevent moisture from coming up through the slab (i.e. rising damp). Polyethylene sheeting is the most commonly used type of plastic for vapour barriers under concrete slabs due to its effectiveness in blocking moisture, affordability, and availability. The thicker the polyethylene, the more effective it is at keeping out water. It is also more durable and puncture-resistant. The recommended thickness for polyethylene sheeting under concrete slabs is between 10 and 15 millimetres.
When installing a vapour barrier, it is important to ensure that the seams between sheets overlap by at least 6 inches and are sealed with tape to prevent any gaps where moisture could enter. It is also important to avoid puncturing the vapour barrier during installation and to repair any visible damage before pouring the concrete.
In addition to protecting the concrete slab from moisture damage, a vapour barrier can also help to reduce the amount of water that wet concrete sucks up from the ground when it is first poured, leading to a stronger bond of concrete. This is especially important for indoor slabs, as moisture can cause slippery concrete floors and damage to the building.
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What type of plastic to use?
Plastic is installed under concrete to prevent moisture from seeping up through the porous concrete and into flooring or anything else in contact with it. This is known as a vapour barrier.
The most common material used under concrete slabs is polyethylene plastic sheeting. The American Concrete Institute (ACI) recommends that the polyethylene vapour barrier should be at least 10-mil thick, while the IRC requires a minimum of 6-mil for heated structures. Thicker poly creates a better barrier and is more puncture-resistant. Polyethylene vapour barriers can be easily placed over tamped earth, sand, and/or aggregate base.
For outdoor slabs, a vapour barrier is not always necessary. However, if you plan to seal the surface with an impermeable membrane or layer, it is recommended to use a vapour barrier to prevent trapped moisture inside the concrete.
When choosing the right plastic to use under concrete, it is important to consider the thickness and durability of the material. Thicker plastic will provide better protection against moisture and is less likely to be damaged over time. It is also important to ensure that the plastic is properly sealed to prevent any punctures or tears that could compromise its effectiveness.
In addition to polyethylene, other materials such as aluminium foil and reinforced plastic sheeting can also be used as vapour barriers. Aluminium foil has the advantage of being completely impermeable to air and moisture, while reinforced plastic sheeting provides extra strength and durability.
It is always recommended to consult with experts or refer to reputable sources for specific guidelines and standards, such as the ASTM E1643 guidelines, to ensure the proper installation and effectiveness of the vapour barrier.
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How to install plastic under concrete?
Plastic is installed under concrete to protect it from moisture damage. Concrete is a porous material, and moisture from the soil can penetrate it, causing it to crack over time and risking structural damage. A vapour barrier, such as a plastic sheet, can be placed under the concrete slab to prevent this.
To install plastic under concrete, follow these steps:
- Start by preparing the ground where the concrete will be laid. This includes ensuring the ground is compacted and level.
- Next, lay down the plastic vapour barrier. The plastic should be a durable, thick variety, such as polyethylene sheeting, with a thickness of at least 10 millimetres.
- Overlap the seams of the plastic sheeting by approximately 6 inches (as recommended by ASTM E1643 guidelines) and seal them using sealing tape, heat welding, or a manufacturer-recommended product.
- Ensure the plastic layer is smooth and flat, providing a stable base for the concrete.
- Before pouring the concrete, check the plastic layer for any tears or punctures and repair them if necessary.
- Finally, pour the concrete over the plastic vapour barrier, ensuring it covers the plastic completely.
It is important to note that the plastic vapour barrier should not be exposed to the elements, as this could reduce its effectiveness. Therefore, plastic under concrete is most commonly used for indoor slabs or those covered by a structure. For outdoor concrete, such as driveways, the use of plastic may not be necessary and could even prolong the curing time. However, some sources suggest that a vapour barrier can still be beneficial for outdoor concrete to prevent moisture from escaping into the ground and to provide a stronger bond for the concrete.
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Is a vapor barrier always necessary?
A vapour barrier is a 0.2mm-thick plastic lining placed under a concrete slab. It is used to prevent moisture from seeping up through the slab and causing rising damp. The plastic sheets also prevent water from passing through the concrete and protect the concrete from cracking and deteriorating over time.
While building codes require a vapour barrier under concrete inside a heated building, it is not always necessary. The American Concrete Institute's (ACI) Committee 302 "Guide for Concrete Floor and Slab Construction" suggests that a vapour barrier may not be necessary where no drainage or soil problems exist and in arid regions without irrigation or heavy sprinkling.
However, most sources recommend installing a vapour barrier to prevent moisture damage, which can lead to structural issues and health hazards. Water vapour can rise through the concrete and cause damage to the flooring and the concrete itself. This can lead to costly repairs or replacements. A vapour barrier can also act as insulation, protecting buildings from extreme temperatures and reducing energy costs.
The type of plastic used for a vapour barrier is important. Polyethylene plastic sheeting is commonly used, with a recommended thickness of 10 to 15 millimetres. Thicker plastic is more durable and less permeable to water. It is also important to properly seal the vapour barrier to prevent moisture infiltration.
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Pros and cons of using plastic under concrete
Plastic is installed under concrete to protect it from moisture damage. Concrete is a porous material, which means that moisture can easily enter a concrete slab from the soil underneath it and cause damage to the flooring on top of the concrete slab, any frames, and the concrete itself. This can lead to serious damage to the building that is built on top of the slab.
Pros of Using Plastic Under Concrete:
- Plastic under concrete creates a vapour barrier that stops moisture from the wet concrete from escaping into the ground under the concrete, creating a stronger concrete slab.
- Plastic under concrete can protect it from moisture damage, as moisture from the soil can penetrate the concrete causing it to crack over time and potentially risking lots of structural damage.
- Plastic under concrete can also prevent contaminants and gases from penetrating the concrete slab and reaching the flooring and walls.
- Plastic under a concrete driveway can extend the lifespan of the driveway, preventing moisture from expanding and deteriorating the concrete.
Cons of Using Plastic Under Concrete:
- Plastic under concrete is not necessary for all types of structures. For example, concrete driveways, outbuildings, and unheated carports do not require a plastic vapour barrier.
- Plastic under concrete may not be effective if it is not properly sealed. Overlapping seams must be sealed with sealing tape or by heat welding, and quality cutting tools must be used to get the plastic sheeting to the right size.
- Plastic under concrete may not be effective if it is not made of the right material. Not every polyethylene sheeting is a vapour barrier, and thicker plastic is more effective at creating an impermeable layer.
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Frequently asked questions
The plastic installed under concrete is called a vapour barrier. It is usually made of polyethylene.
Plastic is installed under concrete to prevent moisture damage. Water vapours can move from areas with high humidity into areas with lower humidity. The vapour barrier prevents water from passing through the concrete and protects the flooring and concrete platforms from getting damaged.
The thickness of the plastic depends on the nature of the project. For residential applications, a thickness of 6 mils is commonly used. In areas with higher moisture levels or for commercial projects, thicker sheets, such as 10 mils or 15 mils, might be preferred.
If plastic is not installed under concrete, the concrete foundation is prone to decay and cracking. The concrete can also get damaged due to the moisture, leading to flooring damage, poor climate control, and increased danger from radon gas.
