August 13th, 2020
Thermography is by now a well-established method in monument preservation for making structures hidden behind walls visible. In Figure 2, the example of a half-timbered house is shown; the faqade has been covered with stucco. The wooden beams and the filler material in the partitions between are clearly visible in the IR image, owing to their differing heat capacities and heat-transfer characteristics.
Thermography is also suited to the detection of other hidden building structures. Among these are the location of bricked-up and plastered-over windows, or finding piping in floor heating systems (Figure 3). Images of this type allow a view through the floor screed and floor coverings and can therefore serve to locate leaks.
View Factor and Thermal Time Constants
Figure 4 shows two neighboring houses, of which an outdoor thermal image was taken around midnight under a clear sky. The house in the background shows distinctly different wall temperatures from different areas, for example comparing the areas AR01 and AR03 in the image.
The cause in this case is not different insulation of the wall areas. A correct interpretation can be obtained only by taking into account the so-called view factor , which describes the radiation exchange between the walls and neighboring objects, the ground, and the night sky. The wall, which faces west, is not screened by any nearby objects, so that its radiation exchange with the cold sky is particularly strong. In technical terms, the night sky has a large view factor. As a result, the outer surface of this wall area (AR01) cools faster than the other wall area AR03, where the view factor of the sky is reduced by the neighboring house.
House walls with different structures also cool at different rates. These different thermal time constants are seen in Figure 4 in comparing the areas AR01 and AR05. The house in the foreground, due to the different composition of its walls, shows a notably higher wall temperature. The insulation of both walls is nevertheless comparable and sufficient; this however can be recognized only by analyzing
indoor thermal images. The house in front has a rear ventilated facade. Due to the high heat capacity of the stonework facade, it has a much longer thermal time constant than the thin stucco layer mounted directly on the insulation of the other house. The image thus shows the remaining heat from solar warming during the previous day.
As we have seen, a correct, detailed interpretation of thermographic images is a complex problem, since a number of different factors influence the results of a measurement. Therefore, the very low-priced thermal analyses being offered today, for example thermography of a house for only 100 €, must be regarded with some skepticism. Analysis” is here often equivalent to simply taking IR images. The subsequent attempts at interpretation of the resulting colorful images without professional experience must then necessarily lead to incorrect conclusions.
Thermography of buildings using infrared cameras is becoming increasingly popular, as good thermal insulation of houses is growing in importance. The resulting garish false-color images contain useful information, but can lead to serious misinterpretation when examined by non-experts. A correct interpretation requires among other things information about the site, shadowing by nearby objects, the weather, and heating of surfaces exposed to the sunlight during previous days. As a rule, interior images are more meaningful. Along with evaluation of thermal insulation, thermography is also useful for visualizing hidden structures, such as covered half-timbering or heating pipes.