Vapour diffusion

Moisture is a problem in the tropics so its important to understand two issues, moisture is transported within the air, and through a process known as Diffusion.

One property of air is the water vapour pressure, for air-conditioned buildings, we are mainly concerned with the difference between the outdoor and indoor water vapour pressure. Why air-conditioned buildings? Cooling and dehumidifying indoor air lowers the indoor water vapour pressure.

In the CIBSE guide or equal, properties of air, look up the water vapour pressure for outdoor air say 35 db/28 wb and an air-conditioned space, lets say 24 degC and 50% rh. That delta P, the pressure difference is the driving force for moisture diffusion through the building envelope, including curtain walls, towards the lower pressure space inside buildings.

Typically, we expect that “Good Ventilation” inside the building will transport that moisture and condense it on the cooling coil. However, if the internal face of the external wall is blocked, with a vapour barrier on the inside or obstacles preventing air movement, moisture will accumulate internally causing mould.

Often vinyl wallpaper or a polymer type paint, essentially a layer of plastic, is used inside buildings, which stalls diffusion, causing moisture to accumulate internally, causing damp and mould as shown in the diagram above. If the room is naturally ventilated the water vapour pressure inside and outdoor are effectively equal, and adjacent air-conditioned rooms would with similar conditions means the delta P is also effectively zero.

So its important to understand the different processes, to solve IAQ problems.