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Science behind mould in air conditioned buildings

this article explains occurance of mould in air conditioning buildings in Asia with vinyl wall covering

Science behind mould in Air Conditioned hotels, hospitals, and commerical buildings 6 June 2016

John Herbert, hong kong energy saving expert, BEAM expert
John Herbert

In the tropics, as incomes rise air conditioning is rapidily changing from a luxury into an every day white applicance. It has been estimated that 700 million air conditioning units will be installed globally over the next 15 years, just consider the future electricity demand!

Mould is already an issue in the tropics, but will this demand for air conditioning be matched by an equal number of damp, mould and IAQ problems? Probably.

science of mould in air conditioned hotels, hospitals, and commercial buildings

A common cause of mould is lining the walls of air conditioned buildings with a vapour retarder, this prevents the natural moisture flow through the structure and thereby creating damp and mould.

I don't need to do a calculation, it is basic science. A result of Air Conditioning, besides space cooling, is lowering the air vapour pressure inside the building, whilst on the other hand, the exterior vapour pressure is significantly higher, this mismatch of pressures forces moisture to flow from outside to inside during the summer months.

Normally (without any vapour retarder) this is not a problem, the ventilation system (air conditioning) in the building will transport that moisture and it will condense on the cooling coil, provided that the wall surface is not covered with an obstacle preventing air flow (and thus preventing moisture removal).

science of mould in air conditioned hotels, hospitals, and commercial buildings

But, problems occur (refer diagram) when the interior face of the external wall is lined with a vapour retarder, for example vinyl wall covering.

The pressure difference is driving moisture inward in summer, but its interupted by the internal vapour retarder, and the building fabric immediately behind the vapour retarder is isolated from the ventilation system, so the air conditioning cannot remove the hidden moisture. Therefore moisture accumulates in the building fabric, causing damp, and its quickly followed by the growth of mould (mold).

science of mould in air conditioned hotels, hospitals, and commercial buildings

And that vapour retarder doesn't need to be vinyl wallpaper. Many materials and coatings are also vapour retarders, for example polymer paints and coatings, PVC coverings, mirror, persex, foil faced materials, plastics, whiteboards and the like, these will halt the moisture flow, causing damp, mould, and building damage.

science of mould in air conditioned hotels, hospitals, and commercial buildings

The solution is simple (1) In air conditioned buildings avoid internal vapour retarder (2) Dont block the air flow that will allow moisture in the space to be removed by the air conditioning system. So simple, but it still happens today.

About the Author

John Herbert is a veteran chief engineer with more than 30 years international engineering experience, educated in the United Kingdom, he worked in the UK and then across Asia for more than two decades engaged by international and local companies. He is a Hong Kong Registered Energy Assessor (REA) and BEAM Professional, and is stationed in Hong Kong.

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