Toby Cambray talks about the risks associated with moisture in buildings, and the modelling techniques – like WUFI® – that can be used to assess those risks.
Interview with Toby Cambray
Toby Cambray co-founded Greengauge building & engineering consultancy in 2010, specialising in sustainable and low energy design. He is a certified Passivhaus Consultant and has completed a PhD on moisture in buildings.
Condensation on windows is a common sight in British homes
In buildings, moisture is water in any form
When we use the term ‘moisture’ it can refer to liquid water, water vapour in the air, or even ice. People might be most familiar with water vapour, and how humidity changes with temperature.
It’s also important to think about materials – especially historic building materials – and how they interact with liquid water. And when you add insulation to buildings, moisture is something that really needs attention paid to it.
Moisture movement in buildings is about equilibrium
In the laws of physics, all things try to minimise their energy. For example, if you have an area of high pressure next to an area of low pressure, the molecules try to ‘even themselves out’ and minimise the total net energy. In a different context, that’s what is happening with moisture. In an area of high vapour pressure with lots of water, the water will try to distribute itself as evenly as possible.
The potential risks from moisture in buildings are varied
Organisms that cause mould or timber decay can thrive where moisture is present. Moisture also leads to problems like corrosion, especially of steel. In heritage buildings, it can lead or contribute to deterioration in masonry, through processes like freeze/thaw damage, or salt transport and crystallisation.
Damp and mould inside a historic building
The two main moisture strategies are moisture open or moisture closed
Putting it very simply, you can either manage moisture or block moisture. Most modern buildings are moisture closed, or moisture blocking. Features like damp proof courses and vapour control layers aim to stop moisture getting to where we don’t want it.
In traditional buildings, when these modern products obviously weren’t available, builders worked with materials in a way that managed moisture – often by absorbing it and holding it, before allowing it to dissipate safely. Porous materials like stone and brick can generally store and transport both liquid water and moisture vapour.
There is still a lot to be understood about moisture in buildings
Toby has been carrying out moisture modelling work for over a decade. Any simulation has shortcomings, so he sees it as the responsibility of practitioners to understand as much as possible about the physics of moisture so they can then understand when models are most useful. Toby acknowledges that he still has lots to learn about the physical processes that calculation models are trying to simulate.
Dew points are not the best way to assess condensation within building structures
While we want to avoid condensation within the structures of our buildings, dew points are not always a great way to understand potential moisture risks. This is especially true in masonry materials and traditional constructions. A dew point is the temperature at which condensation occurs for air of a given humidity.
Simpler calculation methods that rely on dew points, like the Glaser method, don’t reflect the full range of real-world conditions. They don’t take account of rain on the outside of the building, or moisture transport and storage in liquid form. Their limited scope might be okay for some situations, but they are not sufficient for making sensible judgements about many structures, including historic buildings.
For example, you could have a structure that never suffers from condensation, but still gets a serious problem like mould. The calculation method might tell you about the condensation risk, but it isn’t sophisticated enough to calculate that humidity is high enough for mould to form.
WUFI software is a more complete way to simulate moisture movement
It’s not perfect, but it is a more complete approach to modelling moisture movement in buildings and modelling how water moves around within porous materials.
In the software, the user builds up the element to be modelled. They then select a climate file with data about sun, wind, temperature, humidity and rain. Then the internal conditions are inputted, some other parameters are set, and the simulation can be run. And the simulation predicts the temperature and moisture content at any point within the element’s structure. Using the temperature and humidity profiles that it outputs, together with some post-processing tools, it’s possible to assess whether problems like mould could occur.
Temperature and relative humidity data over time
There are still uncertainties in WUFI calculations
As with any software or calculation, nonsense in leads to nonsense out. The WUFI software has a database of several hundred materials. Some are modern products with known performance.
It also includes porous materials. Because these materials have unique combinations of liquid transport properties and vapour permeability, there can be enormous variation even within a single category of materials. For an existing building, some assessment and measurement of the material is needed to then make an informed selection from the database.
Other uncertainties include internal conditions. For example, you might not know if you should be accounting for a densely occupied house with poor ventilation. Toby explains that the advantage of modelling is that you can play around with some of these variations and explore the different issues to understand their effect.
An example of how moisture moves within a wall build-up that has wood fibre insulation
The simulation is quick to run, but interpreting the output is key
Toby says his team likes to spend a couple of days on a project, normally. This gives time to go to site, collect material specimens and analyse them. Once the data is available to enter into the software, the actual simulation can take anything from five minutes to a few hours to run, depending on complexity.
The real value is in the interpretation of the results, and providing a professional opinion and risk assessment. In complex situations, things are much more nuanced than a simple pass or fail. It can often mean going back to a design and altering it to reduce risk further. Toby’s work really comes into its own in more unusual situations with higher risk constructions, perhaps featuring an unusual combination of materials.
There are lots of good sources for help and advice
For simpler enquiries, Toby and the team at Greengauge will often direct people to a variety of reputable suppliers who they have built relationships with over time. Examples include Ecological Building Systems, Back to Earth, Natural Building Technologies (now part of SOPREMA), and Best of Lime.
For WUFI calculations, there are no specific qualifications to look for when engaging someone. There are a small number of training courses, but a lot of it comes down to experience: reading academic papers for background, and looking at buildings on site to understand the difference between simulation and reality.
The most common use of WUFI is with internal insulation
A sophisticated modelling tool like WUFI is most suited to applications that have a lot of variables. That’s particularly the case in retrofit, where we have no control over the existing construction and we need to be careful about what we do to the structure to avoid introducing moisture problems.
In new build construction, we tend to have a good idea of what works, especially if following prescriptive solutions. In that case, there’s no need for complex calculations. BS 5250, the code of practice for moisture in buildings, has guidance about when different calculation methods ought to be used.
An analysis of water content over time
A consistent layer of thin internal insulation can help to manage moisture
Using hygroscopic, capillary active and vapour open materials helps to maintain a wall’s ability to dry to the inside. Generally, the thicker the layer of insulation applied to the inside, the harder it is for that drying to happen. Most people are now using 40mm, 60mm, or possibly 80mm of wood fibre insulation, and that seems to work okay in most places. A continuous layer of 40mm of insulation, with thermal bridges resolved, is much more effective than a thicker layer of insulation that’s hit and miss.
Our podcast with Martin Twamley discusses this subject in more detail.
An internal wall insulation strategy from Steico
Using WUFI can stress test a construction
Good practice when doing a simulation is to include moisture loads or moisture stresses. This is something WUFI allows the user to do which simpler calculation methods can’t. It’s possible to allow for a certain amount of rain coming in through a crack, or a certain amount of warm air moving out through the airtightness layer.
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