Is thermal image possible

Thermography

The thermography of buildings is an important tool for assessing their energetic state with regard to heat losses. Various punctual defects can also be uncovered. The results of such measurements can be used for planning an energetic renovation, for checking its effectiveness or for the targeted repair of individual damage.

Examination of facades

A common application of thermography is to assess the quality of a building envelope from the outside. For this purpose, thermal images are taken when the outside temperature is low and the house is heated normally. The poorer the thermal quality of the building envelope, the higher the temperatures on the outside of the building envelope, while in the ideal case the surface temperatures of a facade are usually close to the air temperature (if the sky is clear, they may even be lower).

Defective or poorly constructed areas of a building envelope, where a particularly large amount of heat escapes, can be easily identified on thermal images (see Figure 6). This includes various types of thermal bridges, which can be both constructional and the result of defects (e.g. leaks due to incorrect installation of windows). In the case of buildings with external thermal insulation, for example in the form of a thermal insulation composite system, the consequences of a non-optimal design (e.g. defective insulation panels, joints or thermal bridges due to dowels) can be detected.

In uninsulated buildings, more serious defects are often uncovered, for example inadequately thermally insulated lines of the central heating system in outer walls or particularly warm spots in radiator niches. Effects of hot air escaping from the building can also be discovered in the form of heated areas above the leak. Under certain circumstances, such effects are not only important in terms of energy, but can also be related to the causes or consequences of structural damage.

For quantitatively meaningful investigations, of course, an exact determination of the surface temperatures is necessary. For this purpose, measurement errors for the temperatures must first be minimized. In particular, it is important to enter a correct value for the emissivity of the respective area. Since measuring the emissivity on site is usually difficult, you often have to rely on tabulated material data or on the fact that they really apply to the respective material. Fortunately, for many materials used in construction, the emissivity does not vary too much; in particular, in the case of paints and varnishes, for example, it hardly depends on the visible color. Another possibility can consist in calibrating the device using a surface temperature measured in some other way (for example with a contact thermometer) or at least checking it.

A large number of sources of error can either falsify the measurement results or lead to incorrect interpretations.

Disturbing external influences must also be avoided, as has already been explained above. In particular, several hours should have passed after the facade was irradiated by the sun; otherwise the effects of previous solar radiation may be misinterpreted as heat losses from the building. Ideally, a measurement should be taken in the early morning before sunrise, when the temperatures are fairly balanced. The building should have been heated normally beforehand, and the room temperature and of course the air temperature outside should be known. Disturbing factors such as B. Tilted windows should of course be avoided in the hours before the measurement. The weather is ideally evenly cold, with an overcast sky (cloudy or high fog), so that the radiation temperature of the background does not vary too much. Wind should not be significant as it affects surface temperatures. If the weather is too mild, the heat flows and temperature differences that occur are smaller and therefore more difficult to determine; Interferences are then more important. The surfaces to be examined should be dry, otherwise even small air movements cause additional cooling by evaporation. Of course, a professional measurement includes the documentation of the circumstances found in each case.

On the basis of a correctly carried out measurement, the heat transfer coefficient of the wall structure can be determined approximately. The amount of heat loss (when there is no wind) can be estimated from the difference between surface temperature and air temperature.

Be careful with ventilated components: thermal images are often hardly meaningful here!

The results of the observation of ventilated components are often hardly meaningful. For example, the tiles of pitched roofs are usually ventilated from the rear (principle of the cold roof) so that their surface temperature hardly deviates from the outside air temperature, even if massive heat losses occur through the circulating air. (Please also note that you cannot see escaping warm air with thermography - at most indirectly, if it heats solid objects.) This problem also occurs with some types of facade. In some cases, more reliable statements are then possible through indoor photos. In any case, knowledge of the building structure and its effects on thermographic measurements is essential in order to be able to interpret the results correctly.

The temperature of window panes is generally difficult to determine with thermography, since the emissivity is lower and reflection occurs. For example, by reflecting the cold night sky, it is possible to determine an apparently very low surface temperature of the window panes. This effect becomes particularly strong when viewed from a larger angle.

Do you need an independent specialist to check possibly false or even manipulated thermographic recordings, or also to check dubious claims and arguments? The author of this article is available as a consultant.

Basically, it should be noted that a large number of possible sources of error, as described above, can falsify the results of thermography. Deliberate manipulations are also possible, for example by entering incorrect values ​​for the emissivity, by measuring under unrealistic conditions (e.g. reduced heating of the rooms) or by incorrectly performed calibration procedures. The comparison of images based on different temperature scales or taken at different outside temperatures can also lead to misleading results. The same applies if surfaces with different emissivities are shown on the same picture: The measurement can then be correct for only one of the materials involved. For example, thermal images of walls are circulating on the Internet, some of which are painted with so-called energy-saving paints (with reduced emissivity) and are intended to prove their effectiveness through the resulting temperature differences, which, however, can be incorrectly measured precisely because of the reduced emissivity.

Of course, the sources of error and manipulation possibilities mentioned do not speak against thermography per se. However, this shows that reliable results can only be expected from a competent and trustworthy person.

Investigations indoors

The detection of structural damage can also be the aim of thermography.

It can also be useful to do indoor thermography. For example, you can localize cold spots where a lot of heat is lost and the risk of moisture and mold damage is high. You can also try to assess which causes of heat loss are particularly serious in order to be able to address them in a targeted manner. If walls are soaked, for example as a result of defective water pipes, you may find correspondingly cooler areas under certain circumstances.

Particularly warm spots can also be of interest. For example, you can trace the course of poorly insulated pipes in the central heating system and the hot water supply. In the case of exposed, insulated lines, for example in the basement, poorly executed areas (e.g. at branches) can be found. You can also try to assess the significance of defects that are already known.

In other cases, overlooked electricity consumers are discovered based on their heat development. Faulty cable connections with increased contact resistance, which cause a fire hazard in the long term, are also expressed in locally excessive temperatures.

presentation of results

Obviously, even the best measurements are useless if their results are not presented appropriately. That is why the thermography service includes the preparation of a corresponding report, the quality of which is of considerable importance for the quality of the overall performance.

If the circumstances of the measurements are not known, the results are of limited value.

First, the circumstances need to be clearly described. This includes, for example, the date and time of the measurements, information on the examined object, the weather conditions (air temperature, humidity, cloud cover, wind) and the equipment used.

The measurement results are described in detail below. This includes not only printing the recorded images (together with the color scales used, which should be appropriate to the temperature range), but also explaining the respective circumstances and interpreting the results. For example, any deficiencies found should be pointed out and an attempt should be made to classify their significance appropriately. Of course, it can be very useful to discuss possible causes and to point out ways to remedy the deficiencies.

The client does not benefit directly from measured temperature curves, but only from correct and helpful interpretations.

It should be noted that temperature curves in no way represent the desired result, but rather the starting point for an interpretation that correctly determines the causes, evaluates circumstances appropriately and leads to meaningful recommendations. For example, in some places increased temperatures on a facade can have very different causes, namely increased heat flows from the inside or also warming influences from the outside, as long as the temperature anomaly is not even caused by a measurement error such as scattered inhomogeneous ambient radiation.

It is important to point out possible sources of error and their estimated effects and, if necessary, to name measures taken against them. If this is missing in the report, another specialist can hardly judge later whether possible disruptive influences have been sufficiently reduced. Ideally, the results obtained are not only correct and complete, but also as far as possible understandable and verifiable.

Costs and benefits

The above explanations make it clear that meaningful thermographic examinations require considerable effort:

Why are thermography services usually so expensive?
  • First of all, this requires quite expensive equipment.
  • The time required for the actual measurement can be a few hours or significantly more, depending on the complexity of the task.
  • A professional evaluation of the results and the writing of a high-quality report takes additional time, usually several hours.

All of this work must be carried out by a specialist who has worked thoroughly in the correct operation of the thermographic camera and the evaluation software, as well as, of course, in the basics of thermography. Unfortunately, it is often difficult for the client to check the contractor's expertise, especially in advance.

Even if the costs are high, the benefits of thermography can be many times greater.

In view of the effort and requirements, it is not surprising that the cost of thermography, for example, of a building from the outside, can easily amount to a few hundred euros. After all, the hassle does not come with taking some conventional photographs such as B. to be compared when a property is taken up by a real estate agent. On the other hand, the results of thermography can also be very useful. In particular, they can be of great help when planning an energetic renovation, in order to address the respective problem areas in a targeted, i.e. effective and efficient manner. That is why it is easily possible that the information obtained through thermography can help save a multiple of the costs incurred. In thermography after construction work has been carried out, it is also very important that defects are discovered in good time for a complaint. Otherwise, they could remain undetected for the time being and, in the worst case, only cause expensive structural damage later - after the complaint period has expired.

Natural thermography

Even without technical instruments such as thermal imaging cameras, temperature effects can sometimes be observed indirectly due to natural phenomena. For example, an inadequately thermally insulated roof can, under certain circumstances, be noticed by the fact that the snow on it, at least in places, melts significantly earlier than elsewhere. Such observations are sometimes called Natural thermography designated. Of course, this can hardly replace real thermography, as it only provides information under very specific circumstances, and only rough clues and no solid measurement data.

Incidentally, an outer roof surface that remains cold - by whatever means it is determined - by no means necessarily shows that there is hardly any heat loss here. For example, heat can be lost through escaping air, which cannot be recognized in this way.

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See also: temperature, thermal radiation, energetic renovation of buildings, thermal insulation of roofs
as well as other articles in the categories basic terms, building services, heating and cooling