vapour Retarders Are a Valuable Tool
Condensation from water vapour diffusion can cause a great deal of damage when trapped inside the wall assembly. Extended moisture exposure can cause wood-based building components to decay and steel structural members to corrode, and wet insulation loses R-Value, making the building less energy efficient. Certain building materials, such as wood and paper, can also support mould growth when moisture meets with dormant mould spores present in the materials. Using the cellulose in these building materials as a food source, mould can cause structural deterioration, poor indoor air quality and maintenance headaches.
Fortunately, by incorporating a vapour retarder into the building envelope and implementing other key moisture management strategies, building professionals can ward off these potential moisture problems.
A vapour retarder is a thin sheet that can be made from a variety of materials and is designed to impede the diffusion of water vapour through the wall assembly and protect the building envelope from condensation damage. A properly installed vapour retarder can also act as an interior air barrier, minimizing the flow of moisture-heavy air into insulated cavities during cold weather.
vapour retarder materials are rated by "perms," the measurement of their water vapour permeance. The lower the perm rating, the more successful they are at impeding moisture transmission.
Throughout the building community, vapour retarders are often interchangeably referred to as vapour barriers, which are defined as any material used to completely bar the transmission of water vapour through walls, ceilings and floors. Most of the materials referred to as vapour barriers, however, do actually permit some vapour transmission, making this label misleading.
The most recent edition of the International Residential Code (IRC) divides vapour retarders into the following three categories:
Class I covers materials most frequently referred to as vapour barriers. These vapour retarders have a permeance level of 0.1 perm or less and are considered impermeable. Examples include polyethylene film, glass, sheet metal, foil-faced insulated sheathing and nonperforated aluminum foil.
Class II vapour retarders have a permeance level between 0.1 and 1 perm and are considered semi-impermeable. Examples include unfaced expanded polystyrene, fibre-faced polyisocyanurate, asphalt-backed kraft paper facing on fibre glass batt insulation and smart vapour retarders. Smart vapour retarders provide the best moisture management, as their permeance changes with the seasons. This allows them to serve as a vapour retarder during the dry winter season and become vapour-open during other seasons when conditions are more humid, keeping the building envelope dry in both seasons.
Class III vapour retarders have a permeance rating between 1 and 10 perms and are considered semi-permeable. This class includes most latex paints over gypsum board, #30 building paper and plywood. Per the International Energy Conservation Code (IECC), Class III vapour retarders are allowed only when design conditions exist that promote drying through the use of ventilated claddings or reduce closed-cavity condensation potential through the use of exterior insulating sheathings.
These classifications help building and design professionals select the best vapour retarder for their project. After selecting the vapour retarder, however, it is important to consult the IECC and ASHRAE Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, for climate-based guidance on the proper positioning of the vapour retarder in the wall assembly.
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