When to WUFI: explaining hygrothermal modelling Including why and when it matters

12 April 2024

Hygrothermal modelling may be more familiar to you as WUFI, the acronym for the software used to carry out the analysis. Don’t be intimidated by either name. Stick with me as I explain what it is, why it’s important and when you should consider seeking out this modelling. Like energy modelling at design stage, the basic premise is simple: experiment in a computer model, not on your client’s building.

Hygrothermal modelling is a mathematical simulation of the heat and moisture flow through a construction assembly. The results indicate the vulnerability of the assembly to rot or mould. Their presence is costly because they cause damage to buildings and the people who occupy them. Sadly, we have far too much experience in New Zealand with buildings failing due to moisture leaks. As I’ve warned previously, we are facing a new wave of leaky buildings and these ones rain on the inside

WUFI Pro calculates the simultaneous heat and moisture transport in one-dimensional multi-layered building components. It accounts for most of the mechanisms for moisture movements/storage in materials with the primary simplification being the need to directly input convective flows (that is, air leaks). Critical phenomena driving the building failures we are experiencing in New Zealand are things such as internal moisture/interstitial moisture, radiation to the night sky, sunlight driven moisture penetration into materials  and cavity cladding/attic ventilation. All these impacts can be taken into account.

Risk factors

Hygrothermal modelling analyses how energy and water moves through and is stored in materials to determine the risks to those materials. Which is the biggest risk will vary depending on the specifics of a particular building. What is it made of? Where is it located? We start with the layers of materials in a specific building assembly then take into account the local climate. How hot and humid does it get inside of this building over the course of a year? What are conditions outside, how hot/cold/sunny/humid is it? It’s important to understand that materials/assemblies that may function perfectly well in one part of New Zealand might have an unacceptably high risk of mould in another region. Our wide array of climatic conditions is very relevant to hygrothermal analysis. Shade and driving rain are crucial environmental factors. WUFI analysis will start with the south-facing wall (in the southern hemisphere) and the wall facing the predominant driving rain. Modelling will be progressed using whichever aspect is under most pressure.

A huge number of variables present themselves, especially with novel assemblies. Inputs need to be conservative. But if the model is too conservative, it will predict failure that never occurs and that is not useful. To solve this problem,hygrothermal modelling is done to agreed engineering standards and failure criteria based on real world experience of assemblies that mould or rot. For example, the hygrothermal modelling standard the Sustainable Engineering team uses most often is ASHRAE 160:2021. This standard provides a defined internal environment based upon the kind of building, number of bedrooms (as a proxy for occupancy rates, which are a key variable), the airflow rates through the ventilation system (or absence of ventilation system) and the climate as well as other boundary conditions needed for modelling.

Especially with novel assemblies, we will use a bracketing approach to help us hone in on the key variables. The illustrations below are from a model I built for a strawbale wall, with external cladding and plaster on both sides of the assembly. Changing any of these has an impact on how all the variables interact—not just the assembly details but the external conditions too. An inexperienced modeller could be quickly overwhelmed by thousands of cases to consider. But the more WUFI work I do, the faster I get at identifying which of the cases are the useful ones to focus on. On this job, which was for a small social housing home in Wellington with high occupancy rates, the internal ventilation rate proved to be absolutely key. This building passes the ASHRAE standard, but only with a MVHR installed. Otherwise, high internal relative humidity drives scarily high internal moisture levels.

Example of dynamic output from WUFI Pro hygrothermal modelling

WUFI has a neat feature I really like: a movie presentation of the data that shows how temperature and moisture levels change over time. One of the practical takeaways from this particular modelling project was confidence that increasing moisture resistance on the interior surface of this wall assembly is a good thing. That is because the building is in a heating-dominated climate. The same strategy would be a disaster in a humid, cooling-dominated climate—Darwin is a good example. That’s why our WUFI modelling is done in relation to a specific site. Mould risk isn’t just about the building assembly, the interaction between materials and their environment is crucially important. 

Interpreting results

We build the assembly up to simulate the environment and then consider the resulting mould index. This is a measure of surface warmth and moisture levels over a period of time. The model will show if mould will flourish or die off in those conditions. If the mould index is sustained below 3.0 anywhere in the assembly—ideally below 1.0 anywhere interior to the air control layer—the assembly passes and is considered low risk.

WUFI Pro output mould model

Not all passes are equal. In rare cases, mould might still develop even though the model gives a passing grade. The results are designed to capture 90% or more of the incidents. If the ASHRAE 160 model reports there’s no mould in the building but we tell you it’s a bit marginal, it means that probably the building will not have mould anywhere but it is possible a few spots might occur.

On the other hand, if there is a lot of margin in that passing grade, it is extremely unlikely you’d find any mould anywhere in the building, short of an external water leak somewhere. That can overwhelm any assembly.

When to WUFI

Our team at Sustainable Engineering Ltd has deep experience in WUFI modelling. A lot of our work to date has unfortunately been analysing completed buildings in order to pinpoint why they have failed and the best route to remedy them. We’d like to prevent problems before they happen. We have pushed for WUFI modelling when clients have come to us with details that experience tells us are at high risk of mould (for example, internally insulating concrete walls). And as described above, we have carried out WUFI modelling for non-standard assemblies. It’s often a day’s work or less; we think it is self-evidently a good investment compared to the financial costs of litigation, remediation and reputational damage.