A vapour barrier (VB) is a material that offers a high resistance to the diffusion of water vapour. It is used to separate an environment which is at a high vapour pressure from an adjacent one at a lower vapour pressure.
For best results, it is important that the vapour barrier be continuous, but it
How do we prevent water vapour from migrating outside?
The main way is through vapour barriers. For many years, designers were taught that the VB was a major requirement for insulated walls in order to control the diffusion of water vapour into the colder reaches of these walls, where it could condense and stain the finished surfaces or, worse, initiate the deterioration of the affected materials. Eventually, the designers would use that in a continuous and airtight manner. It was always possible to achieve that. Current practices still use this.
When it became obvious to researchers in the 1960s that air leakage into the walls and roofs was a more important source of water migration, authorities began calling for a "continuous vapour barrier."
It was found to be close to impossible, considering the materials that were being used mostly polyethylene and other not-so-durable products. Still, vapour barriers need to be sealed so that there is no accumulation of water vapour near the walls.
What is the definition of a vapour barrier? Does not have to be perfectly continuous. The key factor is that it must be located on the warm side.
Unsealed laps or minor cuts do not affect the overall resistance to diffusion significantly. The vapour barrier must also be located on the warm side of the insulation or at least in a location in the wall near enough to the warm side to remain above the dew point temperature of the indoor air during cold weather.
The building code defines it as:
A material that has a water vapour permeance less than 60 ng/ (Pa/s/m2) when measured in accordance with ASTM E96, "Water Vapour Transmission of Materials", using the desiccant method (dry cup).
That plays an important role when selecting materials in the design of a wall assembly.
In a typical wall assembly the VB must be located on the warm side of the insulation. It can be located somewhere else depending on where you are in the country and what are your degree days. There are figures that you can look up for different cities and what building code is applicable. When you study these numbers they will be expressed in a ratio (0.3, 0.35, 0.4 etc) A ratio of say.3 means that the ratio of the outboard to the inboard of insulation of the vapour barrier can't exceed that. This means that you can't have more than 30% in the inboard side. This is usually not a problem as people usually just keep it on the inside. However there are several other materials that can act as vapour barriers.
For example, plywood is water permeance less than 60 ng/ (Pa/s/m2) and technically it acts as a VB. As such when introduced on the exterior of the building it will act as a vapour barrier a will trap moisture within the wall assembly which in turn will cause moisture problems. Because of this most building codes now require that plywood and OSD be installed with gaps at their joints to allow the moisture to escape to the exterior.
Continuity and durability is not as important with vapour barriers as it is with air barriers. Partly because a VB is usually is on the inside and protected from the elements and covered by another material such as gypsum board.
Now if you're wondering why most builders, designers and building departments require that vapour barriers be continuous and be properly sealed and caulked at all joints is because in most cases the vapour barrier is also acting as an air barrier. Because it is located on the warm side of the insulation it does have to be air permeable and as such can act as both an air barrier and a vapour barrier. This does not elevate the fact that another air barrier would be required on the outside of the building to prevent air and water from penetrating the wall assembly.
As you can see selecting proper materials to install on your home is not as simple as looking at the R-value per inch or the price of the product, but also how it will perform within the assembly. Make sure you perform the proper research or speak to an experienced building and design professional.
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