The Importance of Air & Vapour Barriers

The Importance of Air and Vapour Barriers

Eco Spray Insulation - The Importance of Air and Vapour Barriers


Residential buildings, if not properly sealed, can leak air, vapour or water. Problems arising from these leaks can compromise the home’s building envelope, mechanical systems in place and even the building’s structure itself. To address these issues, contractors use the barrier systems in order to prevent or at least control the air, vapour and water leaks. The need for air barriers has been more evident in colder climates, where vapour barriers had already been present in wall systems. By combining these two components in a wall system, the air-vapour barrier system was generated. But air and vapour barriers can only be effective in combination with adequate insulation. Whether you have a new construction in hand or you plan to renovate your house, an effective combination of air barriers and insulation will ensure you the much needed indoor comfort for this season. Insulation contractors recognize the value of a well performing air/vapour barrier which in turn will maximise the performance of the insulation layer, bringing down energy costs and preventing moisture related issues.

Understanding the Wall Systems Construction

The components comprising your wall assembly in combination with the climate of the location of the building will determine the ideal requirements for effective air and vapour barrier system. We might be thinking of the walls of our homes as solid barriers between the home indoor and outdoor environment, but as far as water and air infiltration are concerned, they can be much more penetrable than what they seem. A normal wall assembly for the winter or cold climates would follow an outside brick/wood veneer, followed by an air barrier, an insulation layer, an indoor vapour barrier and lastly the interior wallboard. If these components are not installed properly or their base material is of poor quality, then air or moisture will find its way through the wall assembly and the results might be catastrophic. We experience most of the air leaks in the areas of window and door openings, foundation to wall transitions and walls to roof transition. Air leakage in itself means that it will be more difficult to maintain the indoor climate and preserve the energy of the building, as well as the possibility for water infiltration through the building structure. Even the most perfectly assembled walls are prone to water vapour transmission. Moisture can penetrate inside wall assemblies through a process which is called vapour diffusion. During the vapour diffusion process, the water molecules move through porous materials from areas of higher concentration to areas of lower concentration. All in all, this is a normal process to occur as the wall assemblies and the house itself need to breathe. For as long as the moisture transmission is controlled, the condensation accumulated inside the wall assemblies will evaporate via the vapour diffusion process. But if moisture accumulation is left untreated, it will pose serious problems to the wall structure and to the indoor home environment.

Wall System Construction
The Importance of Air & Vapour Barriers 3

To better understand these two terminologies let us take a simple example from our everyday life which includes a wool sweater, a raincoat and a windbreaker. We use the wool sweater as an insulation form to keep us warm during the winter. But that is only one layer of the “insulation” and it will keep us warm if there are no air movements. If we put a raincoat above the wool sweater, it will block the wind and will keep us warm, but will keep the moisture inside, which eventually will be soaked by the wool sweater. If we wear a windbreaker above the wool sweater, it will block the wind from infiltrating our body, will keep us warm and at the same time will allow for the moisture of our body to evaporate through it.

The same logic follows with air and vapour barriers. The vapour barrier is the “raincoat” in this case, which will block vapour diffusion from indoors to the wall assembly. The air barrier on the other hand is the “windbreaker”, which will prevent air leakages but at the same time will allow air diffusion and evaporate excess moisture. A wall assembly can have only one vapour barrier, but there is not a limit on the air barriers it can have. While a vapour barrier does act as an air barrier, the air barrier on the other hand should not block vapour from diffusing. Thus, a vapour barrier should be placed on the inside of the wall assembly, preventing the indoor warm air from condensing inside the walls. The contrary is valid for locations with a warm climate like the southern U.S. for example where vapour barriers should be installed on the outside of the wall assembly, preventing mould growth as a result of outdoor moist condensation. Either way, the job of the vapour barrier is to prevent humid air from condensing as it meets the cold surface of the walls. For a better comparison of the air and vapour barriers we have compiled here below the main characteristics for each of them.

Air Barriers

  • Block air leaks and prevent rain from entering inside the wall assembly while allowing for the excess moisture to evaporate
  • Allow for air diffusion within the wall assembly
  • Are more flexible to be positioned in line with other components of the wall structure

Vapour Barriers

  • Block air leakages, preventing heat loss and serving as an air barrier
  • Are water resistant, serving as precipitation barriers
  •  Block vapour diffusion, acting like a vapour barrier

The Importance of Air and Vapour Barriers

For many contractors, controlling air pressure and moisture inside the building has become a crucial element in building structures that last long and are energy efficient. Unwanted air leaks or penetrations can bring inside your home unwanted “pollutants” such as dust, moisture, noise, heat or cold. When air and moisture movement from inside and outside of the building is not controlled, then we risk to compromise the energy efficiency and the structure of the building. Here below we will present you five tangible benefits of using air and vapour barriers:

1.   Preventing Loss of Conditioned Air

This is the first impact you notice when your building envelope is sealed properly through air and vapour barriers. In winter we increase the indoor temperature and humidity as compared to the outdoor conditions, and we want the building to maintain the energy for a longer period. A building that is properly sealed from the outdoor weather conditions makes it easier for us to control the indoor climate, which is crucial for winter months.

2.   Lower Energy Bills

A properly sealed house will preserve the energy of the building for long periods and thus it will take less effort for the HVAC system to operate. That the HVAC system will be operating less means that the energy consumption will be lower and so will be the utility bills. On the other hand, buildings that preserve their energy and have it easier to control the indoor climate can function on a smaller HVAC system. The savings from a smaller HVAC system can usually offset the costs of air and vapour barrier installation.

3.   Moisture Control

Air movement inside and outside of the house brings moisture wherever it travels. Proper air and vapour barriers installation will reduce the risk of the moisture being condensed inside the wall assembly where excess moisture accumulation can cause mould growth and wood rotting. Air leakages in the building structure can transport more moisture in and out of the building envelope than what occurs from normal vapour diffusion, so it is important we address these issues. 

4.   Better Indoor Air Quality

Not only the air and vapour barriers will help maintain the indoor climate, moisture and comfort, but they will serve as barriers towards outside pollutants like dust, smog, insects, odours, noises etc. It is no longer a question of whether you should have air and vapour barriers in place, but how to design and implement highly efficient barriers that will last long.

5.   Structural Durability

Buildings are designed and constructed to stand the test of time and last for as long as 100 years or more. However, that does not guarantee that mistakes are not made during the implementation process, causing buildings to show signs of structural damage within the first 10 to 15 years of their lifespan.  It is very important for designers, manufacturers, and contractors to cooperate and ensure that all the building’s components, including the air and vapour barriers, have been perfectly designed and installed. Then we can experience all the benefits and comfort that a well-constructed house has to offer.

Building Science

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