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Moisture and Wall Systems

Moisture and Air Leakage

Problem: The movement of warm, vapor-laden air through the wall system in winter.

Cause: Pressure differences between the interior and exterior of the building.

Winds, the stack effect, and mechanical systems can all induce - pressure differences between the interior of the building and its exterior. Inside air can be driven by these pressures through leaks and cracks in the air barrier system of the dwelling, air leakage is responsible for more than 99% of the moisture movement into the wall cavities.

When the water vapor comes into contact with cooler surfaces inside the wall assembly, the vapor can condense; often it is visible in the form of:

  • frost on the back cavity as water that can promote fungal growth,
  • deterioration of wood framing members,
  • corrosion of fasteners in the wall system,
  • deterioration of the cladding system, cracking of brick, buckling of painted surfaces, cracking of stucco, etc., as shown in Figure 4a and 4b.

Potential Moisture Problem Relating To Air Leakages With Brick Veneer

Figure 4a - Potential Moisture Problem Relating To Air Leakages With Brick Veneer

Potential Moisture Problem Relating To Air Leakages With Stucco Veneer

Figure 4b - Potential Moisture Problem Relating To Air Leakages With Stucco Veneer

Solutions: Provide an effective air-barrier system.

Of primary importance to the long-term durability of walls is the provision of an effective air-barrier system that prevents the movement of moisture inside the house into the cavity.

  • Ensure that the air-barrier system is continuous throughout the building envelope, free of leakage openings, and sealed at all penetrations in the envelope.
  • Structurally support the air-barrier system to reduce deflection, cracking, or movement resulting in the breaking of seals and the deterioration of the air barrier over time.
  • Construct the air barrier system with materials that have a high resistance to the flow of air.
  • When the air-barrier system is installed in a location where condensation might occur, ensure that it permits the passage of water vapor. Materials should have a vapor permeability of greater than 1.04 grains/(ft²•s•in.Hg.).
  • Where the air barrier comprises materials with a low water vapor permeability, polyethylene, aluminum foil, etc., generally position the air barrier on the warm side of the insulation.

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