Vapour And Air Barriers

Many normal activities which take place within a house, such as cooking, dishwashing, laundering and bathing, generate considerable amounts of water vapour which are absorbed by the air in the house, increasing its humidity. If, during cold weather, this water vapour is allowed to pass into the outer shell of the building, the low temperature within the thickness of the shell can cause the water vapour to condense back into liquid or frost. Since wetting of the structure, cladding and insulation is obviously undesirable, some means must be used to contain the water vapour within the dwelling. This is the function of the building component which has traditionally been called the "vapour barrier."

Two mechanisms tend to drive water vapour through the building shell: vapour pressure and air movement.

In the winter, there is more water vapour in the air inside the house than in the outside air. As a result, the difference in vapour pressure tends to force the water vapour to diffuse through the materials making up the shell. Most building materials are, to some degree, permeable to the passage of water vapour, but those classified as vapour barriers (such as polyethylene) have very low permeability and thus are very resistant to this diffusion mechanism.

The second mechanism by which water vapour is forced through the building shell is air movement. There are often differences in air pressure from inside to outside the house created by a stack effect, the operation of fans, or the action of the wind. When the air pressure inside is greater than that outside, air will tend to flow outwards through any holes or cracks in the building envelope, carrying with it the water vapour it contains. It has been recognized that this air movement plays a much greater role in the transmission of water vapour than the diffusion mechanism. The most important aspect of an air barrier is continuity — an air barrier is only as good as it is continuous. Many materials, such as drywall, qualify as an air barrier even though they do not perform as well as a vapour barrier.

Common practice uses polyethylene to perform both functions of an air and vapour barrier. This combination is workable, although it does not overcome the difficulty of creating a continuous air barrier. Certain spots on the envelope, such as headers, openings, services, vent stacks, chimneys, electrical, plumbing and mechanical system penetrations, and unusual framing details, are hard to seal well.

However, once it is well understood that there should not be a direct path from the house interior to the outside through the wall cavity, additional precautions and measures can supplement the function of the material chosen to be the air/vapour barrier. The air barrier must be able to resist wind pressures which occasionally become very strong. Vapour pressure, on the other hand, is not as forceful and can be easily resisted.

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