Chloe Zhang
Whether or not to use plastic sheeting under drywall is a highly debated topic. Some sources claim that plastic sheeting can act as a moisture and air barrier, helping to insulate a room and regulate temperature. Others argue that plastic sheeting can trap moisture between the drywall and the plastic, leading to mould and other issues. The general consensus is that plastic sheeting under drywall should be avoided in hot, humid climates with air conditioning, as it can cause moisture to condense on the inside of the plastic. However, in cold climates, a plastic vapour barrier may be beneficial to prevent moisture from condensing on the outside of the insulation. Ultimately, the decision to use plastic sheeting under drywall depends on various factors, including climate, building codes, and personal preferences.
| Characteristics | Values |
|---|---|
| Plastic Sheeting under Drywall | Used to insulate rooms and prevent drafts |
| Can be used to prevent water damage in basements | |
| Can be used as a moisture barrier to prevent mould | |
| Should not be used in hot, humid climates with air conditioning | |
| Should not be used in combination with paper batts to avoid trapping moisture |
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What You'll Learn
Vapor barriers
Plastic vapor barriers are typically installed on the underside of rafters, between the drywall and the rafters, and under the blow-in insulation. This placement acts as an air and
However, one of the main concerns with plastic vapor barriers is the potential for moisture to become trapped. If the barrier is not properly installed or if there is a gap between the old and new plastic, moisture can become trapped between the plastic and the drywall. This trapped moisture can lead to mold growth, reducing the effectiveness of the insulation and causing potential health hazards.
To avoid these issues, some recommend using alternative materials such as foam board or waterproof tile backer board for insulation and covering areas prone to flooding. Others suggest that a vapor barrier may not be necessary at all, especially in humid climates with air conditioning. In these cases, a vapor barrier on the outside of the building may be more suitable.
The decision to use a plastic vapor barrier depends on various factors, including climate, building design, and local building codes. While some regions, such as Canada and Southern Ontario, require vapor barriers due to their cold climates, other areas with hot and humid summers may find that vapor barriers create more problems than they solve. Ultimately, it is essential to research the specific requirements and considerations for your region and consult with local builders or experts before making a decision.
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Moisture issues
Signs of moisture damage include brown stains, soft or swollen spots, bubbling, discolouration, warped trim, and weakened baseboards. If left unchecked, water damage can lead to structural issues and the growth of mould, which poses health and safety risks. To prevent moisture damage, it is recommended to use a special coating called "primer," which helps keep moisture from penetrating the interior of the wall. Other preventative measures include installing fibreglass insulation between studs and adding polyurethane foam to the top layer of drywall, both of which create barriers against moisture.
In certain cold climates, it may be beneficial to use plastic sheeting as a vapour barrier to prevent heat loss and manage moisture. However, this practice is generally discouraged in humid climates, as it can trap moisture between the plastic and the wall, leading to condensation and potential mould issues. Instead, in these cases, the vapour barrier should be placed on the outside of the building.
If you suspect moisture issues, it is essential to address them promptly. Renting drying equipment or high-volume fans and dehumidifiers can help dry out the affected areas. For severe cases, it is advisable to seek professional assistance to diagnose and rectify the problem, as well as to suggest preventative strategies for the future.
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Insulation
Plastic sheeting under drywall can be used as an air and moisture barrier to insulate a room. However, the general consensus is that this practice is only necessary in extremely cold climates, such as the Arctic, as it can lead to moisture being trapped and cause mould issues.
In very hot and humid climates, a vapour barrier can also create problems. If using air conditioning in a hot and humid climate, moisture will condense on the inside of the plastic unless a double barrier is installed, with another vapour barrier on the outside.
In a southern, humid climate with an air-conditioned building, a vapour barrier is not recommended on the inside. Instead, it should be placed on the outside. This is also true for the ceiling, where a vapour barrier should be placed on the walls but not the ceiling to allow vapour to vent through the attic.
There are alternative methods to insulate a room without using plastic sheeting. For example, foamboard, plastic film over windows, and rubber mats can help reduce drafts. Simple air-sealing measures and spray-applied sealants can also lower energy bills and increase comfort without the need for extensive construction.
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Air leaks
To achieve a high level of airtightness, it is crucial to pay attention to potential leakage points at the perimeter of the drywall. While air cannot leak through drywall seams sealed with paper tape and drywall compound, it can leak through cracks where drywall is screwed to framing lumber. These cracks can be sealed with caulk and gaskets, with most experts recommending polyurethane caulk for sealing drywall to framing.
Additionally, it is important to seal areas like the rim joist and penetrations such as electrical boxes, recessed lights, and plumbing. All large holes, including attic hatches, access holes under bathtubs, exterior walls behind tubs or showers, kitchen soffits, and zero-clearance fireplaces, should also be sealed. The airtight-drywall approach can be combined with other measures such as foamboard, plastic film, and rubber mats to further reduce air leaks and increase energy efficiency.
It is worth noting that the use of plastic sheeting under drywall to prevent air leaks is a controversial topic. Some sources suggest that plastic vapor barriers should be avoided, especially in humid climates, as they can trap moisture and cause condensation, potentially leading to mold problems. However, others argue that plastic sheeting can help keep heat and moisture inside while keeping the cold outside, and it is commonly used in mass-produced mobile homes. Ultimately, the decision to use plastic sheeting depends on the specific climate and building conditions.
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Installation
When it comes to installing drywall, one common question is whether to use plastic sheeting as a vapour or moisture barrier. This decision depends on several factors, including the climate, the type of building, and the specific needs of the space. Here is a guide on how and when to install plastic under drywall.
In cold climates, a plastic vapour barrier can help to keep heat inside and prevent cold drafts. It is essential to seal around outlets, ceiling lights, plumbing penetrations, windows, and exterior doors to create an effective air barrier. However, it is crucial to avoid double vapour barriers, as moisture can become trapped between them, potentially leading to mould issues.
In humid climates with air-conditioned buildings, the use of plastic sheeting under drywall is generally not recommended. Instead, the vapour barrier should be placed on the exterior of the building. Additionally, if you plan to run air conditioning, a plastic vapour barrier is not suitable, as it can cause moisture to condense on the inside of the plastic.
For basements that are prone to flooding, it is not necessary to insulate the lower areas with drywall. Instead, consider using waterproof materials like foam board or tile backer board. Leave a gap at the bottom to allow any water in the wall to exit onto the floor.
If you decide to install plastic sheeting under drywall as a moisture barrier, here are the steps to follow:
- Install the plastic barrier on the underside of the rafters, stapling the membrane in place.
- Use longer drywall screws to fasten the drywall sheets to the rafters, sandwiching the plastic barrier between them.
- Ensure that any seams in the drywall are filled and sanded smooth to prevent moisture from working its way up.
It is important to note that opinions vary on the effectiveness and suitability of using plastic sheeting under drywall, and building codes may vary based on location. Consult with local building experts or professionals for specific recommendations and guidelines.
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Frequently asked questions
It depends on the climate. If you live in a very cold climate, a plastic vapour barrier can be beneficial for keeping the heat in and moisture out. However, if you live in a hot and humid climate and use air conditioning, a vapour barrier could cause moisture to condense on the inside of the plastic, potentially leading to mould issues.
In hot and humid climates, it is recommended to use a vapour barrier on the outside of the building. Construction products designed to allow some moisture vapour to diffuse are also available.
If installed incorrectly, plastic under drywall can trap moisture in the insulation, reducing its effectiveness and potentially leading to mould issues.
