Manufacturing Trend 2011/07-08, Technical Diagnostics Section

"A universal measurement method"

Thermographic methods provide the opportunity to examine heat bridges, insulation deficiencies, hidden building structural and building services elements, condensation, capillary moisture, leaks, and seal failures in more detail.

If the thermographic measurement conditions detailed earlier were followed "(Manufacturing Trend 2011/5.)", all that remains is to understand how to evaluate and analyze the measurement results obtained in this way (graphically represented, i.e., as thermal images). Therefore, in the following, we discuss the appearance and detectability of the most common building structural and building services faults. Heat Bridges, Insulation Deficiencies Heat bridges are relatively easy to recognize: where the highest temperature is observed on an outdoor image – usually well defined – (and there is no local external heat source or reflection), a heat bridge (or crack) is present. In indoor images, the coldest spots usually indicate heat bridges. It is equally easy to determine (with the same external or internal coating) which building element has better or worse insulation properties. Let's see what can be examined:

• renovation needs in old buildings (e.g., condition of windows, wall insulation, joints and connections)
• implementation of external insulation in new and renovated buildings, as well as at connections of subsequent building parts, balconies, etc.
• occurrence of structural heat bridges due to bridging elements (e.g., steel beams)
• condition of joints (especially interesting in prefabricated panel buildings)
• gaps in external walls from building services installations.

Hidden building structural and building services elements

These measurements must be carried out utilizing various weather- and time-dependent heat processes. The "trick" could be measuring after daytime heating (in the absence of subsequent sunlight) (based on heat capacity differences), or based on heat flow due to nocturnal or winter cooling. In all cases, the heat capacity and thermal conductivity differences between the materials under examination (sought) and their surroundings must be exploited according to the desired effect.

For example, with proper heat flow, steel and wooden bridging elements in walls become visible using thermographic devices. (Due to steel's high thermal conductivity and wood's low conductivity and heat capacity, the bridging elements become visible between concrete or brick.) Based on the same principle, building materials with different properties (alterations, extensions, additions, infills) can be made visible, or even wall thicknesses can be determined (e.g., for assessing chimney wall construction).

Discovery of hidden building structural and building services elements. Left: mapping of wooden beams under plaster based on differences in wood and masonry thermal conductivity, right: assessment of chimney masonry wear based on wall insulation thickness dependence

It is also possible to use thermographic tools to locate heating pipes and hot water lines. These inspections must be carried out during the heating phase, before the surface reaches a homogeneous temperature distribution. With this method, the laying of heating systems (positioning), pipe density (e.g., for underfloor heating, wall heating), as well as pipe length and tightness, and the air-tightness of heating elements and pipes can be non-destructively checked.

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 moisture in the air condenses as liquid on surfaces at this temperature. This can naturally happen inside the wall, as the wall itself has air- and moisture-permeable properties, and the temperature gradient between the exterior and interior can reach the temperature corresponding to the dew point. Possible causes of condensation damage: * inadequate insulation (primarily noticeable at room corners or bridging elements, concrete sills) * poor building operation (inadequate heating or ventilation).

Once we know at what temperature objects (walls) experience condensation due to reaching the dew point under given air temperature and humidity, based on the listed environmental parameters, it can be determined from internal thermal images where condensation, mold growth is expected. Moreover, depending on the structure of the wall, not only the risk of condensation (and mold growth) can be identified, but it can also be calculated how long it would take for the building material or insulation to become damp – while maintaining the current use of the room. (Moistening would lead to almost complete loss of insulating properties, so the process must be stopped at all costs.)

The parameters essential for evaluation:

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

Capillary moisture and dampness

During thermographic inspection, the temperature decrease caused by the heat dissipation due to evaporating moisture (the required amount of evaporation heat) can be detected. Such inspection naturally requires a thermal camera with particularly good thermal resolution. The following phenomena can be found using this method:

• moisture rising or infiltrating from the ground (capillary moisture)
• dampness due to improper sealing of roof connections or gutter pipes
• dampness caused by improper sealing of sewage pipes.

Leaks, imperfections

Thermographic leak detection is based on the physical laws of heat conduction: if the temperature of the medium (mostly water) flowing in the pipe system is higher than its surroundings (heating or hot water pipes, underfloor heating, etc.), heat conduction occurs through the surrounding materials to the external (observable) surface. Thus, besides the location of the pipeline, the temperature rise caused by the exiting liquid in the surrounding material becomes visible using thermographic devices. It is always valid that imperfections can only be detected with thermographic devices if a temperature difference is created at the leakage site, which can be sensed on the observable surface through heat conduction. To detect small leaks, the amount of exiting medium must be increased using pressure boosting devices (in addition to applying maximum temperature). Leaks in cold water pipes can only be found if hot water can be connected to them.

Since the exiting medium naturally flows within the surrounding material and accumulates in any existing cavities, the highest temperature rise occurs where the medium can remain in larger quantities and transfer the amount of heat corresponding to its heat capacity to the surrounding material. Therefore, the largest heat effect is not always visible where the leakage occurs, but where the exiting medium accumulates.

Furthermore, finding leaks is complicated if the surface visible with the thermal camera is shiny (reflective, polished, glazed) because the surface's heat radiation-reflecting property (low emissivity factor) makes it difficult to detect small temperature differences. A completely different problem arises when leaking pipes are hidden behind multilayer cover structures (e.g., behind insulation). In such cases, the presence of insulation makes it impossible to identify the location of the leak unless the exiting medium flows through the insulation towards the surface to be measured.

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

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