August 13th, 2020
In the course of the energy-efficiency discussion, buildings in the private as well as the industrial sector have become a focus of attention. Since they are usually heated using fossil fuels, better thermal insulation automatically results in a reduction of CO2 emissions. Furthermore, fuel costs play an increasing role. This has resulted in political efforts to promote better building insulation (for German regulations, see e. g. [6,7]). One of the results is “energy passports”, which document the energy consumption of buildings.
Fig. 1 Infrared images of buildings such as this one are frequently found in the media. The false-color representation is reasonable, but its interpretation is demanding. Untrained users often reach completely invalid conclusions.
The introduction of legal limits for heat losses has also necessarily led to the development of methods for testing the thermal properties and thus the insulation of buildings [8-11]. Thermography has come into use in this field since the 1990’s for the following applications:
• Locating thermal bridges. This includes finding hidden structures such as half-timbering behind a stucco facade;
• Locating water intrusions and moisture;
• Identifying leakage from pipes, for example in floor heating systems;
• Quantitative determination of so-called heat-conductivity coefficients.
In order to be able to interpret thermographic images correctly, one must register a series of additional data before and during the measurements. A summary of the basic rules for structural thermography is given in Chapter 6 of reference . Among these are general factors such as the preparation of the parts of the building to be investigated, but also geometric data. The latter include the absorption of the radiation by the atmosphere, the radiative influence of neighboring objects via the so-called view factor, and also the geometric resolution of the images.
Owing to their large masses, buildings or parts of buildings often have rather long thermal time constants, up to several hours. Therefore, weather conditions before and during the thermo-graphic investigations are important. Measurements on a dry morning before sunrise are optimal for quasistatic conditions, providing that it was cloudy and there was no precipitation during the night or the day before. This guarantees that the sun has not warmed up particular parts of the building by direct solar radiation. Furthermore, it ensures that there was no noticeable radiative cooling into the clear night sky. However, such conditions are not often available, so that as a rule, compromises must be made.
Exterior thermal images are often taken only to give an additional overview. Occasionally, for example with rear ventilated facades, they are not at all useful. Quantitative thermographic structural analyses are usually made from within the building, since many thermal signatures become visible only there.
It makes sense to take visible-light photos from the same perspective as the infrared images, in order to visualize the mapping in the test report. This report should take into account all the relevant ordinances and norms. It is usual to mark points, lines or surface areas in making a quantitative analysis of the images. This is done either directly in the camera or in a later analysis of the images. When all the parameters are correctly adjusted, these marked regions show either minimal, maximal or average temperatures. In order to capture the thermal signatures of small structures quantitatively with high precision, the optical system of the IR camera must provide the necessary geometric resolution .
Examples of Thermal Bridges
In the residence shown in Figure 1, one can clearly discern a reddish-white spot at the upper right edge of the dormer window. This is the thermal signature of an energy-effective heat leak, as can be demonstrated by comparison to other, similarly-constructed dormer windows on the same house (not visible in the picture). The cause is a lack of insulation. Interior thermography showed in the first instance that the temperature did not drop below the dew point when the outside temperature was around freezing; however, with still lower outside temperatures, this could happen. In addition, this heat leak caused such high additional heating costs that its repair would pay for itself within a few years. The rectangular dark areas, by the way, are reflections of the cold night sky from windows and a solar heating installation on the roof.