The difference between air barriers and vapour barriers
The job of a vapor barrier is to prevent vapor diffusion, and the job of an air barrier is to stop air leakage through differences in air pressure. A wall system should have one vapor barrier, but can have many air barriers. A vapor barrier can act as a very effective air barrier, but an air barrier does not (and should not) always stop vapor from difusing.
A wool sweater for example, is a good choice of insulation and will keep you warm when there is no air movement, but will allow the wind to howl right through it. A wool sweater with a raincoat will keep you warm but hold moisture inside and soak your insulation. A wool sweater with a windbreaker will keep you warm, stop the wind from stealing your heat, yet allow moisture to difuse through it.
So think of a windbreaker as an air barrier, and a raincoat as a vapor barrier. That is about as far as I can stretch the human to house analogy, hope it helps.
Since warm air expands, there is more space between its molecules compared to cold air. Water vapour is found in that space. When warm air cools as it passes through your walls, it contracts and squeezes out the moisture, leaving you with condensation.
In order to prevent condensation from forming, a vapour barrier should be placed on the warm side of your insulation to stop warm, moist air from condensing on a cold surface inside your wall.
In cold climates 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, vapor barrier should be installed on the outside of the insulation, especially where there's air-conditioning involved to prevent condensation and mold.
In both cases, the vapour barrier is tasked with preventing warm, humid air from shedding its moisture as it meets a cool surface, no matter which direction it is travelling.
The most important thing to realize is that there is no fixed rule regarding vapour barriers. Building practices should always be determined by the climate in which you are building.
How water vapour travels:
There are two main ways moisture will pass through your walls that you should be concerned about — air leakage and vapour diffusion. These are two completely different things, with two completely separate solutions.
Vapour diffusion is the process of moisture passing through breathable building materials, like drywall and insulation. Vapour barriers are there to prevent that from happening.
Air leakage is due to air pressure differences 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 drop in temperature causes air to contract, and water vapour turns to liquid. Since the warmer the air is the more moisture it can hold, where the dewpoint will be in your wall is determined by the difference in temperature from indoor to out, and the amount of moisture in the air (RH - Relative Humidity).
The job of both air barriers and vapour barriers is to prevent moisture from forming at that critical point, they just do it in completely different ways.
The rule for vapour barrier installation in cold climates is to have it on the interior with at least 2/3rds of your insulation on the outside of the vapour barrier. Air barriers on the other hand can come in the form of house wrap (WRBs), tightly sealed sheathing, insulation that slows airflow, and well-sealed gypsum board (drywall).
To explain this further, Gypsum board (drywall) is vapour permeable, but stops air flow. This means water vapour can diffuse through it, but air cannot pass through it. So if you were to have a home with no windows and no vapour barrier but simply a sealed gypsum board box all around, you would have an airtight seal with no moisture carried through by air transport.
The key factor here, is that the amount of vapour molecules that will pass through that gypsum board box is insignificant compared to the moisture that will pass through if you cut just one small hole in it and had an air pressure difference.
The need for a proper air seals in homes is grossly underestimated, and too much faith and focus is put on the vapour barrier. According to the U.S. Department of Energy, "air movement accounts for more than 98% of all water vapor movement in building cavities.”
If you think of how a polyethylene vapour barrier is installed, it will be cut, stapled and taped, then have nails and screws put through it to install strapping and drywall, along with breaches due to electrical wires and boxes. In most cases, the vapour barrier will be perforated thousands of times during the building process.
But a perforated vapour barrier would actually not be a problem if you have a tight air seal. Like that gypsum board box, the amount of water vapour that can pass through a ripped and torn vapour barrier is insignificant as long as the air seal is intact.
Can a house be too airtight? No it cannot.
Unfortunately, air barriers are really not given the attention they should be in regards to the building envelope. In large residential developments, air barriers are often not even on the radar. Crews come and go, and in the interest of mass production, some standard practices can be detrimental to the performance of the final wall system.
A proper air barrier is one of the most important elements of a successful building enclosure, and one of the most overlooked. Given the amount of heat loss due to air transmission and the potential moisture damage from air leaks, air barriers should be getting a lot more attention than they are.
- Green Building Advisor - Understanding Air Barriers, Vapor Barriers and Drainage planes
- Building Science Corporation - Air Barriers vs. Vapour Barriers