EcoHouse Canada 2 - Air Barriers - The overlooked and essential component of building envelopes

Air barriers and vapour barriers are not the same thing. Which one is more important would probably  surprise most builders. While many variables are at play that affect actual numbers, building scientists estimate that as much as 100 times more water vapour can be carried out through walls by air leakage than will be carried out by vapour diffusion. That makes air barriers 100 times more important than vapour barriers. So why is no one talking about them? Because we are too busy talking about
vapour barriers.

By Mike Reynolds

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What is most urgently needed, is first of all a realization that controlling vapour diffusion and controlling air leakage are two completely separate concepts. The intent of a vapour barrier is to control the diffusion of water vapour through solid building materials. The purpose of an air barrier is to prevent air leakage through individual holes. Both barriers prevent moisture transport through wall assemblies, but they do so in completely different ways.

An air barrier is not something you walk into a building supply store and buy; it is a design concept, and arguably, a commitment. Products that can act as air barriers are drywall, sheathing, some types of insulation, house wrap, even the polyethylene we normally install as a vapour barrier, but only if you set out with the intent of creating an air barrier and seal them all properly.
This means they will need to be accompanied by an arsenal of tape, gaskets, spray foam and caulking. Carelessly installing your choice of air barrier products pretty much ensures that you won’t have one, as there are few aspects to building enclosures that are less forgiving than a faulty air barrier.

There is an unfortunate myth that persists in mainstream residential construction that a certain amount of holes in the polyethylene ‘air/vapour’ barrier isn’t a bad thing, in that it allows fresh air into your home. Very old and leaky homes are often held up as examples to defend this theory, as they were bone dry with no mould in sight.

Dry they were alright, because they leaked like sieves between every board. That constant exodus of hot air, combined with an endless supply of cold, dry air made the accumulation of mould an impossibility, evidenced by the chapped lips and nosebleeds of occupants.

The modern day addition of polyethylene to wall assemblies means homes are now infinitely tighter than they were in the past, and so they hold a lot more moisture inside. There will always be an air pressure difference from inside to out, and air will move through those random holes left in the vapour barrier. And it will be bringing with it a whole lot of water to deposit in your wall.
Granted, homes need fresh air, but letting it come in randomly through holes is unreliable and lacks any consistency. You will either get too much fresh air and waste energy, or not enough, trapping stale, moist air inside your home. In either case, you are directing all of your interior moisture through a completely arbitrary number of holes. Without fail, this will cause moisture damage, allow debris to collect in your walls, and provide a very convenient opening for insects to enter.

A durable, high performance home is well sealed and relies on a Heat Recovery Ventilation unit [HRV] to manage relative humidity and provide fresh air for occupants. An HRV unit is essential in a sealed home, and will without question save you money and protect your investment.

Where and when should you install an air barrier?
Anywhere and as often as you like.

A building envelope should only ever have one vapour barrier, and it goes on the warm side of the insulation. Air barriers on the other hand, can be anywhere in the wall assembly, and you can have as many as you like as long as they are permeable to water vapour [with the exception of your acting vapour barrier].

It is highly unlikely that your air barrier will be flawless, so by all means create a second and a third. This is what is referred to as the ‘belt and suspenders’ approach.  As the overall goal is to slow the migration of air through your wall, each additional air barrier will only increase the effectiveness of the previous one.

If you successfully negotiate the five essential components listed on previous page, congrats, you have a very airtight house. And consequently, your vapour barrier [whatever you choose that to be] is relatively unimportant in comparison.
According to the Canadian National Building Code [CNBC 9.25.4], vapour barriers are not required to be continuous, free of holes, taped, sealed or even overlapped. As long as you have an effective air barrier in place, you could install a sheet of poly so riddled with holes that only 90% of it remained intact, and it would stop 90% of moisture diffusion.

Somewhere along the way most builders started to treat vapour barriers and air barriers as if they were one and the same thing. They are not. But because the role played by each of these two barriers is misunderstood, they are often not being effectively applied within the wall assembly.

Understanding the importance of air barriers and the consequences of not having one is relatively new in the field of building science, dating back to the early 80s. Unfortunately though, the last 30 years has seen little or no movement towards prioritizing them. Contrary to popular belief, vapour diffusion just isn’t that big a deal, whereas air leakage is.

It is time to start putting our efforts where they are most urgently needed, and that starts with using the term ‘air barrier’ in every phase of building from design to finishing. In fact, in most of Canada you would be best to omit a vapour barrier altogether during construction and just put on a vapour retarder primer when work is complete.

Yes, that means a poly-free house. It saves money, meets code and performs better. More on that in the Summer issue of ecoHouse.

Mike Reynolds is a former home builder, a LEED for Homes Green Rater and the editor of Ecohome.net .

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