Moisture Movement via Air Flow

Air flow accounts for almost all of the moisture movement into the wall.

Rate of flow is a function of:

• Size and number of cracks and holes
  • Influenced by the air barrier system
• Pressure difference across the envelope
  • Influenced by air flow effects
• Indoor/outdoor temperature difference

The amount of moisture that flows into the building envelope is mainly determined by the amount of air flow across the envelope. Air will either infiltrate or exfiltrate across the envelope depending upon the location of the neutral pressure plane.

The location of the neutral pressure plane varies constantly depending upon the wind effect, the stack effect, and the effects of flues and mechanical equipment. As a result, houses with the same equivalent leakage area may perform differently.

The best way to ensure good overall performance is to minimize the number of air leakage points throughout the building envelope by means of a continuous air barrier.

Things to keep in mind when dealing with moisture flow and insulation:

• When insulation is added, the building exterior becomes much colder
• Cold air is able to hold much less moisture than warm air
• As warm, moist air cools in the cold outer layers of the building, the water vapour it holds may condense as liquid
• The temperature at which water condenses is called the DEW POINT

Condensation in the building envelope can reduce the effectiveness of insulation and even cause rot, peeling paint, buckled siding, spalling and efflorescence on brick and concrete walls, mould growth and other problems.

As absorbent insulation becomes wet, its thermal value drops, thereby intensifying the problem and moving the dew point toward the source of the flow of moisture (i.e. toward the interior surface of the wall).

The frost and condensation end up on the backside of the sheathing, not at the original dew point location (Building Science Insights; BSI-049: Confusion About Diffusion; by Joseph Lstiburek; 2011)