What does ‘vapour permeable’ mean?

At the moment there is a lot of confusion about air barriers and vapour barriers. The BCA now calls for vapour permeability in air barriers so it’s about time we understood what vapour permeability means and how it works. The following is a simplified explanation.

An easy way to grasp the core of the concept is to think that air is BIG and vapour is small. Vapour is liquid gas suspended inside air. Vapour is a small *component of air. This is why vapour can pass through holes that air can’t.

A vapour permeable air barrier stops the Air from getting through but lets the vapour through. (This is how a Gore-Tex jacket works).

The supplied image is my crude attempt at representing this concept graphically. Air is too big to pass through the membrane but vapour is small enough to pass through.

perm1

A vapour permeable air barrier is great for energy efficiency by having the ability to stop air from passing through. At the same time it allows vapour to pass through which is great for moisture management. e.g. in a heating climate the vapour drive goes from inside to outside. A vapour permeable wall wrap will stop air movement, (allowing the insulation to work) but allow the moisture laden air from inside to pass through and condense on the outside of the building envelope where it can harmlessly drip away or dry.

A vapour barrier on the other hand does not let air through but it also stops vapour from getting through. Vapour barriers are good underneath a concrete slab on ground but are potentially treacherous for a wall system, hence the update in the BCA.

Please note: Foil wall wrap with holes punched in it, (often called breather wrap) is not vapour permeable. This type of wall wrap performs badly as an air barrier due to the holes, and also performs badly with vapour by not letting it through, (apart for the tiny spots where the holes are). In a heating climate situation as described above this type of wall wrap will result in condensation occurring on the inside surface of the wall wrap which will in turn wet the insulation, cause structural decay, mould and potential health issues for the occupants.

Just remember, vapour needs to be dealt with properly. It either wants to get in or get out. If it can’t do either of these it will condense. (There’s a bit more science to it than that but you get my point). It has been said that vapour is like a disobedient child. If you don’t let it do what it wants to do it will do it anyway and make your life a misery.

LESSON: Understand vapour permeability and choose the right product for the right application.

*Components of air: By volume, air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapour, on average around 1% at sea level, and 0.4% over the entire atmosphere.

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