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July 19, 2018 - 1 min

Understanding the Difference Between Permeable and Non-Permeable Air Barriers

Published by Jean-François Côté

To each project a solution: it is sometimes difficult to navigate through all the products on the market. SOPREMA will thus attempt to summarize in a few words the main principles of building science in relation to the waterproofing of walls.

Walls : Understanding the Building Envelope​

Components which separate the inside of a building from the outside of it are included in what is called the building envelope. The functions of this barrier are the following:

  • Watertightness
  • Air and vapour tightness
  • Control of heat gains and losses

To ensure the smooth operation of the envelope, it is essential to get good continuity between components.

Understanding Air Leaks and Water Condensation ​

Air leaks are a common problem concerning the building envelope. These air leaks are essentially the result of uncontrolled air movement through the building envelope. Air leaks can cause many problems: Condensation, moisture, mould growth and lower air quality inside the building.

When talking about condensation, it is important to know that warm air can hold more water vapour than cold air. When warm air cools, the percentage of relative humidity increases and can reach a saturation level called the dew point. Then, if the air cools even more, the water vapour surplus which cannot be contained in the air turns into water droplets, thus forming condensation.

Air leaks going through the building envelope are caused by air pressure coming from one or more sources, such as wind pressure, the stack effect, or pressure created by heating, cooling and ventilating mechanical units.

Using an Air Barrier To Avoid These Problems

The use of an air barrier becomes necessary to control air leaks. Air barriers are generally divided into two categories: permeable and non-permeable.

Choosing the right type of wall is very important when designing a building. In order to make the most informed choice, it is important to understand what the walls will have to go through during the building’s service life. Let’s compare the different types of walls that we can find in a cold climate market

Wall With a Continuity of Insulation​

The first type of wall that we can find is the one with a continuity of the insulation. In this case, the whole structure of the building is kept on the warm side, which greatly reduces thermal bridges. This wall is the most energy efficient and comfortable. The installation of an air/vapour barrier is recommended when constructing these walls. Here’re the main features of this type of wall membrane :

  • Resistant to : air leaks, water infiltration and diffusion of water vapour
  • Must always be set on the warm side of the insulation
  • Are considered air/vapour barriers

Hybrid Wall​

The second type of wall that we can find is the hybrid, where insulation is in wall cavities and on the cold side of the permeable air barrier. Insulating materials are added in the cavity between studs, thus moving the dew point inwards. In this assembly, the vapour barrier and air barrier functions are separated. A vapour barrier is installed on the warm side of the insulation and the air barrier is positioned outside the structure. Here’re the main features of this type of wall membrane :

  • Good protection against air leaks and water infiltration in the building envelope
  • Allows the diffusion of water vapour
  • Are not considered vapour barriers