What is an air barrier?

Approximately 100 times more water vapour is carried into our walls by air leakage than is carried by vapour diffusion. That makes air barriers 100 times more important than vapour barriers, so why is no one talking about them?

Air barriers are essential to a properly functioning wall system
Air barriers are essential to a properly functioning wall system © U.S. EPA

Approximately 100 times more water vapour is carried into our walls by air leakage than is carried by vapour diffusion. That makes air barriers 100 times more important than vapour barriers, so why is no one talking about them?

We don't talk about air barriers because we are too busy talking about vapour barriers. 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, insulation, house wrap, even the polyethylene we normally install as a vapour barrier can act as an effective air barrier, but only if you set out with that in mind and properly seal it.

The difference between air barriers and vapor barriers © Ecohome

What the above diagram attempts to illustrate, is how much moisture goes through a small hole, and how comparitively little would go through drywall if you didn't even install a vapour barrier. This should indicate where our priorities should be in order to keep our walls dry.

Air leakage is a serious problem for both new and older buildings, and can:

  • Increase heating and cooling costs
  • Cause moisture damage in walls
  • Allow pollutants and debris to collect in wall interiors
  • provide openings for insects and rodents to enter

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 the air barrier.

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 as plentiful as you like. It is highly unlikely that your air barrier will be flawless, so by all means have 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 help, and increase the effectiveness of the previous one. 

The five essential components of an air barrier:

The Institute for Research in Construction (IRC) at the National Research Council (NRC) lists the five basic requirements for an effective air barrier as being:

1. Continuous: no gaps, cracks or openings.

2. Impermeable to the leakage of air.

3. Rigid: supported in both directions from wind pressure.

4. Durable: must last the life of the building.

5. Repairable: access for repair if damaged.

A sampling of air barrier materials and techniques:

House wrap:

Also known as Water Resistive Membranes (WRBs), brand names include Tyvek and Typar.  While its intent is largely as a breathable membrane to protect against water penetration behind cladding, it can be an effective air barrier if installed as such. Water vapour can pass through easily, but air cannot.

Polyethylene:

For good or bad, almost all home construction in Canada includes a polyethylene vapour barrier. If that is your method of choice, with a little extra effort it can also act as your air barrier. For a truly successful polyethylene air barrier, it would need to be protected against wind pressure by being supported in both directions. One sure fire way to accomplish that would be to sandwich it between two solid materials like drywall  for example, but this route unfortunately adds a measurable amount of cost.

Airtight drywall approach (A.D.A):

With the help of gaskets and sealants, gypsum board (drywall) is a very effective air barrier, so long as the joints are sealed with drywall compound and tape; and all junctions at the floor, roof and openings are sealed with gaskets and appropriate sealants.

Breathable insulated structural sheathing:

Worth noting first -  as a material, OSB (oriented strand board) is an effective air barrier. But if the joints are sealed properly it also acts as a vapour barrier, which is a no-go on the exterior of homes anywhere in Canada, as you will trap moisture inside your wall assembly.

There are however, insulated sheathing materials that can provide an air barrier along with a thermal barrier while still allowing walls to breath and therefore not trap water vapour in your walls.

Excel II © BP

A product has to be pretty sustainable for us to plug it; that said, we are fans of Excel II from Building Products of Canada Corp (BP). It replaces exterior sheathing, and installs directly to the studs. Excel II has high recycled content, contains no VOCs, formaldehyde or ozone depleting agents. The R Value is 1.5, enough to break the thermal bridge at each stud. And when joints are taped it makes an excellent air barrier.

There are many products and techniques that can make for a great air barrier, but most importantly we need to prioritize it during every step of the entire building enclosure process. 

Think of it as if you are trying to seal a leaky bucket and make it watertight. The water will easily find any holes you overlook, and eventually leave you with an empty bucket. Granted, a house will never run out of air, but it will also never run out of moisture to send through any holes in the air barrier.