Wall water vapour speaks to what is wall water vapour, why wall water vapour and types of wall water vapour.
This is when air passes through a building assembly, the water vapour in the air is transported through the assembly as well.
Where significant air pressure differentials exist between one side of an assembly and the other, the amount of water vapour transported through a building assembly and the other, the amount of the amount of water vapour transported through a building assembly by uncontrolled air leakage can be one or two orders of magnitude greater than that transported by water vapour diffusing directly through building materials.
By controlling the f low of warm, moist air toward the cooler side of a building assembly, air barriers can play an important role in protecting against condensation within the assembly.
When designing air barrier systems, the water vapour permeability of the air barrier material must be considered. For example in a heating driven climate, an air barrier material located toward the outer, cooler side of an insulated building assembly must be vapour permeable to prevent the trapping of moisture within the assembly.
Traditional building paper and breathable building wraps are good building choices for use in this application. By contrast, a bituminous membrane with low paper permeability would be a poor choice.
As a general rule, air barrier materials located on the cooler, lower vapour pressure side of a building assembly should always be vapour permeable. Conversely, air barriers located air barriers located on the warmer, high vapour pressure side of an assembly, may consist of materials with low vapour permeance and be designed to function as both an air barrier and a vapour retarder.
The National Building Code of Canada (2005) sets quantifiable air permeance limits for air barrier materials and requires the control of air leakage with continuous air barrier systems in most buildings.
The International Energy Conservation Code (2006) has more limited requirements for controlling air leakage, calling for the sealing of gaps through building envelope assemblies but without setting measureable criteria. At the time of this writing, several US states have adopted more comprehensive air barrier system requirements based on Canada’s model code, and it is likely that requirements for such sytems will continue to spread in the United States over the coming years.