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 moisture risk in existing and new buildings is a growing branch of building physics, as we understand more the role that ‘hygrothermal‘ activity – the movement of heat and moisture through buildings – plays in the health and structural stability of our buildings.
The ways to predict and design out this moisture risk are not straightforward. To make this clearer, it is useful to understand the differences between the available assessment methods.
Heat and moisture transport are intrinsically-linked physical phenomena. This close relationship makes their assessment complex and assessing them separately would not lead to the same results as assessing them simultaneously. This makes hygrothermal assessments complex.
Methodologically, there are essentially two approaches to dealing with this complexity in assessments:
- 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, and what are its limitations?
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 because it can take into account only some of the heat and moisture transport mechanisms that take place in a building and therefore it is only applicable under specific conditions. More specifically, some its limitations are:
- Exclusion of external climate parameters such as precipitation, solar radiation, etc.
- Exclusion of liquid transport.
- Exclusion of the 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? Is it the sound of a dog’s bark …? Is it a new advanced form of WiFi…? The hottest new DJ on the electronic dance music circuit…? These are all questions that people naturally ask when they are told that a WUFI assessment is required for their project.
The answer to these questions is no. WUFI is an acronym that stands for ‘Wärme und Feuchte Instationär’, which translates in English to “Heat and Moisture Intransient,” and it is a family of software designed to conduct the most advanced analysis of the hygrothermal conditions (the combination of heat and moisture) 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 and provide 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 substantial impact on the hygrothermal behaviour of an element, such as;
- 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.
Lastly, WUFI goes even further to also allow assessment of the impact of imperfect construction by allowing one to simulate water penetration on the outer portion of the assembly and varying rates of air leakage from the room on 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 building constructions, particularly when 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. The inappropriate use of the Glaser method facilitates and maintains misconceptions.
It is clear that in order to get a realistic insight of how a build-up behaves hygrothermally and whether moisture-related risks exist or not, under simulated conditions that are as similar to the real-life ones as possible, WUFI modelling is essential. 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 above.
Senior Building Physics Consultant | Certified Passivhaus Designer
Enhabit offers hygrothermal simulation and condensation risk analysis (including both WUFI and Glaser modelling). Please get in touch for advice on the moisture risk of your project.