Module 4 · Evidence-Based Diagnosis
4.6 Data Logging and the Moisture Excess
9 min read
In any contested condensation case the argument is the same: the landlord says the occupants generate too much moisture and ventilate too little, the occupants say the building is cold and cannot be heated. Both can be partly right, and no amount of visual inspection settles it. A month of logged data, reduced properly, does — and the reduction is a single derived quantity most surveyors never calculate.
Learning objectives
By the end of this lesson you will be able to:
- Deploy loggers so the data can answer the question.
- Derive the moisture excess and interpret it.
- Read a trace for generation, ventilation and heating patterns.
- Present logged data in a way a non-technical reader can act on.
Key terms
- Moisture excess
- Internal mixing ratio minus external, in g/kg. How much more water the indoor air holds than the outdoor air — the direct measure of net indoor generation against ventilation.
- External reference logger
- A logger recording outdoor temperature and humidity at the property. Without it the moisture excess cannot be calculated at all.
- Ventilation rate
- How fast indoor air is replaced. Inferred from how quickly the mixing ratio falls back after a generation event.
- Diurnal pattern
- The daily cycle of temperature and moisture. Its shape reveals occupancy, heating schedule and whether the space clears overnight.
Read the rest of Evidence-Based Diagnosis
Module 1 is open to everyone. The remaining six modules — building science, each damp mechanism in depth, meters and thermal imaging, the full inspection method, and report writing — are free too. We just ask for an email so you can pick up where you left off, and so we can send your certificate when you finish.