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.

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.

A vapour barrier (or vapour retarder) is typically a plastic or foil sheet used for damp proofing to prevent condensation from forming in various building assemblies such as walls, roofs, foundations and floors. In a typical commercial building or home, vapour barriers or vapour diffusion retarders can improve energy efficiency and comfort, while also preventing problems from moisture and other damages to the house and her members.

What is an Air Barrier?

Air barriers are systems of materials designed to control airflow between a conditioned space and an unconditioned space. The air barrier system is the primary air enclosure boundary that separates indoor (conditioned) air and outdoor (unconditioned) air. Air barrier systems also are defined as the location of the pressure boundary of the building enclosure.

The difference between air barriers and vapour barriers

1. Air barrier:

• Resist air leakage and rain penetration while allowing the diffusion of moisture in the form of vapour.
• Allow the walls of a structure to “breathe”.
• Offer designers more flexibility in positioning of the barrier within wall assembly.

2. Vapour barrier:

• Resist air leakage, acting as an air barrier.
• Resist rain penetration, acting as a precipitation barrier.
• Resist vapour diffusion and serving as a vapour barrier.

The purpose of a vapour barrier is to prevent vapour diffusion, and the job of an air barrier is to stop air leakage through differences in air pressure.Is a must for a wall system to have one vapour barrier, but can have many air barriers. A vapour barrier is at the same time a very effective air barrier, but an air barrier does not (and should not) always stop vapour from diffusing.

• A wool sweater for example, is a good choice of natural insulation and will keep you warm when there is no air movement, but this won’t happen if there is wind going through it. 
• A wool sweater with a raincoat will keep you warm but at the same time, it holds moisture inside and soaks your insulation.
•  A wool sweater with a windbreaker normally will keep you warm and it will stop the wind from stealing your heat, yet allow moisture to diffuse through it.

So in this case a windbreaker is an air barrier, and a raincoat as a vapour barrier. 

When warm air it will cool as it passes through your walls,then it contracts and squeezes out the moisture and in this way  leaves you with condensation. 

In order to prevent condensation from forming, a vapour barrier should be placed on the warm side of your insulation and it will be able to stop moist air from condensing on a cold surface inside the wall.

In cold climates countries like Canada, for most of the year, the vapour barrier should be on the inside of the insulation. In hot climates like the southern U.S. for example, vapour barriers should be installed on the outside of the insulation. 

In both cases, the vapour barrier will prevent warm, humid air from forming moisture as it meets a cool surface.

There is no fixed rule regarding vapour barriers. Building practices should always be determined by the climate.

How water vapour travels?

Air leakage and vapour diffusion are two main ways that moisture will pass through your walls. These are two completely different things, with two completely separate solutions.

Vapour diffusion is the process of moisture passing through breathable building materials, like insulation and drywall and the vapour is there to prevent that from happening.

Air leakage pressure the air difference between indoors and out, which forces air through any holes in your air barrier.

Where the problem arises?

The dewpoint in a wall is the point where the temperature causes air to contract, and water vapour will turn to liquid. Since the warmer the air is, the more moisture it can hold, where the dew point will be in your wall, is determined by the difference in temperature from indoors to out, and the amount of moisture in the air.

The job of both air barriers and vapour barriers is to prevent moisture from forming at this critical point, but they do that in completely different ways.

Why Do Air Barriers Really Matter?

Preventing the Loss of Conditioned Air. For most consumers, the biggest reason “why air barriers are important is comfort. Controlling interior temperature leads to comfort.

Lower Utility Bills. Maintaining conditioned air means less energy is needed to recondition the air and that means lower utility bills. And since all building systems must perform well together to optimize the energy efficiency of a home, the savings can add up.

Preventing Moisture. Wherever air moves, water vapour can follow. If air sealing is installed properly, this reduces the risk of water vapour moving into the wall system where prolonged exposure can result in moisture issues such as wood rotting and mold.

Improved Indoor Air Quality. Air barrier systems help  keep out pollutants such as suspended particulates, dust, allergens, insects, odors, noise and more.

Why Do Vapour Barriers Really Matter?

Vapour barriers are relatively cheap. Floor failures are not. The most important reason you should use a high-performance vapour barrier is to protect the expensive floor covering in your building.

A proper vapour barrier installation can limit your liability. The concrete foundation of your building may feel rock-solid. Water vapour will always move from a high relative humidity to a low relative humidity – even through concrete. That’s why nearly every expert in the concrete industry recommends a below-slab vapour barrier.

Your indoor air quality depends on it and so do the building inhabitants. Water helps the mold grow in any environment; but while you might be quick to clean up after a spill to avoid mold growth.So in this case you will eliminate the odors in your home.

Give your HVAC system a break – and maybe save some money in the process. One of the critical ways many HVAC systems work in buildings is by removing moisture from the air and reducing the relative humidity of the building envelope. By forgoing the installation of a below-slab vapour barrier, you ensure a greater amount of moisture entering the building from underneath your building’s foundation.

A high-performance vapour barrier prevents long-term curling. This might be the biggest misconception when it comes to vapour barriers: their impermeability to moisture does not allow a newly-placed slab to dry evenly out of the bottom of the slab, as it does from the top. This disproportionate drying may lead to minor, short-term curling in the hours and days it takes for a slab to dry. However, the far greater risk to a slab is long-term curling over the lifetime of the concrete foundation, which a high-performance vapour barrier helps prevent.