Building structures are complex systems that must be designed to withstand various environmental conditions such as temperature fluctuations, moisture infiltration, and air infiltration. The durability and performance of building assemblies rely heavily on how heat and moisture move through the structure, making it essential to have an effective and reliable method of analysing this movement. Fortunately, computer-aided hygrothermal analysis and modelling have made it possible to understand how heat and moisture move through building components. This helps designers and builder ensure that their buildings will meet the required performance and durability standards.
Heat and moisture movement through building assemblies is a complex phenomenon that can lead to serious problems such as condensation, material degradation, and even mould growth. Condensation occurs when the relative humidity in air reaches dew point temperature, causing moisture to form on the surfaces of the building components. The location of condensation can vary depending on the type of assembly, the materials used, and the climate conditions. For example, in cold climates, the condensation point is usually on the inside of the exterior surface of the building assembly, while in warm and humid climates, it is often on the interior surface. When condensation occurs, it can lead to material degradation, including rot, spalling concrete/brick, or even mould growth, which can compromise the integrity and performance of the building assembly.
Proper ventilation, vapor diffusion and drainage are essential for allowing a building assembly to dry and preventing the accumulation of moisture that can lead to these issues. This is particularly important for wall assemblies that have insulation and a vapor barrier. In such assemblies, if there is no ventilated cavity, moisture that gets into the assembly through gaps, such as around windows, doors, other penetrations, or vapour drive from warm to cold can become trapped, leading to the accumulation of moisture and potential mould growth. In contrast, having a ventilated cavity with proper drainage allows moisture to dissipate and the assembly to dry, reducing the risk of condensation and associated problems.
The importance of a well-sealed home cannot be overstated when it comes to controlling interstitial condensation through small gaps in assemblies. Poorly ventilated buildings with inadequate sealing can have an increased risk of condensation, leading to moisture-related issues. However, buildings that are both well-sealed and adequately ventilated can be very effective at managing condensation. This is why the building envelope sealing Verification Methods (H6V3 of Volume Two (2019: V22.214.171.124) and J1V4 of Volume One (2019: JV4)) are updated to include requirements for continuous ventilation in homes that are well-sealed.
One tool that can be particularly helpful in hygrothermal analysis is WUFI (Wärme und Feuchte instationär, or Transient Heat and Moisture) software, developed by the Fraunhofer Institute for Building Physics in Germany and Oak Ridge National Laboratory in the US. WUFI software calculates heat and moisture transfer through a selected portion of the building envelope. We can achieve compliance with the NCC if we model building assemblies in accordance with criteria set forth in ASHRAE DA07 "Criteria for Moisture-Control Design Analysis in Building." WUFI can adequately model a component of an existing or new structure by considering the material properties, orientation of the analysed section, and historical or future climate data for the structure's location.
At SUSTAINABUILDING PTY LTD, we understand the importance of hygrothermal modelling and proper ventilation in ensuring the durability and performance of building assemblies. We specialize in performing hygrothermal modelling of proposed assemblies using WUFI 1D software, which can provide quick initial reviews of assemblies and more in-depth consulting to improve or fix the assemblies' performance. Our trained staff can use the WUFI Pro 6.6 software to assess your areas of concern and offer solutions to moisture drive problems.
Image: Implementation of a ventilated facade with a vapour permeable Weather Resistive Barrier (WRB) on the SUSTAINABUILDING demonstration home.
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