Difference Between Air Barrier vs. Vapour Barrier

Difference Between Air Barrier vs. Vapour Barrier

Difference Between Air Barrier vs. Vapour Barrier


What is a Vapour Barrier?

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.
  1. 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.

Building ScienceHome InsulationSpray Foam Insulation

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