Sheathing Membrane
Sheathing membranes are sheet materials (such as asphalt impregnated paper, spun bonded polyolefin (house wrap) or other materials) that are installed as a component of the exterior walls of a building to protect against rain and wind entry. Sheathing membranes are often referred to as water-resistive barriers.
At least one layer of sheathing membrane must be installed behind siding, stucco, or masonry veneer cladding. The membrane must also conform to the performance requirements of CAN/CGSB-51.32M “Sheathing, Membrane, Breather Type”. Sheathing membranes act as drainage planes for any bulk water that gets past the cladding. The Building Code refers to the membrane as the second plane of protection within the building assembly. The sheathing acts to intercept all precipitation that gets past the cladding (the first plane of protection) and direct any precipitation to the exterior of the assembly. To resist water entry, the membrane must be sealed at all penetrations (including services like windows, doors, ventilation ducts, piping, wiring and electrical outlets). It must also be compatible with other components it comes into contact with.
One of the most important differentiating properties between the various sheathing membranes is their vapour permeability. The material’s vapour permeability can help to reduce inward vapour diffusion that sometimes results from cladding materials that absorb and store moisture, like brick. Lower vapour permeance sheathing membranes help to control the inward flow of vapour. On the other hand, low vapour permeance materials can impede the drying of the assembly to the outdoors.
If these membranes are also being used as the air barrier, then they must be continuous and sealed throughout the building envelope to ensure that air is restricted from moving through the assembly. In this situation, care must be taken at all penetrations to ensure that they are properly air sealed. When moist interior air enters the assembly in winter, there is potential for condensation within the cavity. Likewise, in some climates warm, moist exterior air that enters a cavity in an air-conditioned building, can also result in condensation.
Building paper is a sheet material used as a sheathing membrane within the building envelope to reduce air infiltration and water ingress. It consists of a heavy paper, usually impregnated with bitumen. Building paper is typically applied inboard of exterior finish materials in wood-frame construction. This material is not permeable to liquid water; however, it is relatively permeable (greater than 60 ng/Pa.s.m2 ) to water vapour. So, when it is installed on the outside of the assembly, it resists the ingress of rainwater but does not significantly impede the outward drying of the assembly. For an example of how a wall assembly with building paper, and its corresponding information, is displayed on the Effective R Tool, please see Wall ID 8521.
House Wrap is a sheet material applied to a building exterior to reduce wind infiltration and water ingress. When properly detailed and installed it can also act as an air barrier. Spunbonded Polyolefin (SBPO) is one of the many available house wraps. This synthetic material is highly vapour permeable (much, much greater than 60 ng/Pa.s.m2) but is not permeable to liquid water. In other words, when installed on the outside of the assembly, it does not significantly impede the outward drying of the assembly. For an example of how a wall assembly with house wrap, and its corresponding information, is displayed on the Effective R Tool, please see Wall ID 8731.
Liquid applied water resistive barrier is a barrier that is either painted or sprayed onto the outside of the wall system to reduce entry of bulk water into the assembly. A liquid applied water resistive barrier is commonly used when installing EIFS. By spraying, painting, or trowelling the water resistive barrier onto the assembly, it is fully and permanently affixed to the assembly. For an example of how a wall assembly with a liquid applied water resistive barrier, and its corresponding information, is displayed on the Effective R Tool, please see Wall ID 8646.
References
Building Paper. (2013). In Glossary of housing terms (Rev. ed. of: A glossary of house-building and site-development terms, 1982 ed.). Ottawa: Canada Mortgage and Housing Corporation
Dupont. (2019). DuPont™ Tyvek® Fluid Applied WB+™. Retrieved from https://www.dupont.com/content/dam/dupont/amer/us/en/performance-building-solutions/public/documents/en/43_d100066_enna_tyvek_fluid_applied_wb+_data_sheet_abba.pdf
Spunbonded Polyolefin. (2013). In Glossary of housing terms (Rev. ed. of: A glossary of house-building and site-development terms, 1982 ed.). Ottawa: Canada Mortgage and Housing Corporation
Lstiburek, J. (2001, September 15). RR-0105: Brick, Stucco, HOUSEWRAP and building paper. Retrieved April 27, 2021, from https://www.buildingscience.com/documents/reports/rr-0105-brick-stucco-housewrap-and-building-paper/view
National Research Council Canada. (2015). National Building Code of Canada 2015 s. 9.27.3.3.(1)
National Research Council Canada. (2015). National Building Code of Canada 2015 s. 9.27.3.2.(1)
Sheathing Membrane. (2013). In Glossary of housing terms (Rev. ed. of: A glossary of house-building and site-development terms, 1982 ed.). Ottawa: Canada Mortgage and Housing Corporation