Easier ways to gather data to predict mould growth VTT model adapted to use room rather than surface temperature

18 February 2024 by Jason Quinn

Modelling mould growth in domestic environments using relative humidity and temperature is a useful technical paper, as it seeks to modify the VTT model of mould growth on surfaces to make it easier to use in built homes. The research also suggested the existing VTT model underestimates the risk of mould in domestic settings. 

The authors surveyed a large number of English homes and then curve fit the data using the VTT model to use room temperature and room relative humidity (RH) data rather than surface temperature and humidity.This matters because it’s hard to measure surface values as it varies within a single building. Different surfaces are at different temperatures.

There are some valuable real world applications that flow from expanding the existing model of mould growth to allow for generalisation to air readings of RH and temperature in a domestic setting. For instance, mould risk could be monitored in real time using only simple sensors. That data could then be used to increase ventilation and/or heating or to add active dehumidification to the home.

The VTT model is so-called because it was developed at the VTT Technical Research Center of Finland. It takes as inputs surface temperature, surface relative humidity and material properties to calculate mould growth when conditions suit and includes mould dieback as conditions cause mould to decrease. This allows the use of dynamic hygrothermal models such as WUFI to predict the conditions at any point in a building construction and then use the VTT model to predict the mould growth likelihood at that location. Sustainable Engineering Ltd’s team uses this in our practice, via the ASHRAE 160 standard for hygrothermal modelling.

Survey responses on the presence of visible mould growth and mouldy odour in 300 homes located in Cornwall, in England’s south-west ,were compared to the VTT model results along with measured temperature and humidity. The results indicated that the VTT model under-predicted mould growth and odour. From the paper: “Mould indices were most successful at predicting occupant responses when the model parameters encouraged higher vulnerability to mould growth compared with the original VTT model. A lower critical RH level, above which mould grows, a higher sensitivity, and larger increases in the mould index all consistently increased performance.”

The key point in that technical jargon is that when the room has an interior RH of 50% or higher, the VTT model will start accumulating mould growth. Spikes above 50% RH are not an issue but spending months above 50% will (according to these results) correlate with mould. This is a lower limit than I’m used to using and is likely due to poor thermal bridge design in the construction of the homes that were surveyed.

Tamaryn Menneer, Markus Mueller, Richard A. Sharpe, Stuart Townley
Modelling mould growth in domestic environments using relative humidity and temperature
Building and Environment
Volume 208, 2022, 108583,ISSN 0360-1323,

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