Vapor Barriers, Air Barriers, Water Barriers, Weather Barriers – What Are They?

There seems to be a fair amount of misunderstanding regarding what the purpose of these materials are and where they are found on a building. First, it is important to note that these materials are just part of a system that makes up a control layer on a building. Each control layer is intended to perform a certain function.

A vapor barrier restricts the passage of water vapor and is often seen as a brown Kraft paper face on fiberglass insulation. However, this layer can also include a wide range of materials ranging from 6-mil polyethylene sheets to high performance liquid applied (sprayed or painted-on) coatings. The proper location of these materials on a building wall can vary depending on the climate, insulation placement, and wall cladding system used.

An air barrier is intended to reduce air from passing from one area to another such as from the inside to outside of the building. This is often a sheet or liquid applied material.

The Building Codes now require the use of a water resistive barrier under the building cladding. Often you see a white sheet material (Tyvek or similar) under bricks or siding (vinyl or cement board) to fill this role. But there are many other materials that are water resistive barriers such and 15# building felt (i.e., tar paper), and liquid applied (I.e., painted or sprayed on) materials.  The liquid applied materials are generally a high-tech silicone or “rubber” coating that seals the sheathing.

A weather barrier is also commonly called a building envelope. It is the layer intended to keep the weather (i.e., sun, rain, wind, etc.) out of the building. This system can include more than just a “waterproofing” material, it can also include the roof, windows, and exterior walls of a building.

Many of the systems available today are advanced materials that prevent liquid water from entering a building while allowing evaporated water (vapor) to pass. Also, to further confuse things some materials can be classified/utilized in more than one layer (i.e., a roof membrane can function as an air barrier, vapor barrier, water resistive barrier, and weather barrier)

Most buildings constructed before 1980 do not have a water restive barrier under the cladding. These would benefit from the addition when re-cladding project is performed. Depending on the material used, the new layer can reduce the potential for water related structural damage and air infiltration, which can reduce the air leakage into a building (draftiness) and thus the energy usage for the conditioning of the building interior.

For more information consult the Air Barrier Association of America (Airbarrier.org), which is the organization that tests, specifies, and defines these various materials. A design professional should be consulted before selecting the material to be used and where it goes on a wall, as more harm than good can a result if a quality material is applied in the wrong place. Finally, even the best control layer will not provide the full effect of its intended purpose if the materials are not installed properly. So, inspection of the installation and proper detailing is a must.