Advisory Note - Combating High Humidity in Commercial Buildings

A Service to A.G. Coombs Group Clients

Combating High Humidity in Commercial Buildings High humidity is one of the more challenging and persistent issues facing commercial buildings today. Longer humid spells, warmer temperatures and more frequent rainfall are creating conditions where indoor moisture levels remain elevated for extended periods.

For building owners and operators, this shift brings a very real set of risks: Elevated indoor humidity creates favourable conditions for mould growth, accelerates deterioration of interior finishes, contributes to corrosion and moisture-related degradation of HVAC equipment, and places additional strain on mechanical systems, driving up operational costs.

This Advisory Note sets out the issues and risks and provides key strategies for effective humidity control.

Effectively managing humidity has become essential — not only for occupant comfort and indoor air quality but also for protecting the longevity and value of building assets.

Many commercial buildings continue to face persistent humidity problems due to incomplete HVAC system commissioning, inadequate maintenance practices, or misunderstandings around ventilation control requirements. Without targeted and proactive strategies, minor humidity issues can quickly escalate, leading to costly remediation, tenant dissatisfaction and reputational impacts.

Successfully addressing humidity requires coordinated and strategic collaboration among building owners, facility managers, HVAC specialists and occupants. In a similar way to building system tuning, robust humidity management practices support sustainability objectives, NABERS Indoor Environment ratings and Green Star certification efforts.

The risks of doing nothing are significant:

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Costly mould remediation, often tens of thousands of dollars per tenancy

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Premature corrosion or fouling of cooling coils and/or chilled beams, ductwork and fittings including grilles

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Increased tenant complaints and potential reputational impact

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Higher carbon emissions and energy costs from unnecessary reheat or over‑ventilation

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Negative impacts on NABERS Indoor Environment and Green Star ratings.

Even short periods of unmanaged humidity can create long‑term issues for both building performance and asset value.

‘How a Cooling Coil Works’ Adapted from Leonardo Drs, Inc. (Marlo Heat Transfer Solutions), 17 April 2023, How a Cooling Coil Works, www.marlocoil.com/how-a-cooling-coil-works

Key Strategies for Effective Humidity Control Optimise HVAC System Performance Many HVAC systems have the inherent capability to manage indoor humidity effectively, yet this potential often remains underutilised due to insufficient commissioning or system tuning.

Regularly reviewing and optimising chilled water setpoints, supply air temperatures and reheat strategies against benchmarks ensures your HVAC system can effectively control humidity levels. Incorporating these reviews into seasonal maintenance schedules provides ongoing comfort and energy performance benefits.

Advisory Note: Combating High Humidity in Commercial Buildings

Recommission and Verify Air Handling Unit Moisture Removal Performance As buildings operate, age and conditions shift, air handling units (AHU) can change from original design intent. Air filters load up and are replaced, sometimes with different media, cooling coils can become partially obstructed, fan speeds get adjusted, or air and water balancing slips over time. None of these issues seem dramatic on their own, but together they change air velocity and coil temperatures — and that can reduce the AHU’s ability to remove moisture and reduce the air’s latent heat load.

A good first step is to review how the AHU is performing today.

Understanding current airflows, coil condition and air-off temperatures against the original design helps reveal whether the unit is still providing the level of dehumidification (also known as latent cooling) the building needs. From there, a few targeted corrective actions can make a substantial difference:

• Confirm chilled-water coil performance, ensuring air-off temperatures are operating as per the design intent

Primary Chilled Water Outside Air (OA)

OA Damper

Heating Coil

Filters

Cooling Coil

Primary Hot Water

Supply Air

Reheating Coil

Centrifugal Fan

Supply Damper

Primary Hot Water

‘Schematic of 100% Outside Air AHU with Dehumidification Coil’ Adapted from M. Baglione, Cooper Union Engineering Faculty. Building Sustainability into Control Systems - Air Handling Units. https://engfac.cooper.edu/melody/417

• Clean and review the coil and surrounding components, repairing insulation and removing any bypass paths that undermine latent capacity

• Address any identified airflow issues, including fan setpoints, filter condition, and any balancing problems that may have crept in over time.

To keep performance on track, a full check and recommissioning of the AHU every five years is recommended. This resets the AHU to its design operating conditions, verifies airflow and balancing and ensures moisture-removal performance is maintained through the most humid periods of the year.

Regular recommissioning not only stabilises humidity control — it protects finishes, supports energy efficiency and helps the overall HVAC system operate reliably and as intended.

Maintain Outside Air Dampers and CO₂ Sensors In many HVAC systems these dampers modulate to allow greater amounts of outside air into the system when outside air conditions are favourable. As such outside air dampers can play a critical role in controlling the volume of humid air entering your building. Without proper inspections, testing and maintenance, these dampers can inadvertently admit excessive outside air when conditions are less than favourable, raising internal moisture levels.

Routine damper inspections and adherence to ventilation standards, such as Australian Standard AS/NZS 1668.2, are important in preventing unintended moisture infiltration.

Additionally, some HVAC systems monitor internal CO₂ levels to help control how much fresh outside air is drawn into the system. Ensure that CO₂ sensors are regularly calibrated and maintained. Incorrect sensor readings can trigger unnecessary outside air intake, increasing indoor humidity levels. Accurate, calibrated sensors reduce this risk, improving both comfort and efficiency.

Manage Ventilation and Building Pressurisation Maintaining a slight positive internal pressure is one of the most effective ways to prevent humid air from infiltrating through gaps, risers and doorways. When buildings become negatively pressurised — often from unbalanced supply and exhaust airflows — moist outdoor air is continuously drawn inside, leading to condensation and mould.

To reduce this risk, it is recommended the following actions are undertaken:

• Balance supply and exhaust airflows and verify during seasonal or five-year recommissioning

• Monitor building static pressure in the Building Management and Control System (BMCS) and trend it alongside outdoor humidity to detect infiltration issues early

• Incorporate floor-level pressure control within the BMCS, using strategically placed pressure sensors and trim-control logic to maintain consistent pressurisation across multiple zones

• Ensure entries, airlocks and foyers operate correctly, particularly in humid or coastal locations.

Where possible, employ enthalpy-based economy control rather than temperature-only systems. Enthalpy control measures both the heat and moisture content of air, limiting outdoor-air intake on humid days while still enabling free cooling when conditions are suitable.

Use Analytics and Proactive Maintenance Continuous monitoring is important for catching humidity issues early.

Tracking supply-air humidity, cooling coil air-off temperatures and internal humidity on a floor-by-floor basis will provide a clear picture of how well the system is actually removing moisture, both at the AHU and in the occupied space.

Advisory Note: Combating High Humidity in Commercial Buildings Modern BMCS platforms can add significant value to this monitoring data by applying rules-based algorithms to detect patterns — such as coil performance dropping, outdoor-air dampers drifting, or zone humidity rising despite cooling - and trigger smart alarms that alert operators before conditions escalate.

Common Challenges to Avoid Humidity initiatives often fail due to:

• HVAC designs that don’t explicitly account for moisture control • Poorly maintained outside air dampers and CO₂ sensors • Lack of seasonal system review and recommissioning or focused tuning to adjust for changing occupancies and changing climate

• Limited use of analytics to detect early warning signs.

To stay ahead of humidity risks:

• Schedule a humidity audit or focused building tuning session before peak humidity season

• Include humidity thresholds and seasonal reviews in maintenance contracts

• Engage an HVAC specialist to confirm ventilation and reheat strategies align with best practice

• Leverage BMCS and analytics to monitor RH trends and prevent costly remediation.

Fan curve and system curves for clean and fouled coil.

Adapted from Siegel et al. (2002, p.5) Dirty air conditioners: Energy implications of coil fouling. Proceedings of the 2002 ACEE Summer Study on Energy Efficiency in Buildings. 1.

Strategically placed sensors around coils, drains, ductwork and key floor areas can support this, providing the system with additional data to identify issues early and accurately.

With intelligent analytics and targeted alarms, buildings can respond to humidity problems proactively instead of reacting after mould, condensation or energy waste has already occurred.

Engage Building Occupants and Service Providers Tenants and service providers can play an important role in managing humidity day to day. Simple observations — musty odours, visible condensation, damp spots or unusual comfort complaints — often provide the earliest signs that something isn’t quite right. Encouraging occupants to report these issues promptly helps prevent small problems becoming major remediation works.

It’s also important that cleaning and maintenance teams understand the impact of their activities. Blocking air paths with equipment, closing grilles or disabling ventilation systems during after-hours work can all upset airflow and pressurisation, allowing humid air to infiltrate the building. Clear communication and basic awareness training go a long way in keeping the system operating as intended.

Taking a proactive approach to humidity control is not optional — it’s part of responsible, future-focused building management.

The buildings that perform best are those that treat humidity as a core operating parameter, not a seasonal nuisance. With the right combination of tuning, recommissioning, sound ventilation strategies and analytics, buildings can stay comfortable, resilient and energy-efficient even as climate conditions continue to change.

A.G. Coombs Advisory is available to support building owners and facility teams who want to strengthen their humidity-management approach and safeguard long-term asset performance.

For further guidance on Combating High Humidity in Commercial Buildings, please contact:

Michael Kettle Senior Technical Specialist A.G. Coombs Advisory +61 458 771 598 mkettle@agcoombs.com.au

By involving building users and service providers, facility teams gain an extra layer of early detection and support — strengthening the overall humidity-management strategy.

Published Dec 2025. While every effort has been made to ensure the accuracy of information in this publication, the A.G. Coombs Group assumes no responsibility for errorsor omissions for any consequence of reliance on this publication. © 2025 A.G. Coombs Group and all related entities. All Rights Reserved.