A vapour barrier on walls is most effectively installed on the warm side of the insulation, typically towards the interior of the building, directly behind the interior finish material like drywall. This strategic placement is crucial for preventing warm, moist air from migrating into the wall cavity, where it could condense and cause significant moisture-related issues within the structural elements.
Understanding Vapour Barriers
A vapour barrier, also known as a vapour retarder or VCL (Vapour Control Layer), is a material used in building construction to resist the diffusion of moisture (water vapour) through the wall, floor, and ceiling assemblies. Its primary purpose is to control moisture flow from areas of high vapour pressure to areas of lower vapour pressure.
Why are they needed?
- Prevent Condensation: Warm, moist air from inside a heated home can pass through walls and, if it encounters a cold surface within the wall cavity (like sheathing or framing in winter), it can condense into liquid water.
- Protect Building Materials: This condensation can lead to serious problems, including:
- Mold and mildew growth
- Rotting wood framing
- Degradation of insulation effectiveness
- Corrosion of metal components
- Compromised indoor air quality
- Enhance Durability: By controlling moisture, vapour barriers contribute to the long-term durability and structural integrity of the building.
The "Warm Side" Rule for Wall Installation
The general rule for installing a vapour barrier is to place it on the warm-in-winter side of the wall assembly. This means it typically goes on the side of the insulation that faces the heated interior space. This placement ensures that any warm, moist air from inside the home is blocked before it can penetrate deep into the wall cavity and reach colder surfaces where it might condense.
- Key Principle: The goal is to stop warm, moist air from entering the structural elements of the wall. By placing the vapour control layer on the warm side of the insulation, you create a barrier against this moisture migration.
However, the "warm side" can vary depending on the climate zone:
1. Cold Climates (Heating-Dominated)
In regions where heating is the predominant concern for most of the year, the interior of the building is consistently warmer and more humid than the exterior.
- Placement: The vapour barrier should be installed on the interior side of the insulation, directly under the drywall or other interior finish.
- Example: A 6-mil polyethylene sheet is typically stapled to the interior face of the wall studs, covering the insulation, before the drywall is installed.
2. Hot, Humid Climates (Cooling-Dominated)
In climates where cooling dominates and humidity is high outside for much of the year, the exterior air can be warmer and more humid than the conditioned interior.
- Placement: In these cases, a traditional interior vapour barrier can sometimes cause problems by trapping moisture that might enter from the exterior. Often, a "vapour retarder" with a lower perm rating (allowing some moisture passage) or no vapour barrier at all on the interior is preferred. Instead, a vapour retarder might be placed on the exterior side of the insulation or within the wall assembly to manage moisture coming from the outside.
- Note: The term "vapour barrier" (Class I) is often avoided, and a "vapour retarder" (Class II or III) is used instead, sometimes on the exterior side of the wall sheathing, or even a continuous insulation board which acts as a vapour retarder. Building codes for these regions often emphasize the use of an exterior air barrier and adequate ventilation over a strict interior vapour barrier.
3. Mixed Climates
These regions experience both significant heating and cooling seasons.
- Placement: The approach is more nuanced. Often, a "smart" or "variable-permeance" vapour retarder is used. These materials change their permeability based on humidity levels, allowing the wall to dry out in different directions depending on the season.
- General Practice: A Class II (mid-range permeability) vapour retarder on the interior side is a common solution, providing enough resistance to manage winter moisture without trapping summer moisture.
Typical Vapour Barrier Placement in Wall Assemblies
The precise location of the vapour barrier depends on the specific wall construction, but the principle remains consistent: on the warm side of the insulation.
Here's a common sequence for a typical framed wall in a cold or mixed climate (from interior to exterior):
- Interior Finish: Drywall, plaster, paneling.
- Vapour Barrier: Polyethylene sheeting, kraft-faced insulation, or a "smart" membrane.
- Wall Studs/Framing: Wood or metal studs, with insulation typically installed between them.
- Insulation: Fiberglass batts, mineral wool, spray foam, or rigid foam boards.
- Sheathing: Plywood, OSB, or fiberboard.
- Exterior Weather Barrier (House Wrap): Tyvek, felt paper, or a liquid-applied membrane. This acts as an air barrier and a secondary rain barrier, allowing vapour to escape outwards.
- Exterior Siding/Finish: Vinyl, brick, stucco, wood siding.
Table: Wall Assembly Layers and Vapour Barrier Placement
| Layer Order (from Interior to Exterior) | Description | Vapour Control Function |
|---|---|---|
| Interior Finish | Drywall, plaster, paneling | Aesthetic and fire rating |
| Vapour Barrier | Polyethylene film, smart membrane, kraft-faced insulation | Primary vapour control layer |
| Wall Studs/Framing | Wood or metal studs | Structural support |
| Insulation | Fiberglass, mineral wool, spray foam | Thermal resistance |
| Sheathing | Plywood, OSB, rigid insulation (can be vapour retarder) | Structural support |
| Exterior Weather Barrier/House Wrap | Tyvek, felt paper | Air barrier, rain control |
| Exterior Siding/Finish | Brick, vinyl, stucco | Aesthetic and weather protection |
Key Considerations for Effective Installation
Proper installation is as crucial as correct placement. Any gaps or tears can significantly reduce the effectiveness of the vapour barrier.
- Continuity: The vapour barrier must be continuous across the entire wall surface, extending from the bottom plate to the top plate and connecting with the vapour barrier in adjacent walls, ceilings, and floors.
- Sealing Penetrations: All penetrations through the vapour barrier (e.g., electrical boxes, plumbing pipes, vents) must be meticulously sealed with compatible tapes or acoustical sealant.
- Overlaps: Overlap seams by at least 6 inches and seal them with construction tape designed for vapour barriers.
- Attaching: Use staples or fasteners to secure the material to the framing, ensuring it's snug but not overly taut to avoid tearing.
- Window and Door Openings: The vapour barrier should be cut and carefully sealed around window and door frames, integrating with flashing details.
- Building Codes: Always consult local building codes, as specific requirements for vapour barriers (including material type and placement) can vary significantly by region and climate zone. For instance, Energy Star guidelines often provide best practices for residential construction.
Types of Vapour Retarders
Vapour retarders are classified by their "perm rating," which measures how much water vapour can pass through them.
- Class I Vapour Retarder (0.1 perm or less): Considered a true "vapour barrier." Examples include polyethylene film (6-mil), aluminum foil. Best for very cold, heating-dominated climates.
- Class II Vapour Retarder (0.1 < perm ≤ 1.0): Common for many mixed climates. Examples include kraft-faced insulation batts, asphalt-impregnated paper.
- Class III Vapour Retarder (1.0 < perm ≤ 10): Often referred to as a "vapour-permeable membrane" or simply a vapour retarder. Allows some drying. Examples include latex paint, gypsum board. Used in hot, humid climates or as part of a "flow-through" wall design.
For optimal performance, select the vapour retarder class appropriate for your specific climate zone and wall assembly design.
Common Mistakes to Avoid
- Incorrect Placement: Installing a vapour barrier on the wrong side of the insulation (e.g., exterior side in a cold climate) can trap moisture within the wall cavity, leading to rot and mold.
- Lack of Continuity: Gaps, tears, and unsealed penetrations render the vapour barrier largely ineffective.
- Using the Wrong Material: Selecting a vapour barrier that is too impermeable for a hot, humid climate can prevent the wall from drying out.
- Trapping Moisture: In some scenarios, especially with continuous exterior insulation, an interior vapour barrier may not be necessary or could even be detrimental if it creates two impermeable layers, trapping moisture. A common recommendation from organizations like Building Science Corporation is to avoid having two vapour barriers in a wall assembly.
