Module 6 · Retrofit and Moisture Risk
6.4 Hygrothermal Modelling and Its Limits
9 min read
Hygrothermal simulation models heat and moisture transport through a construction over time, hour by hour, accounting for liquid transport, storage and real weather. It is genuinely better than a steady-state calculation, and it produces graphs that look authoritative in a report. Both of those things are true, and the second is why practitioners need to be able to interrogate a model rather than quote it.
Learning objectives
By the end of this lesson you will be able to:
- Explain what transient simulation adds over a steady-state calculation.
- Identify the input assumptions that most affect the result.
- Interrogate a modelling report rather than accepting its conclusion.
- State when modelling is worth commissioning and when it is not.
Key terms
- Steady-state (Glaser) method
- Monthly-average vapour diffusion calculation ignoring liquid transport, moisture storage and real weather. A screening tool, and treated as one.
- Transient simulation
- Hour-by-hour modelling of coupled heat and moisture transport, including liquid transport, storage and driving rain, over multiple years.
- Material data set
- The measured properties a model uses for each layer. Generic entries for a material class can differ substantially from the actual material on site.
- Boundary condition
- The assumed internal and external climate. Includes internal moisture load, which is frequently the input the result is most sensitive to.
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