Building Performance – Lesson 2
2. Moisture moves from warm to cold.
As a warm weather based example, rain falls on a brick veneer wall and is absorbed. The sun comes out, heats up the bricks and drives the water to the interior side of the brick where it is cooler. From there it either drains out via the weep holes or as a vapour moves out through the wall system.
Air conditioning can pull this moisture through to inside the building where the air is cooler. Interior and exterior temperature differences help drive the movement.
If you don’t design the wall system with the appropriate materials for your climate this moisture could condense inside the wall cavity causing mould growth and structural decay.
To manage this process we need to select an exterior weather barrier, and interior finishes that allow the wall to dry, along with barrier materials that effectively manage the rain and water vapour.
I intentionally gave a summer example because in Victoria we usually we think of the winter scenario. In North America cladding such as brick, weatherboard & fibre cement are known as ‘reservoir’ cladding. The term “reservoir’ recognises that these materials have the capacity to hold moisture for extended periods. This capacity to retain water can particularly cause the problem of inward vapour drive where air conditioning is used. This is not a problem in cold climates such as Canada, parts of Northern U.S.A and Europe, but in ‘air conditioning dominated’ climates such as much of mid & Southern U.S.A, and Australia it is something worth thinking about.
Just because you have a predominant winter heating load doesn’t mean you should ignore summer.
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