Evaluating the Mould Index in the Building Envelope Using WUFI Pro (AIRAH DA07)
- Shahram Derakhshan
- Mar 16
- 4 min read
Condensation risk analysis is a critical aspect of building performance modelling, particularly in Australia, where varying climates demand different approaches to moisture control. With the release of NCC 2022, the importance of robust moisture risk assessments has increased, particularly in aligning with international standards and industry best practices. This article compares the condensation risk assessment methodologies outlined in NCC 2022 with those based on ISO standards, AIRAH DA07/ASHRAE 160, and WUFI PRO.
NCC 2022 and Condensation Risk Analysis
The National Construction Code (NCC) 2022 includes new provisions for condensation management, particularly in energy-efficient buildings. It emphasises:
Vapour control layers: Ensure the correct placement and permeability to prevent interstitial condensation.
Ventilation requirements: Encouraging proper airflow within building cavities.
Material selection: Consider the hygroscopic properties of materials to avoid moisture accumulation.
Climate-specific considerations: Recognising that warm and humid zones require different solutions than colder climates.
Class of Buildings: Condensation risk considerations apply to residential (Class 1 and 2) and commercial (Class 3 to 9) buildings, particularly those with high insulation levels and reduced air leakage.
While the NCC outlines general condensation risk requirements, it does not prescribe a specific calculation method for assessing interstitial condensation. Instead, it aligns with broader industry standards, such as ISO 13788 and AIRAH DA07.
NCC 2022 Performance Requirements and DTS Provisions
The NCC 2022 provides condensation management provisions under both Performance Requirements and Deemed-to-Satisfy (DTS) Provisions:
Performance Requirements (Condensation Risk Management): Ensure that building elements are designed and constructed to manage moisture accumulation and limit conditions conducive to mould growth.
DTS Provisions: Provide prescriptive solutions such as ventilation openings, vapour-permeable membranes, and drainage paths.
The NCC 2023 Handbook on Condensation Management offers additional guidance and outlines strategies for moisture control in various Australian climates.
ISO 13788: Glaser Method and Its Limitations
ISO 13788 describes a simplified steady-state method (often called the Glaser method) to assess interstitial condensation risk based on:
Vapour diffusion calculations under steady-state conditions.
Monthly climate data and simplified thermal resistance assumptions.
The assumption is that moisture only moves by diffusion (ignoring capillary transport and absorption effects).
While ISO 13788 provides a straightforward and standardised method, its biggest limitation is excluding transient moisture transport. It does not account for:
Hygroscopic buffering effects of materials.
Dynamic climatic changes (e.g., daily temperature fluctuations, rain absorption, and drying effects).
Capillary action and material sorption.
Due to these limitations, many industry experts prefer dynamic simulation tools for more accurate moisture risk analysis.
AIRAH DA07 and ASHRAE 160: A More Comprehensive Approach
AIRAH DA07 (Condensation in Buildings) and ASHRAE 160 (Criteria for Moisture-Control Design Analysis in Buildings) outline more detailed methodologies, incorporating:
Hygrothermal simulations considering both vapour diffusion and material sorption.
The impact of internal moisture sources and ventilation strategies.
Mould growth risk assessment based on environmental conditions (e.g., temperature and relative humidity).
While these guidelines offer a more detailed approach than the ISO Glaser method, they still rely on empirical models rather than fully dynamic simulations. This is where WUFI PRO becomes a valuable tool for Australian projects.
WUFI PRO: A Dynamic Hygrothermal Modelling Solution
WUFI PRO is a transient heat and moisture simulation software that allows for:
Dynamic climate data inputs (hourly weather files rather than monthly averages).
Simulation of both vapour diffusion and capillary transport.
The effect of driving rain, solar radiation, and material wetting/drying cycles.
Long-term mould growth prediction based on the Mould Index Model (ISO 13788 does not include this feature).
WUFI PRO is particularly beneficial in Australian climates, where:
Warm and humid regions (e.g., Brisbane, Darwin) require consideration of inward vapour drive from external humidity.
Cooler regions (e.g., Melbourne, Hobart) require insulation placement strategies to prevent internal condensation risks.
Mixed climates (e.g., Sydney, Perth) require seasonal adaptability in material selection and ventilation strategies.
References and Industry Guidance
For professionals seeking further information, the following references provide valuable insights into condensation risk management:
NCC 2022 and 2023 Handbook on Condensation Management (Australian Building Codes Board)
AIRAH DA07: Best practices for moisture control in Australian buildings
ASHRAE 160: Moisture control criteria for North American climate zones (useful for comparative analysis)
ISO 13788: Hygrothermal performance of building components and elements
WUFI PRO User Manual: Guidelines for accurate moisture simulation in construction materials
Conclusion
Condensation risk assessment is essential to ensuring the durability, energy efficiency, and indoor air quality of buildings. While ISO 13788 provides a fundamental starting point, the limitations of steady-state methods make dynamic simulation tools like WUFI PRO more suitable for accurately assessing moisture risks in Australian climates.
By leveraging WUFI PRO, building professionals can make informed decisions on vapour control, insulation strategies, and ventilation solutions to mitigate condensation-related issues. Deratec has extensive expertise in WUFI PRO modelling and can assist with detailed condensation risk analysis for roofs, walls, floors, and other building envelope elements. Contact us to learn how advanced hygrothermal simulations can enhance your project's performance and compliance with NCC requirements.
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