Humidity and Mould in Buildings: A Malaysian Fix

The Industry Code of Practice on Indoor Air Quality 2010 sets a total fungal count limit of 1,000 cfu/m3 for Malaysian workplaces, and a recommended comfort band of 40 to 70% relative humidity. Buildings that fail the first number have usually been living outside the second for months without anyone logging it. Humidity and mould in buildings is almost never a sudden event. It is the slow accumulation of hours when the air was wet enough for spores already sitting in carpet backing, ceiling tiles and duct dust to start feeding, and nobody was measuring during those hours.

That is the core problem in tropical facilities. IAQ assessments happen on a working weekday with the plant running. Mould grows on Sunday night.

Why does mould grow in air-conditioned Malaysian buildings?

Mould needs moisture, an organic surface and time. Temperature barely matters in Malaysia because every month is growing season. What decides whether a gypsum wall stays clean or turns black is the moisture available at the surface, not the number on the wall-mounted thermostat.

Outdoor air here typically sits close to saturation, with dew points commonly in the mid-20s Celsius through most of the year. Every door opening, every unbalanced toilet exhaust, every leaky riser and every hour of unfiltered infiltration pushes that moisture inside. Air conditioning is the only thing removing it. When the ACMV system stops removing moisture, even for a weekend, the building reverts toward outdoor conditions, and surfaces that are still cool from the previous run period sit at local humidity far above whatever a room sensor would report.

The 40 to 70% band is a comfort range, not a mould threshold

ICOP IAQ 2010 recommends 23 to 26 C and 40 to 70% RH for acceptable indoor conditions, aligned with the indoor design conditions in MS 1525. Those are comfort recommendations. They are not a guarantee of no growth.

Treat 70% as a ceiling you are not supposed to touch rather than a target you may drift up to. Room air at 70% RH will produce much higher humidity right at a cool surface, and mould responds to that surface condition. A perimeter wall behind a filing cabinet, the back face of a ceiling tile under a sweating pipe, or the inside of an AHU casing can be in the growth range while the space sensor still reads a compliant number. Practical operating advice for tropical offices: hold the occupied band nearer 55 to 65% RH, and treat any zone logging above 70% for sustained periods as a defect to investigate, not a comfort complaint to dismiss.

AHUs switched off overnight and over weekends

From Friday 6pm to Monday 8am is roughly 62 hours with no dehumidification. Infiltration continues. Moisture stored in furniture, files, plasterboard and carpet releases back into the air. By Monday morning the space is wet, the returns smell musty, and the chillers spend the first two to three hours pulling out water instead of delivering comfort.

The instinct is to run everything all weekend, which is expensive and unnecessary. The better approach is a humidity setback: keep a defined RH ceiling active outside occupied hours and let the control system stage minimum AHU operation and cooling only when that ceiling is crossed. That usually means short, infrequent runs, a fraction of full occupied energy, and it stops the rebound cycle that feeds mould. It also protects paper records, server rooms and any area with wood or fabric finishes.

Two more things worth checking before you touch schedules. First, confirm toilet and kitchen exhausts are not left running unattended after hours, because they pull humid outdoor air into a building with nothing conditioning it. Second, confirm the building is not depressurised during unoccupied mode for the same reason.

Oversized cooling that short-cycles and never dehumidifies

Cooling capacity and drying capacity are not the same thing. A system removes sensible heat by lowering air temperature and latent heat by condensing water out of the airstream at the coil, and only the second one controls humidity. If you have ever stood in a space that felt cold and clammy at the same time, you have met the difference in person, which is explained in more detail in why a room can be cool but humid.

Oversized units make this worse in a specific way. The unit satisfies the thermostat quickly, shuts down after a short run, and the coil never stays wet long enough for meaningful condensate to form and drain. Worse, moisture already on the coil re-evaporates into the space when the fan keeps running after the compressor or valve closes. The result is a building that hits its temperature setpoint all day and still logs 72% RH.

The same failure appears in chilled water systems when supply temperature is reset upward too aggressively to save energy. Raise it far enough and the coil surface no longer sits below the entering air dew point, latent capacity goes to zero, and the space quietly gets wetter while the energy dashboard shows an improvement. Any chilled water reset strategy should be interlocked with zone humidity, not run on temperature alone.

Other common contributors: fan speed pushed too high on VSD-equipped AHUs so face velocity carries moisture off the coil, and blocked or improperly trapped condensate drains that let water sit in the pan and become its own biofilm reservoir.

Cold surfaces, condensation and hidden growth

Most mould in commercial buildings is not on the wall you can see. It is above the ceiling. Damaged or missing insulation on chilled water pipes, uninsulated valve bodies and flanges, poorly sealed duct joints and cold AHU casings all condense water into the plenum, which is often also the return air path. That means growth in the plenum feeds spores directly back into the occupied space.

When investigating a fungal count exceedance, open the ceiling. Look at pipe insulation jackets, drain pan condition, coil face cleanliness, filter loading and any water staining on tiles or gypsum. A high count with no visible growth in the room almost always resolves to something above it.

The fix hierarchy: monitor, schedule, then maintain

Work in this order.

Monitor continuously. A single assessment tells you the condition on one day. Trend data tells you the building's Monday morning peak, the weekend rebound, and which zones sit above the band. This is also the evidence base an employer needs when demonstrating that IAQ is being assessed and maintained, as covered in the ICOP IAQ 2010 compliance guide and in the code itself, published by DOSH.

Fix the schedule. Add an unoccupied RH ceiling, interlock chilled water reset with humidity, correct after-hours exhaust operation, and check fan run-on behaviour.

Then maintain the hardware. Clean coils, correct filters, clear and trapped drains, repaired insulation, verified outdoor air rates near the ASHRAE 62.1 office guidance of approximately 5 cfm per person plus 0.06 cfm per square foot. Maintenance is essential, but doing it without fixing scheduling means repeating it forever.

CobiNeural is built for the first step. It logs CO2, temperature, humidity and related IAQ parameters continuously alongside energy, water and chilled water, sends WhatsApp and email alerts when a zone crosses its threshold, and produces the reporting trail for compliance reviews. It runs standalone or as an overlay on an existing BMS, PLC or SCADA, so you can see the weekend rebound without replacing controls.

If your last IAQ assessment came back clean but the complaints keep coming, the data you are missing is from the hours nobody was in the building. Request a demo and we will show you what a full week of humidity trending looks like in your zones.