Alexei Volkov
When it comes to constructing concrete countertops, one important consideration is whether to use a vapour barrier, such as a layer of plastic, underneath the concrete. The primary purpose of a vapour barrier is to prevent moisture from permeating the concrete slab and causing damage. Concrete is inherently porous, allowing moisture to migrate upwards from the soil below and leading to potential issues such as cracking and ruined flooring. While some argue that a vapour barrier is unnecessary for outdoor slabs, it is particularly crucial for indoor slabs to prevent moisture from affecting the building's interior. The use of plastic sheeting, specifically polyethylene plastic with a thickness of 10 to 15 millimetres, is recommended to create an effective moisture protection layer. However, opinions vary, with some suggesting that plastic can prolong the curing process and cause issues with tile installation. Ultimately, the decision to use plastic under a concrete countertop depends on specific circumstances, local regulations, and personal preferences.
| Characteristics | Values |
|---|---|
| Prevent moisture damage | The plastic acts as a vapour barrier, preventing moisture from the soil from entering the concrete slab and causing damage. |
| Strengthen concrete | Plastic can help reduce hairline cracks by reducing the amount of water absorbed by the concrete during curing. |
| Comply with building codes | In certain areas, using plastic under concrete slabs is required by code to prevent moisture-related issues. |
| Protect plumbing | A barrier between plastic piping and concrete is necessary to prevent the concrete from eating away at the plastic and causing leaks. |
| Drawbacks | Plastic may prolong the drying time of concrete and potentially cause issues with tile installation. In areas with freeze/thaw cycles, plastic can delay the removal of bleed water. |
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What You'll Learn
Plastic prevents moisture damage to concrete countertops
Concrete is a porous material, and as such, it is susceptible to moisture damage. Water vapours move from areas with high humidity (such as the soil underneath a concrete slab) into areas with lower humidity (like the building above the slab). This movement of moisture can cause serious damage to the concrete slab and the building constructed on top of it.
Plastic sheeting placed underneath a concrete slab can act as a vapour barrier, preventing moisture from the soil from permeating through the slab. This protective layer will also reduce the amount of water that wet concrete sucks up from the ground when it is first poured. By stopping this movement of moisture, plastic sheeting helps to prevent moisture damage to concrete countertops.
The use of plastic sheeting as a vapour barrier is especially important for indoor slabs, as moisture can ruin the flooring above the concrete slab. However, it is also beneficial for outdoor slabs, particularly if the surface is sealed with an impermeable membrane or layer. In this case, a vapour barrier is recommended to prevent ground moisture from becoming trapped within the concrete slab.
It is crucial to use the correct type and thickness of plastic sheeting. Polyethylene plastic sheeting with a thickness of 10 to 15 millimetres is commonly used, as it is durable and almost impermeable to water. Overlapping the seams by 6 inches and sealing them with tape or heat welding ensures that moisture cannot infiltrate the concrete slab through gaps in the plastic barrier.
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Plastic sheets are a vapour barrier
Concrete is a porous material that easily allows moisture to pass through it. This can cause damage to the concrete itself, as well as any flooring or framing above it. Plastic sheets are often used as a vapour barrier to protect against this.
Vapour barriers are a critical component in construction, insulation, and moisture control. They are designed to prevent water vapour from penetrating walls, floors, and crawl spaces, which helps reduce mould growth, structural damage, and energy inefficiencies. The thickness of the plastic moisture barrier determines its durability, flexibility, and suitability for different applications. For example, 2 Mil plastic is lightweight and best for temporary protection or light-duty applications, while 20 Mil plastic is the ultimate in durability, suitable for industrial use where maximum strength is required.
When using plastic sheets as a vapour barrier under concrete, it is important to use quality cutting tools to get the right size for your concrete slab. Overlapping the seams by 6 inches and sealing with sealing tape or by heat welding will ensure that the protective layer is properly sealed and will prevent moisture infiltration. The most common material used under concrete slabs is polyethylene plastic sheeting with a recommended thickness of 10 to 15 millimetres.
However, it is important to note that not all poly sheeting is a vapour barrier. For outdoor slabs, a vapour barrier is typically not essential. In some cases, a vapour barrier can even be detrimental, such as in areas subject to freeze-thaw cycles, where it could prolong the time needed to get rid of bleed water. Additionally, if an indoor slab is not properly sealed, an interior vapour barrier can make it harder for the cavity to dry out. Therefore, it is important to consider the specific conditions and requirements of your project when deciding whether or not to use a plastic vapour barrier under a concrete countertop.
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Plastic can prolong bleed water evaporation
Concrete is a porous and permeable material that easily allows the movement of water. When concrete is poured, water rises to the surface, a process called "bleeding". The evaporation of this bleed water from the surface of the concrete is critical to its strength. The longer concrete wet-cures, the stronger it will be.
Plastic under concrete acts as a vapour barrier, preventing moisture from entering or escaping the slab. In some cases, this can be beneficial. For example, in areas with high humidity, a vapour barrier can prevent moisture from rising up through the concrete and into a building. Similarly, when concrete is curing, it is important to maintain its internal moisture so that it can properly hydrate and gain strength. Covering it with plastic can trap evaporated moisture and slow down the curing process.
However, plastic under concrete can also prolong the time needed for bleed water to evaporate. If the concrete is finished before the bleed water has dissipated, surface de-lamination can occur. This can lead to a soft, dusty finish called "laintance".
Therefore, it is important to consider the specific conditions and requirements of the concrete countertop before deciding whether or not to lay plastic underneath it. While plastic can help prolong bleed water evaporation and prevent premature drying in some cases, it may also trap too much moisture and hinder the evaporation process, affecting the final strength and appearance of the concrete.
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Plastic prevents water vapour from entering buildings
Whether you should lay plastic under a concrete countertop depends on several factors, including the climate and the potential for moisture damage. Some sources recommend using plastic under concrete to prevent moisture damage, especially for indoor slabs. Others argue that it is unnecessary, especially in outdoor settings.
Vapor retarders are particularly important in cold, hot, or humid climates to prevent moisture damage and improve energy efficiency. In cold climates, vapor retarders should be installed towards the interior of the building, while in hot and humid climates, they should be placed towards the exterior. In relatively mild or balanced climates, a vapor barrier may not be necessary.
Vapor retarders can be installed as membranes or coatings. Membranes are typically thin, flexible materials, but can also include thicker sheets. Coatings, such as elastomeric coatings, can be applied to interior or exterior surfaces and provide a vapor barrier and waterproofing.
When installing a vapor barrier, it is important to seal all seams and penetrations to ensure effectiveness. Even a small hole can render the barrier useless. Vapor barriers should also be installed continuously and as perfectly as possible to prevent moisture from infiltrating the building.
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Plastic can prevent hairline cracks in concrete
Concrete is a porous and permeable material, and as such, it is prone to moisture damage. Water vapours move from areas with high humidity into areas with lower humidity, and this movement can cause damage to the concrete and any materials above it. Moisture can enter a concrete slab from the soil underneath, and this can cause cracking in the concrete and any flooring on top of it. This is especially true for indoor slabs, where the damage can be more extensive.
To prevent this, a vapour barrier is often placed under the concrete slab. Plastic sheets are a great option for this purpose, as they are commonly used and easily available at most building supply stores. The plastic will prevent moisture from the soil from permeating through the slab. It will also reduce the amount of water that wet concrete sucks up from the ground when it is first poured. This is especially important when pouring concrete in dry conditions, as the concrete will suck up any moisture present and this can lead to hairline cracks.
Hairline cracks are very common in freshly placed concrete and are caused by the rapid depletion of moisture from the fresh concrete within its plastic state. These cracks are very small, about 0.003 inches (0.08 mm) in width, and can be very shallow. They are usually caused by variations in air temperature, concrete temperature, relative humidity, and wind velocity at the surface of the concrete. These factors can cause high rates of evaporation, leading to the concrete drying too quickly and cracking.
To repair hairline cracks, the crack is usually filled with a sealant or concrete caulk. The ideal sealant will have low surface tension and low viscosity, allowing it to penetrate deeply into the crack. However, repairing hairline cracks can be challenging, as it requires skill and practice to make the cracks disappear. Since most hairline cracks don't affect the structural integrity of a countertop, sealing and concealing the crack are the main goals of a repair.
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Frequently asked questions
Yes, you should put plastic under a concrete countertop to prevent moisture damage. Plastic acts as a vapour barrier, stopping moisture from the soil from moving up through the concrete and causing damage.
Concrete is porous, which means that moisture can easily enter a concrete slab from the soil underneath. This can damage the flooring on top of the concrete, any frames, and the concrete itself.
The most common material used under concrete slabs is polyethylene plastic sheeting with a recommended thickness of 10-15 millimetres.
No, a vapour barrier is not essential for outdoor slabs. However, if you plan to seal the surface with an impermeable membrane or layer, it is recommended to use a vapour barrier to prevent ground moisture from moving upwards through the slab.
Some people argue that plastic can prolong the time needed to get rid of bleed water and may cause surface de-lamination. It may also cause the concrete floor to not dry completely, which can create problems when laying tiles over the slab.
