Canada
The National Building Code of Canada requires an air barrier system encompassing the entire building envelope, a vapor barrier if condensation is expected, and control of precipitation (Chapter 5, Environmental Separation, Sections 5.4-5.6). Multiple standards are listed in the code that identify performance requirements for building materials. New materials for which a standard has not yet been written undergo technical evaluation by CCMC (Canadian Construction Materials Centre), who publishes evaluation reports which verify compliance of a material or assembly with the intent of the code. Some air/moisture barrier materials have been shown to meet the requirements for air leakage as a material component of an air barrier system and are either listed or currently under evaluation by CCMC.

Typical Wall Assemblies with Fluid Applied Air/Moisture Barriers for Climate Zones in North America

The model wall constructions illustrated below are examples of wall constructions that incorporate a fluid applied air/moisture barrier in two climate zones of North America. In each case the fluid applied air/moisture barrier functions as an air barrier and moisture barrier material over the sheathing that:

• Protects the sheathing from moisture damage during construction
• Minimizes air leakage into the wall cavity and to the interior environment from warm humid outside air in hot humid climates (and during summer months in cold climates)
• Protects the sheathing against incidental moisture that may occur outboard of the sheathing but behind the cladding while in service
• Minimizes air leakage from the interior towards the exterior in cold climates

The fluid applied air/moisture barrier has a unique advantage as compared to building wraps beneath non-contact siding such as brick veneer with a cavity because it is in effect structural and does not tear and lose its effectiveness with negative wind gusts during construction or while in service.

Note, in hot humid climates it is important to:

• Use a water vapor permeable interior wall covering to permit drying to the interior and to prevent condensation immediately behind the interior wall covering.
• Use unfaced batt insulation to permit water vapor diffusion and drying to the interior
• Pressurize the interior space with conditioned (dehumidified) air so that warm humid outside air is not drawn to the interior
• Use a low permeance rigid insulation on the exterior to resist vapor diffusion to the interior, especially if porous cladding like brick veneer is used

Note that the rigid EPS insulation (as opposed to XEPS insulation) is chosen because it is vapor permeable. The vertical grooves in the insulation drain incidental moisture. The insulation is adhesively attached to the fluid applied air/moisture barrier to prevent thermal bridging that would occur if it was attached with metal fasteners, and, to minimize penetration with mechanical fasteners. The installation of the wood siding over strapping creates a cavity to promote drying of the wood in the event it gets wet during construction or while in service.

In cold climates the vapor retarder is essential (unless mechanical controls are in place to adequately control interior relative humidity conditions in winter). The vapor retarder minimizes water vapor diffusion to the exterior during winter months. However, it is essential to eliminate leaks, condensation, or any other source of moisture in the frame wall cavity, given that the vapor retarder on the interior and the fluid applied air/moisture barrier and insulation on the exterior create a very "tight" construction with limited drying potential.

Note, in cold climates it is important to:

• Insulate on the exterior, particularly when metal studs are used, to prevent:
  • Telegraphing (ghosting) of metal studs on the interior or exterior wall surfaces
  • Heat loss via conduction through the metal studs
  • A dew point from occurring in the metal stud cavity and condensation which can lead to corrosion
• Adjust the type and/or thickness of the rigid insulation to prevent a dew point in the frame cavity and condensation on or within the wall sheathing. As the size of the stud cavity increases and the thickness of batt insulation increases the dew point moves further to the exterior with the risk of the sheathing becoming a condensing surface.
• Provide a neutral or slightly negative indoor pressure to prevent exfiltration of warm humid air into cold walls

Note that each of the above model wall constructions illustrates a design strategy that incorporates a fluid applied air/moisture barrier, and other design considerations for the effective control of moisture in the wall assembly. As each building is different and has its own unique set of materials, climate, and interior conditions to consider, these model wall assemblies should be taken as a guide relative to any specific project. Appropriate adjustments in materials, and their position in the assembly should be made. The overall design strategy must include prevention and control of rain water penetration, minimizing the risk of condensation caused by water vapor diffusion or air leakage, and maintaining proper mechanical controls of the interior environment.

Fluid applied air/moisture barrier materials are effective components in wall assemblies that control moisture by minimizing air leakage and protecting water-sensitive components from moisture. Their material properties such as water vapor permeability, UV resistance and mold resistance must be taken into account. They are cost effective alternatives for moisture control in wall assemblies that have several performance advantages over building wraps and traditional asphalt saturated felt or paper moisture protection.