If you are trying to get to grips with retrofitting a traditional house, you may have come across the term Hygrothermal Modelling. What is it ? and What do I need to know ?

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Hygrothermal modelling is a computer simulation that is used to assess different wall constructions and to predict whether there is a risk of condensation and mould growth. There are a number software packages available that enable architects to carry out the assessment. WUFI and Delphin are perhaps the most popular.

Condensation and mould and wall or ceiling surfaces are the result of excessive moisture within a property. When air in the building flows across a cold surface, it transfers its moisture to that surface, which is visible as water droplets. Surface condensation is a problem that can be seen fairly easily, what is more problematic is moisture that occurs, deeper within the wall fabric, out of sight. High levels of moisture cause timber to rot and stonemasonry to deteriorate. When insulation is added to a stone wall, the risk of interstitial condensation increases. The more insulation you add, the greater the risk.

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Carrying out an assessment to predict whether condensation will result from the amount and/or type of insulation that you are planning to use in a renovation is important.

There are a few challenges that need to be considered carefully;

• Computer models are not the same as real life. They can provide guidance, but they are not an accurate prediction of conditions with a real building.
• They rely on a number of assumptions about climate, the physical properties of materials, peoples lifestyles, quality of workmanship during construction, etc.
• They require expertise to understand the assessment process and the results of the analysis.
• They can also provide a false sense of security and there is a real danger that we believe the numbers produced, just because they come from a computer and that the results are more accurate and reliable than they area. This can lead designers to develop solutions that have minimal tolerances, to maximise insulation level.

There is an assumption amongst many designers that as long as the materials that are used are breathable, everything will be alright. Our regulators also struggle to understand how traditional masonry works and often insist that vapour barriers are installed to try and eliminate moisture movement.

The consensus amongst conservation experts is that it is important to retain some heat within the core of the masonry wall in order for it manage moisture effectively. Trying to improve the thermal performance too far will result in problems.

Treat any computer simulation with caution and make sure you understand what the number mean and what assumptions have been made. Keep as wide a margin for safety as you can and make sure you keep on top of your annual maintenance.

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MAAC studio are accredited conservation architects, PAS 2035 Retrofit Co-ordinators and Principal Designers working with traditional buildings throughout Scotland. 

If you need independent advice give us a call ?