Building Performance – Lesson 3
3. Moisture moves from more to less.
This rule primarily applies to moisture in the form of vapour. This movement happens via two processes:
– Diffusion (3% of the vapour problem)
– Air flow (97% of the vapour problem)
DIFFUSION is the movement of moisture in the vapour state caused by either a difference in vapour pressure and/or temperature. In warm, weather when the outside humidity is high, the moisture wants to move from outside to inside where the air conditioning is removing the moisture from the interior space.
On a frosty winter’s morning when there is less moisture in the outside air, moisture wants to move from inside where lifestyles generate more moisture, to outside where there is less moisture.
So if moisture moves from more to less, the type and permeability of the wall system materials is important.
For example, in a winter scenario, if a non-permeable wall wrap is installed, moisture moving from the interior of the house to the outside may condense on the interior side of the wall wrap. This condensation occurs when the moisture reaches its dew point at the cold inner face of the wall wrap. This moisture problem may cause mould and decay inside the wall system.
The other significant way moisture moves from more to less is via air flow. Moisture can be carried by air flow into or out of the home through holes and cracks in the building envelope. This air flow is caused by:
– Stack effect
– Mechanical systems
STACK EFFECT is created when there is a temperature difference between inside & outside. The cold air leaks into the building through cracks and unsealed openings. Warm air leaks out of the building through similar cracks and unsealed openings in the ceiling.
WIND. When the wind blows it creates a pressure difference across the building. On the windward side air leaks in. On the leeward side air is drawn out. This air flow can transport energy and moisture in either direction.
MECHANICAL SYSTEMS. Central heating fans and exhaust fans can cause a difference in pressure. They can either draw moisture in or allow it to flow out. By building a tight envelope and controlling mechanical pressures we can reduce moisture flows through the building.
Sealing gaps, cracks & penetrations is important. We can solve 97% of the moisture problem by providing an air tight envelope.
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