Omar Khalid
Whether or not you need plastic over drywall insulation depends on a variety of factors, including your local climate and the type of insulation you are using. Plastic sheeting is typically used as a moisture barrier, and opinions vary on whether it is necessary or effective. Some sources claim that plastic creates a vapor barrier that keeps interior moisture from getting trapped in the insulated walls, while others argue that it traps moisture and causes mold problems. The general consensus is that it is important to avoid double vapor barriers, as any moisture that gets through one may condense between them.
| Characteristics | Values |
|---|---|
| Plastic over drywall insulation | Creates a vapor barrier to prevent moisture from getting trapped in the insulated walls |
| Can cause moisture to get trapped inside the house, leading to potential mold issues | |
| May not be suitable for climates with high humidity outside | |
| Can be used in bathrooms or specific regions with cold winters and dry summers | |
| Kraft paper insulation | Acts as a vapor barrier and is preferred by some contractors |
| May not provide a sufficient vapor barrier by itself | |
| Vapor barriers | Required in certain regions, such as Canada, to prevent condensation |
| Should be installed properly to avoid moisture problems |
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What You'll Learn
Plastic sheeting is a moisture barrier
The use of plastic sheeting as a vapour barrier to protect drywall insulation is a common topic in construction and remodelling forums. Some contractors and remodelers recommend using plastic sheeting as a vapour barrier to protect the insulation from moisture. One contractor mentions stapling 6-mil plastic between the insulation and the sheetrock, creating a vapour barrier that prevents interior moisture from getting trapped in the insulated walls. This is especially important in cold climates, where water vapour can condense in the insulation and ruin it.
However, others caution against the use of plastic sheeting, arguing that it can trap moisture between two vapour barriers and cause moisture problems, such as wet walls during heating seasons. One remodeler mentions that drywall does not allow much moisture to pass through, but punctures in the drywall and plastic for outlets, switches, or junction boxes can render the plastic ineffective.
The type of plastic sheeting used is also important. Construction-grade polyethylene sheeting, for example, has a tight cell structure that can retard the passage of small molecules like radon, methane, and water vapour. However, not all plastics are created equal, and the lowest grade of construction-grade polyethylene sheeting may have defects that affect its effectiveness as a moisture barrier.
To ensure the best results, some experts recommend using engineered films specifically designed as vapour retarders. These films are made from 100% virgin resins and have excellent quality control, ensuring that they do not decay when used in contact with soil or granular fill under concrete slabs.
In summary, while plastic sheeting can be used as a moisture barrier over drywall insulation, it is important to consider the potential for trapping moisture and the quality of the plastic sheeting used. Checking with local inspection departments and consulting with insulating contractors can help determine the most appropriate solution for a particular construction or remodelling project.
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Vapor barriers are required in some areas
The type of building material used is another factor that determines the need for a vapor barrier. For example, in regions with mixed-humid, hot-humid, mixed-dry, hot-dry, and marine climates, vapor barriers are used with hygro-thermal analysis. However, they are not recommended for very cold, subarctic, or arctic regions. Additionally, the use of certain materials, such as brick over building paper and vapor-permeable sheathing, can create challenges when combined with exterior vapor barriers.
The purpose of the structure and the specific location within it are also important considerations. For instance, vapor barriers are often used in bathrooms to prevent moisture issues. However, the use of vapor barriers in other areas of the house, such as the whole house, may not be advisable due to potential moisture problems.
Furthermore, local building codes and regulations play a significant role in determining the requirement for vapor barriers. In Canada, for example, installing a vapor barrier on the warm side of a wall in cold climates is standard practice and code. Checking with local inspection departments or building officials is essential to understanding the specific requirements for a given area.
In summary, the need for vapor barriers varies across different regions and is influenced by factors such as climate, building materials, the purpose and location within a structure, and local building codes. While vapor barriers can provide benefits in certain areas, such as preventing moisture problems and increasing energy efficiency, they should be used judiciously to avoid potential issues like trapped moisture or condensation.
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Plastic vapor barriers can cause moisture problems
Plastic vapor barriers are commonly used to prevent moisture problems in homes. However, it is now understood that these barriers can actually cause moisture issues, particularly when used in certain climates or without proper installation.
Vapor barriers were introduced to address wall moisture problems that arose with the use of insulation, which lowered the temperature of wall surfaces and led to condensation. The original belief was that vapor barriers could stop outward vapor flow and prevent moisture buildup.
However, it is now recognized that mid-twentieth-century building science overstated the effectiveness of vapor barriers in preventing moisture issues. In reality, bulk water intrusion from deficiencies in drainage planes, flashing, and other moisture management details are often the primary causes of moisture problems.
One issue with plastic vapor barriers is that they can trap moisture within walls, leading to potential damage. While they block water vapor from passing through, they also hinder the wall cavity's ability to dry out. This trapped moisture can result in prolonged exposure to water, causing more significant damage over time.
Additionally, the use of plastic vapor barriers in specific climates, such as cold weather, can be problematic. In cold conditions, the presence of a vapor barrier on the interior side of a wall can prevent drying to the drier indoor space, leading to moisture accumulation. This issue is further exacerbated when air conditioning is used during hot and humid weather, as the vapor barrier blocks the escape of humid air that enters the wall cavity.
To mitigate these issues, it is crucial to consider the local climate when selecting a vapor barrier. In cold climates, less permeable barriers (Class I or II) should be used on the warm side of the insulation, while in hot and humid climates, more permeable barriers (Class II or III) should be chosen for the cool side. Proper installation is also essential to prevent condensation problems, as gaps or leaks in the vapor barrier can allow interior air to carry water vapor into the wall cavity.
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Plastic vapor barriers can cause mold
Plastic vapor barriers are often installed to prevent moisture from getting trapped in insulated walls. However, if not properly installed, they can have the opposite effect and trap moisture, creating an environment conducive to mold growth.
Mold requires moisture to grow and can become dormant when it dries out. A vapor barrier is intended to prevent moisture from reaching the insulation. However, if the vapor barrier is installed on the warm side of a wall in a cold climate, it can cause condensation, providing the perfect environment for mold to thrive. This condensation occurs when water vapor travels through the barrier and condenses as it reaches a cold enough temperature, known as the dew point.
While drywall itself is not highly permeable to moisture, punctures in the drywall and plastic for outlets, switches, or junction boxes can allow air with interior moisture to pass through. This can result in trapped moisture between the vapor barrier and the drywall, leading to potential mold issues.
In addition, the use of plastic vapor barriers has been associated with moisture problems, particularly in areas with significant temperature variations. Remodelers often encounter wet walls during heating seasons when plastic vapor barriers are utilized. This indicates that the plastic may not effectively prevent moisture penetration and can even trap moisture, creating conditions favorable for mold development.
To mitigate the risk of mold growth, it is crucial to ensure proper installation of the vapor barrier and provide adequate ventilation. Allowing one side of the wall to breathe prevents moisture from becoming trapped and reduces the likelihood of condensation. Proper ventilation helps to regulate moisture levels and inhibits the formation of condensation, thereby deterring mold growth.
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Plastic vapor barriers are standard practice in cold climates
In cold climates, the vapor barrier is placed directly behind the drywall to protect the wall structure from the hot, humid air of the house. The vapor barrier is installed on the warm side of a wall, which, in cold climates, is toward the interior of the building. This is because water vapor travels through until it hits a cold enough temperature (the dew point) and becomes water. Drywall is not very permeable to moisture, but with a big enough area and enough time, the vapor will soak the inside of the wall.
However, there is some debate on the use of plastic vapor barriers. Some sources claim that plastic vapor barriers are not necessary and can even cause moisture problems by trapping moisture inside the wall. It is important to properly install all layers and keep bulk moisture out of wall cavities to prevent condensation and mold issues.
In addition, the use of plastic vapor barriers may depend on the specific climate zone and the type of insulation used. For example, in some cases, a vapor barrier is not needed if using unfazed 'Rockwool Comfortbat' insulation in a cold climate. It is always recommended to check with local building codes and professionals to determine the appropriate course of action for a specific project.
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Frequently asked questions
It depends on the climate in your area. Plastic over drywall insulation can cause moisture to be trapped inside your house, which can lead to mould. However, in some cases, it can help to prevent moisture problems. It is recommended to check with your local inspection department to see if vapor retarders are required in your area.
Kraft paper or Tyvek can be used as a vapor barrier instead of plastic.
It depends on your local climate. If you live in a cold climate, a vapor barrier on the warm side of a wall is standard practice to protect from vapor condensing in the insulation.
Using plastic over drywall insulation can cause moisture to be trapped, leading to potential mould problems. It can also affect the breathability of your house.
Plastic sheeting is used as a moisture barrier to prevent water damage and mould growth.
