Mold in the Building: How to Measure and Analyse

Clock icon Created: September 3, 2020

Salimus was contacted to investigate poor energy performance, high humidity levels and mold formation in the building. We performed walk though, talked to maintenance personnel, conducted air tightness test using TEC  fans ( performed investigation with Microfogger 2 smoke pencil to find leakages, measured pressure differences between spaces, did recommissioning of the building HVAC system and issued the final report. The client is happy with the result, the building envelope is properly sealed, and ventilation rates are adjusted. But what about mold? Surely, it should be a scientific way to determine the “favorable conditions of mold growth” and remediate the issue.

Connection between Water Activity and Microbial Growth

There is the connection between building dampness, mold growth, and health concerns. But there is no definition of what “damp” in the pure scientific way. The damp obviously means wet and can be somehow measured by the amount of moisture in the wall material. What researchers have found back in the fifties is that the mold growth is controlled by water activity aw and not moisture content.

Back in 1953 Australian-born microbiologist William James Scott pioneered studies on the water relations of microorganisms in food and has shown that microorganisms have a limiting aw level for growth. It is now generally accepted that aw is more closely related to the microbial, chemical, and physical properties of foods and other natural products than is total moisture content.

Scott related the relative vapor pressure of food to the thermodynamic activity of water, using the definition aw = p/po, where aw is the water activity derived from the laws of equilibrium

thermodynamics, p is the vapor pressure of the sample, and po is the vapor pressure of pure water at the same temperature and external pressure.

Water activity is a measure of the energy status of water. Microorganisms cannot utilize low energy water for their growth, regardless of how much water is present. The practical lower limit for molds growth is a water activity around 0.70 – 0.80 and toxin and spore production stops at even higher water activities.

Measuring Mold in Buildings

Therefore, a water activity measurement is the most appropriate test to determine if a building is damp enough to support the growth of mold. Water activity is easily measured on laboratory samples using advanced bench top instruments, but in situ measurements can be more difficult because vapor equilibrium between the gas and liquid phases is necessary to determine water activity.

Except for existing moisture, thermal bridges also play an important role to mold formation. Thermal bridges exist where there is either a discontinuity in the insulation or a bridging element with a higher thermal conductivity. The heat transfer in these areas is significantly higher compared to the surrounding area, resulting in a lower surface temperature. Condensation of the moisture contained in the air occurs on those surfaces which temperature is below the dew point of the air, creating dump spots where mold can grow.

Mold can be dangerous for the health and cause respiratory illnesses. That is why humidity, dew point and thermal bridges/insulation assessment is critical for all buildings. Warm temperatures, ideally between 21-32 degrees °C, and indoor relative humidity above 80%   are a major stimulant for mold growth.

Calculating the water activity on the wall:

aw-value =(𝑃𝑠𝑎𝑡(𝑇𝑖)∗𝑅𝐻[%]) / (𝑃𝑠𝑎𝑡(𝑇𝑠𝑖) ∗100)


Psat(Ti)= Saturation pressure at ambient air temperature [hPa]

Psat(Tsi)= Saturation pressure at surface temperature [hPa]

Tsi= room-side surface temperature

Ti= inside air temperature

The saturation pressure Psat of water can be calculated with the Magnus-formula:

𝑃𝑠𝑎𝑡(𝑇)[ℎ𝑃𝑎]=6,112ℎ𝑃𝑎∗𝑒𝑥𝑝(17,62∗𝑇 / (243,12 °𝐶+𝑇) ), for −45°C≤𝑇≤60°𝐶 ,

gO Measurement-System

Salimus is using GreenTEG’s new gO Measurement-System to measure aw-value.

gO Measurement System for cloud-based U-value, R-value, temperature, and humidity measurements

The system can track both wetting and drying processes and follows water infiltration into building materials as well as the dry down progress during remediation. In addition, because the system is tracking water activity, it will not only be able to show water movement, but it will also indicate whether mold growth is possible when water infiltration has occurred. The system utilizes easy to install water activity sensors and a data logging system that can be set up anywhere.

The gO Measurement-System is a wireless system. It consists of a base station which receives the measurement data from up to 16 measurement nodes via LoRaSC2. This sends them over the mobile network into the cloud (Microsoft Azure Hosting). From there, all data can be conveniently monitored and evaluated. In particular, the multi-channel option offers the advantage that several U-values, humidity and temperature measurements can be done in parallel with one system. This significantly simplifies the performance of several simultaneous measurements.

Overview main functions and features

  • Real time data monitoring
  • Real-time status indicator of the measuring nodes and the base station
  • Personal login account
  • Combination of the sensors for U-value, R-value and AW-value assessment
  • Status indicator for U-value measurement regarding the ISO compliance
  • After the measurement, all data is presented in a structured report
  • csv-measurement data can be downloaded
  • Simple data management and cost control
  • Highest data security
  • Works with any internet-ready device
  • Continuous updates and maintenance

In situ measurements

For the data acquisition, an optimum measurement period is chosen according to ISO Norm 9869 to record accurate results for the U-value, the minimum test duration is 72 hours (3 days) if the temperature is stable.

left: thermographic imaging measurements, center: gOMS node type 1 on the inside, right: gOMS node type 2 on the outside
The graphs show the gOMS measurement results for temperatures, U-value evolution, relative humidity and aw-value in the bedroom right corner (wall

Analyzing the data Salimus can tell why there is mold in the building and whether it be attributed to structural defect or insufficient ventilation or something else.

Salimus will provide comprehensive mold remediation strategy from engineering point of view: what should be done and to what extend (how much to increase U value of the subject wall, how to balance the ventilation system…)

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