Mould in Air Conditioned Buildings in Asia

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Mould in
Air Conditioned
Buildings
by John Herbert

Diffusion

Diffusion
Across Asia the hot & humid climate means
the outside air has higher vapour pressure
(VP) than the air inside air conditioned
buildings having a lower vapour pressure (VP).
This difference creates a pressure gradient
(P) across the building envelope, causing
moisture to be transported through the building
envelope in a process known as Diffusion.

Diffusion
Higher Vapour
Pressure (VP)
Lower Vapour
Pressure (VP)
Air Conditioned Buildings
building envelope
diffusion

Diffusion
Moisture passing through the building
envelope would normally transported by air
circulating in the room.
Moisture, transported by the air, when it
reaches the air conditioning cooling coil, the
moisture is condensed to water, and drained
away. So moisture does not accumulate in the
building envelope.

Diffusion
building envelope section - NTS
room air
circulation
transfers
moisture
high VP
33 degC
80% rh
3,680 pa VP
low VP
24 degC
60% rh
1,752 pa VP
moisture
OUTSIDEINSIDE

Diffusion
However, if a vapour retarder such as vinyl
wallpaper is lining the inside face of an external
wall of an air conditioned room moisture
becomes trapped behind the vapour retarder,
effectively isolated from the room air circulation.
Moisture accumulates beneath the vapour
retarder creating damp and leading to growth of
mould.

Diffusion
Where damp and mould occurs in a space
without any air movement that rules out
moisture transport by air, for example behind
vinyl wallpaper or other materials impervious to
air.
Therefore, moisture transportation by Diffusion
is the likely culprit.

Diffusion
room air
circulation
cannot
transfer
moisture
high VPlow VP moisture
OUTSIDEINSIDE
moisture trapped behind
vapour retarder
moisture and mould
vinyl wallpaper

Diffusion
The challenge is not just limited to vinyl
wallpaper, any vapour retarder, for example
• Material with aluminium foil facing layer
• Millwork, furniture with plastic laminate
• Vinyl's and plastics e.g. whiteboard
• Polymer based paints and coatings
• Glass, including mirrors

Diffusion
mould
vapour retarder
room air
circulation
cannot
transfer
moisture
OUTSIDEINSIDE
moisture trapped behind
area of vapour retarder

Diffusion
Decoration of air-conditioned rooms often
means the walls are finished internally with
plasterboard, and windows are often trimmed
with decorative millwork.
Often finishes inadvertently include a vapour
retarder for example foil faced thermal
insulation, the vapour retarder traps moisture,
and being isolated from the circulation of room
air moisture accumulates causing mould.

Diffusion
unventilatedcavity
plasterboard
insulation
room air
circulation
cannot
transfer
moisture
OUTSIDEINSIDE
moisture trapped in cavity
by vapour retarder

Mechanical
Ventilation

Negative Pressurisation
Mechanical Ventilation with exhaust fans
drawing air from washroom, bathrooms, and
kitchens create a (-ve) negative pressurisation
in the building.
Replacement air is drawn through any open or
undercut doors, and open windows. And
through any defects including leakage caused
gaps around doors, windows, eaves, etc.

Air Conditioned Building
negative
pressure
(-ve)
infiltration
Exhaust air fan(s)
create (-ve) pressure
building envelope
EAF

Where the outdoor air condition is, lets say, 33
degC & 80% rh, the dewpoint temperature of
the air is approx. 28.5 degC.
That means if the outdoor air impinges upon a
surface with a temperature below the
dewpoint temperature moisture in the air will
condense, causing damp and potentially
mould growth.

typical room air
24 deg C 60% rh
outdoor air
33 deg C 80% rh
dewpoint temp
28.5 deg C
condensationzone

Its understood that water ingress whether
through diffusion or infiltration creates damp
inside buildings, causing damage to building
fabric and finishes, and then growth of mould
impacting the health of its occupants.
Therefore strategies are needed to account for
moisture and mitigate the risk.

Positive Pressurisation
To manage the uncontrolled ingress of outdoor
air, a PAU filters and dehumidifies the outdoor
air delivering conditioned air into the building.
If the quantity of conditioned air is greater than
the exhaust air, this creates Exfiltration
characterised by the intention leakage of
conditioned air and preventing ingress of hot &
humid outdoor air.

This leakage of conditioned air is intention, to
minimise the risk of hot and humid outdoor air
causing condensation.
Often witnessed as conditioned air (cool air)
spilling through the entrance door out to the
street.

positive
pressure
(+ve)
exfiltration
Exhaust air fan(s)
building envelope
EAF
PAU(s) create
(+ve) pressurePAU

Conclusions
Air movement inside buildings can be
complex, particularly in high rise buildings,
therefore its important that every space in the
building is analysed room by room, and floor
by floor.
Lift shafts and operation of lift cars pushing air
through the shaft particularly in high rise
buildings causes air movement that may
impact the design strategy.

positive
pressure
(+ve)
exfiltration
Exhaust air fan(s)
building envelope
EAF
PAU(s) create
(+ve) pressurePAU
infiltration
diffusion

Conclusions
In addition external airflow (wind) creates a
varying pressure on the building depending on
its direction and strength which should also be
considered when formulating the model to
mitigate the risk of damp and mould in
buildings.

John Herbert
@johnherbert
+(852) 2335 9830

Mould in Air Conditioned Buildings in Asia

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