Internal Barriers

Air, Vapour and Weather Barriers - General Information

There are three basic mechanisms by which moisture enters or leaves a building:

• Diffusion:
  Moisture moves from an area of high humidity to an area of lower humidity through porous materials such as gypsum wallboard and sheathing. It is a process which occurs over a large area and a vapour barrier must be provided in the building envelope to reduce moisture movement by diffusion.
• Air leakage:
  Air leakage is the movement of moisture as a result of humid air leaking through the building envelope. Air leakage takes places through gaps and discontinuities in the envelope and has the potential to deposit a large volume of moisture in wall and roof cavities. An effective air barrier is required in the building envelope to prevent moisture movement by means of air leakage.
• Rain penetration:
  Rain penetration is the entry of precipitation into the building envelope. A weather barrier, ideally a rainscreen, and other techniques are used to prevent infiltration.

Increased interest in the energy efficiency of buildings has led to building techniques which alter the way moisture-laden air infiltrates and exfiltrates. Moisture which escaped through building assemblies was not as likely to condense in assemblies because the thermal gradient in building assemblies was different. With high insulation requirements, escaping moisture may condense in building assemblies where it can reduce insulation values and otherwise damage the building fabric.

Figure 7.1 * Air Moisture and Weather Barriers for Walls

Air moisture levels are defined by relative humidity. A relative humidity of 100% indicates that the air is holding as much water vapour as possible for a given temperature. A relative humidity between 30 and 50% is the optimum range for comfort. Higher relative humidity may be required, for example to preserve museum artifacts, or in computer rooms to reduce static electricity. Low relative humidities will in increased static and dust, and some people may experience discomfort.

Warm air is capable of holding more moisture than cold air. Moisture diffuses from the warm, moist side of building components to the colder side where the air retains less moisture.

When warm air is cooled, the decrease in temperature means that the amount of moisture that the air is capable of holding decreases. If the temperature is reduced too far; the air will no longer be capable of holding all of the moisture, and some of the vapour will turn to liquid water and be deposited on the cold surface. This process is called condensation and the temperature at which condensation occurs is called the dew point.

In cold climates, condensation from diffusion usually originates from the inside during the cold months. The reverse may occur in very hot, humid climates where air-conditioning is used extensively.

Exfiltration of warm, moist air by air leakage is the major cause of moisture deposition. It occurs as a result of a pressure differential between the building environment and the outside. Moisture may be carried through the envelope in either direction depending on whether a positive or negative pressure occurs in a building at any given time.

Rain penetration of the building envelope may, of course, occur in both hot and temperate climates.

The NBCC requires that all thermally insulated wall, ceiling, and floor assemblies be provided with both a continuous barrier to prevent the leakage of air, and a barrier to prevent the diffusion of water vapour into the building assemblies. (Figure 7.1)

The NBCC also requires that wall sheathing paper be applied to the exterior of the sheathing to prevent rain penetration. For wood-frame construction, a polyethylene sheet (properly supported and caulked) is usually used to serve as both the air and the vapour barrier.

It is important that cladding and other weather-shield materials not admit water, but also have the permeability to allow the escape of moisture which has bypassed the interior air and vapour barriers.

Sheathing, building paper or wrap, and cladding all allow the transfer of vapour in small quantities. However, it is important to construct with wood products having low moisture contents, and to minimize the amount of rain water trapped in assemblies during construction to ensure that excessive moisture in the wall cavities does not damage the building envelope. High initial moisture content is also generated in buildings as a result of the curing of concrete, and the drying of gypsum wallboard compound and paint.

* These drawings are only for showing the placement of the vapor barrier within the wall.