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A Comparison of Air barriers vs. Vapour Barriers

December 25, 2017

The purpose of an air barrier is to block air leakage. The purpose of a vapour barrier is to prevent vapour diffusion. In a conventional wall system both the vapour barrier and air barrier should be present and in good condition. When comparing air barriers vs. vapour barriers, it’s important to understand the differences as well as the attributes.

Air barriers and vapour barriers each have a specific purpose and role in the “building envelope”. As such, the installed insulation does one job (thermal protection); the air barrier does another job (blocking air movement); and the vapour barrier does another job (preventing moisture from accumulating). All three components work together.

Air barriers are an integral part of the “building envelope”

Controlling airflow throughout the “building envelope” has a direct influence on the movement of indoor heat and moisture. In comparing the purpose of air barriers vs. vapour barriers, it’s important to understand that airflow carries air moisture, affects indoor air quality, and has an impact on how thermal energy is used. 
An air barrier is a system that controls the airflow between the indoor space (conditioned space), and the outdoor environment (unconditioned space). In some dwellings (multi-unit) air barrier systems also function as a smoke barrier or a fire barrier. In a typical residential home, the air barrier will separate house from garage.

An effective air barrier system should be impermeable to any airflow. It should be continuous throughout the entire “building envelope”. And it should be able to withstand the various forces weather and climate. Finally, for the long term, an air barrier system should be resilient over the lifespan of the residential home or building.

The difference between air barriers vs. vapour barriers

In any building structure, moisture related issues are the primary cause of wear and tear. So whether it’s a residential home or a commercial building, it’s critical to install moisture control measures. At the same time, it’s important to understand the difference between air barriers vs. vapour barriers – it will alleviate confusion, while highlighting the salient aspects.

A building can rarely be too air tight (as an example, once a certain level of airtightness is achieved, mechanical ventilation is introduced into the 'house as a system'). Which is why an effective air barrier will stop air leakage, resist air pressure fluctuations, and keep indoor climate well regulated. Simply put, an effective air barrier system keeps the outdoor air out, and the indoor air in. With a vapour barrier, the idea is to prevent vapour diffusion and avert the potential for condensation to collect.

Professional contractors install air barriers and vapour barriers so that they work in tandem to be effective as a “system” – controlling moisture accumulation; resisting air movement; and ensuring an airtight “building envelope”. And while uncontrolled airflow isn’t the only cause of moisture issues, the more airtight a building, the less reliance on vapour barriers.

What’s most important, especially in a residential home, is to install high performance products that are multi-functional, and will effectively address airflow and air moisture. This is where professional advice and installation expertise will pay dividends down the line.

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