What is WUFI?
Methodologies for Measuring Moisture Risk in Buildings
Whether you’re building a new home or retrofitting an old one, the last thing you want is a damp, mouldy building. Understanding the moisture risk in new and existing buildings is a growing branch of building physics that looks to understand the role ‘hygrothermal’ activity – the movement of heat and moisture through buildings – plays in the health and structural stability of our buildings.
It is not straightforward to predict and design out this moisture risk. Therefore, it is useful to understand the differences between the available assessment methods.
While heat and moisture transport is an intrinsically linked physical phenomenon, this close relationship makes their assessment complex. Assessing them separately does not lead to the same results as assessing them simultaneously. Therefore, there are essentially two approaches to dealing with this:
– Steady-state, Glaser method (BS EN ISO 13788): Works by making a number of assumptions to simplify and decouple the equations.
– Transient-state, WUFI (BS EN ISO 15026): Works by solving the equations numerically, using computer simulation
What is the Glaser Method?
The Glaser method, as codified in BS EN 13788, is a procedure to assess the condensation risk in building fabric. The original Glaser method, developed by Dr. H. Glaser, a German engineer, was first published in 1959, as an assessment method for diffusion transport processes in the thermal envelopes of cold rooms. Being a simplified calculation procedure, the Glaser method has substantial limitations as it only takes into account some of the heat and moisture transport mechanisms that take place in a building, not all, and therefore it is only applicable under specific conditions. Furthermore, it excludes:
– External climate parameters such as precipitation, solar radiation, etc.
– Liquid transport
– Hygroscopic moisture capacity of materials
As a result, the Glaser method should only be used to compare, at the design stage, alternate forms of constructions which are:
– Not subjected to wind-driven rain
– Made only with materials with a low water storage capacity
What is WUFI?
So, what is WUFI? WUFI is an acronym that stands for ‘Wärme und Feuchte Instationär’, which translates in English to “Heat and Moisture Intransient.” It comprises a family of software designed to conduct the most advanced analysis of the hygrothermal conditions in a building envelope.
The WUFI software is a numerical simulation tool developed to provide a more realistic depiction of the hygrothermal conditions of each examined scenario. It provides results that allow the assessment of a number of moisture-related risks, such as mould growth, interstitial and surface condensation, timber decay, corrosion, and freeze-thaw deterioration.
It gives a more realistic depiction of the hygrothermal conditions because WUFI takes into account all the heat and moisture mechanisms and material properties that may have a substantial impact on the hygrothermal behaviour of an element. These varying conditions include:
– Heat storage in dry building materials and absorbed water
– Heat transport by moisture-dependent thermal conduction
– Latent heat transfer by vapour diffusion
– Moisture storage by vapour sorption and capillary forces
– Moisture transport by vapour diffusion
– Moisture transport by liquid transport (surface diffusion and capillary flow)
Additionally, this type of simulation accounts for the following climatic conditions:
– Internal and external temperature
– Internal and external humidity
– Solar and longwave radiation
– Precipitation (normal and driving rain)
– Wind speed and direction
WUFI also allows assessment on the impact of imperfect construction. It does so by allowing a simulation of water penetration on the outer portion of the assembly, as well as varying rates of air leakage from within the inner portion.
The Benefits of WUFI Modelling
As explained above, the Glaser method’s simplified, steady-state approach excludes several hygrothermal transport processes from consideration. All of these are of particular importance in the hygrothermal assessment of traditional and new build constructions, particularly if they are internally retrofitted with insulation. Using the Glaser method to assess conditions and building fabric that are outside of the method’s scope can generate results that do not resemble, and can even contradict, the more accurate results of numerical simulation with WUFI.
It is clear that in order to get a realistic insight into how a build-up behaves hygrothermally, and whether moisture-related risks exist or not, WUFI modelling is essential. This is because it provides simulated conditions that are as similar to real-life ones as possible.
This is the case whether an existing building element is retrofitted, or a new-build construction is designed, and a moisture-risk has been suspected. It can also be used to assess the feasibility of a prefabricated construction element in different climate zones within the UK.
Generally speaking, construction types that are considered high-risk ones are often build-ups with internal wall insulation or flat roof build-ups.
When Do You Need a WUFI Calculation?
When should a hygrothermal performance assessment be considered essential? To understand whether an examined build-up involves any moisture-related risks there are three sources of information and guidance in the British standards:
– BS5250 – Code of practice for control of condensation in buildings
– BS EN 13788 – Hygrothermal performance of building components and building elements
– BS EN 15026 – Hygrothermal performance of building components and building elements – Assessment of moisture transfer by numerical simulation
– BS5250 includes a number of standard details for each building element, and it is the first source of information that needs to be consulted. If your specific build-up is included in the listed cases, then the standard will direct you to the required assessment method. This could be either assessment by inspection, assessment by using BS EN 13788 (Glaser method), or assessment by using BS EN 15026 (WUFI).
It is, however, very likely that your examined build-up is not listed in the standard. In this case, the assessment method should be determined based on the feasibility and the pros and cons of each method, as described within this article.
Written by Kostas Megagiannis