Building Thermography

Detecting Moisture in Building Envelopes

One of the most important elements of buildings in terms of energy efficiency is their external "envelope" - the walls, windows, and roof. It is essential for these elements to have the best possible thermal insulation properties (lowest thermal conductivity and lowest emissivity). However, if the moisture content in the insulation materials increases for any reason, their thermal conductivity also increases, naturally leading to a deterioration in their insulation properties. Moreover, moisture entering building materials - especially in indoor spaces - can sometimes lead to harmful mold growth.

The causes of moisture can vary: external water ingress (rain, roof leaks), seepage (e.g. due to damage to the floor's waterproof layer), cracks in pipes, or leaks in seals, condensation, etc. Depending on the location of the moisture, the presence of moisture, its potential causes, and the environmental conditions under which it can be detected vary. A clear distinction must be made between detecting indoor and outdoor moisture: thermographic inspections need to be carried out at different times and in different ways.

Detecting Outdoor (Walls, Roofs) Moisture

If, for example, flat roofs or walls are soaked through, their increased thermal conductivity due to water content can be detected during winter weather (heating season). In other words, assuming a temperature difference of 15 to 20 K between the interior and exterior, it can be shown on thermal images that in the soaked areas, more heat escapes from the interior due to their reduced insulation properties. This is visible in warmer areas on external thermal images captured with a thermal camera, and colder areas on internal images - just like any other type of thermal bridge. Leaks resulting from the building structure (or its construction defects) can be distinguished on the affected surfaces based on their irregular geometry. It is important to note that external images can only be taken in rain- and snow-free, windless weather conditions, and absolutely free of sunlight (nighttime). For indoor images, ensuring the presence of the aforementioned temperature difference is sufficient, although this difference must be maintained for at least a week prior to the measurement. Conducting such measurements in summer conditions is much more challenging (almost impossible) because strong daytime sunlight significantly heats up the building's walls, meaning these surfaces are practically never in a steady thermal equilibrium even at night. This means that surface temperatures (and the internal heat gradient of building materials) are constantly changing, and careful consideration is needed to determine whether the visible temperature difference in the thermal image at the time of capture is caused by differences in sunlight intensity on the surface or the absorbance properties of the surface - or the increased heat capacity of wet surfaces. Since it can be assumed that strong sunlight dries out moisture in the surface layers of the material during the day, neither the altered heat capacity nor the thermal conductivity due to moisture in the building materials can be utilized for evaluation. Therefore, summer measurements of external envelope (external walls, roofs) moisture are highly uncertain and are clearly NOT recommended.

Indoor Moisture, Pipe Leaks

Moisture can be found not only on the exterior of the building but also inside - of course, for different reasons than on external walls or roofs. Most commonly, we encounter condensation, pipe leaks, or moisture seeping from the building's foundations. These can be detected with a thermal camera throughout the year (if we know how), albeit with significant physical limitations. Let's start with detecting condensation. The air inside a building always contains a certain amount of moisture in vapor form. The air's moisture retention capacity depends on its temperature. The dew point is the temperature at which the vapor in the air condenses and forms condensation on surfaces at that temperature. This can naturally occur inside walls as well, as on the one hand, the wall itself has air- and vapor-permeable properties, and on the other hand, the temperature gradient between the exterior and interior can reach the dew point temperature within the wall. Mold growth can occur with 80% indoor humidity and room temperatures of 18 to 25°C. Detecting condensation and seepage (e.g. from below) is based on the same physical fact that the moisture accumulated in materials continuously evaporates, cooling the surface through heat dissipation from evaporation - thus making the surface cooler than other parts of the surface (whether it is an indoor or /nighttime/ outdoor measurement). If condensation or seepage leads to complete saturation of the building material, then due to the created thermal bridge, lower temperatures can be sensed on the affected inner surfaces of the external walls, and higher temperatures on the external surfaces. The cause of condensation also deserves a separate paragraph: The air inside a building always contains a certain amount of moisture in vapor form. The air's moisture retention capacity depends on its temperature. The dew point is the temperature at which the vapor in the air condenses and forms condensation on surfaces at that temperature. This can naturally occur inside walls as well, as on the one hand, the wall itself has air- and vapor-permeable properties, and on the other hand, the temperature gradient between the exterior and interior can reach the dew point temperature within the wall. Possible causes of condensation damage:

• inadequate insulation (primarily noticeable at room corners or lintels, concrete sills);
• poor building operation (inadequate heating or ventilation).

Once we know at what temperature objects (walls) condense due to reaching the dew point at a given air temperature and humidity, based on the environmental parameters listed below, it can be determined through internal thermography where condensation and mold growth are expected. Moreover, depending on the structure of the wall, not only the risk of condensation (and mold growth) can be identified, but it is also possible to calculate how long it would take for the building material and insulation to become damp while maintaining the current use of the space. (Moisture penetration would naturally lead to almost complete loss of insulating properties, therefore the process must be stopped in any case.)

The essential parameters for evaluation:

• indoor and outdoor air temperature and relative humidity at the time of measurement
• ventilation and room usage at the time of measurement
• typical use of the examined space (living room, office, bedroom, warehouse, etc.)
• climatic data (e.g., lowest winter temperature, highest relative humidity)
• volume of the examined space, ventilation speed, moisture load (e.g., from human respiration)

Let's also consider the thermographic detectability of leaks in pipelines: this can be done in winter and summer, with the "simple" condition that if the temperature of the medium flowing in the pipeline (mostly water) is higher than its surroundings (heating or hot water pipes, underfloor heating, etc.), heat conduction occurs through the surrounding materials to the outer (observable) surface. Thus, during heating, the location of the pipeline becomes visible first using thermographic devices.

Meanwhile, the warm liquid escapes at the leak site, so it can spread along the pipeline between the layers of surrounding materials sideways and downwards. Heat conduction now starts from the warm liquid in all directions, including towards the visible surface. Finally, with thermographic devices, it can be determined whether the heat distribution on the surface differs from the "normal" heat distribution of the intact pipeline. However, it is always valid that leaks can only be detected with thermographic devices if a temperature difference exists or can be created at the leak site, which can be sensed on the observable surface through heat conduction.

Summary

Outdoor moisture measurements can only be carried out during the heating season (while adhering to the above conditions), indoor moisture and certain leaks can be detected year-round (but with different methods). Condensation leading to mold growth (by its occurrence) can naturally only be detected with thermographic devices in suitable weather conditions – mostly in winter. The best and most reliable measurement results can always be achieved in winter weather and with the building in a heated state. Of course, alongside the appropriate expertise, the necessary high-quality thermal camera must not be forgotten: for detecting moisture, the most sensitive thermal cameras are essential. Only cameras with a temperature resolution of 0.08 K or better are capable of providing accurate, reliable measurements; with weaker devices, the minimal heat differences caused by evaporation are hardly recognizable. (Checking the qualification and the presence of the appropriate camera is also very important when using service providers – there are many companies providing incompetent work with inadequate equipment, whose thermal images are mostly completely incomprehensible!)

Rahne Eric (PIM Ltd.) pim-kft.hu, termokamera.hu

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