In air conditioned buildings, the exterior vapour pressure will be higher than the vapour pressure in the air conditioned room, therefore the vapour and moisture flow is high to low (blue line in the diagram above)
Now normally this is not an issue, because a decent ventilation system will transport moisture back to the air conditioning cooling coil, it is condensed, and safely drained away.
However, problems occur when a vapour retarder is installed (red line in above diagram) interrupting the flow. Moisture becomes trapped, and even the best ventilation system cannot transport it to the cooling coil. Instead what happens, damp, moisture, and then mould growth occurs (left of red line in diagram above).
And we already know the problems associated with mould, it causes poor indoor air quality, and health issues for the occupants. If the air conditioning room also happens to be under negative (relatively) pressure, as often occurs in hotel rooms, the situation is compounded because the vapour quantity is increased, accelerating the process.
This phenomenon is not limited to Hong Kong, it occurs in hot tropical climates, whether you are in Singapore, Indonesia, Shanghai, or Hong Kong.
Furthermore, it’s the type of problem that’s not immediately apparent, mould doesn’t just appear overnight, and it is unlikely to surface within the defects liability period (DLP).
So it is obvious, lining the external wall of an air conditioned space with any vapour retarder must be avoided at all costs. But sadly it still happens today, increasing maintenance and costs for building owners and operators.
Kelcroft based in Hong Kong provides specialist advice, including energy, built environment, and green building consulting services, advising owners and design teams, to lower the cost of building operations.
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