Role played by infrared thermography

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Aims and Objectives

The aim of my dissertation project is to investigate the role played by infrared thermography in assessing the many defects within a building envelope. Consequently, this semester I have completed background research on infrared thermography and the areas associated with it (i.e. infrared camera, building defects that can be identified, electrical defects etc). From this research, I have produced a detailed literature review and have condensed it for the purposes of this report.

The literature review has provided me with basic knowledge and understanding of how infrared thermography works, therefore, next semester I will be able to undertake experimental work with confidence. Shortly, I plan to take accurate thermograms of a series of buildings with an infrared camera. After interpreting the data within the thermal images correctly, I will identify and locate any defects within the building's envelope. Then, a list of repairs will be produced for each building. Subsequently, the different buildings analysed will be compared and conclusions drawn. Also, for one of the buildings I plan to complete an air tightness test as well as an infrared thermography survey to calculate the air tightness of the structure and determine the location of any air leakage pathways.

Literature Review

1.Introduction

Infrared thermography is an invaluable non destructive technique which uses high performance infrared cameras to detect and measure the amount of infrared radiation emitted by an object based on its temperature, this information is then presented in the form of a thermal image (known as thermogram).

All objects with a temperature above -273°C emit infrared radiation, except objects that absorb all radiation (known as a blackbody) [1]. The amount of infrared radiation emitted by an object varies with its temperature, for example, as the temperature of an object increases so does the amount of infrared radiation emitted [5]. For this reason, thermograms show thermal anomalies of objects by distinguishing warmer areas from colder areas, this provides a use for infrared thermography in various industries throughout the world.

Infrared radiation is in the invisible part of the electromagnetic spectrum, therefore, Figure 1.1[9] illustrates where the infrared region is in comparison to the rest of the electromagnetic spectrum.

2.Theory

Recent advancements in technology have enabled modern infrared cameras to become more practical and easier to operate for the user. This is because they are built much smaller and lighter (more portable) compared to previous older infrared camera systems. Figure 2.1 shows a typical example of a recent infrared camera.

In addition, modern infrared cameras are now much more cost effective and affordable compared to older models and therefore, infrared thermography has increased in popularity as a diagnostic tool. One of the reasons that older infrared cameras were so expensive was the detector within the infrared camera was required to be cryogenically cooled by using liquid nitrogen [2]. Figure 2.2 [2] shows a cryogenically cooled camera from the early 1990's which cost around $60,000.

Typical prices of infrared cameras on the market today vary from around £4000 to £40,000 [2] depending on the accuracy and quality of the camera. Also, it must be remembered that a budget must be set aside for general maintenance and repairs of the device [2].

Key features of infrared cameras that affect the thermogram's quality and accuracy include the thermal sensitivity, spatial resolution and whether the camera is radiometric. The better these features are, the more precise and expensive the infrared camera will be.

The thermal sensitivity represents the smallest temperature difference that can be found at a given temperature. For example, modern infrared cameras can resolve temperature differences of approximately 0.1°C (100mK) at 30°C [2]. Also, the spatial resolution of an infrared camera refers to the number of pixels (per unit area) of the thermal images it produces [2]. In addition, radiometric infrared cameras are cameras that have been calibrated so that temperature can be deduced from the amount of infrared radiation detected [2].

Several factors can affect the information collected within thermograms which will lead to an incorrect interpretation of the results. To prevent this, an in depth understanding and experience of infrared thermography is needed. Subsequently, the right conclusions can be drawn from the thermal images and hence, relevant effects of parameters can be avoided and ignored. The parameters that affect infrared thermography include, the distance that the thermogram is taken away from the target object, atmospheric attenuation, the angle of vision, shadows cast onto the building, solar radiation, ambient air temperature, weather conditions such as wind and rain and also the emissivity and reflectivity of various materials [1] and [10].

Emissivity refers to how efficiently the surface of a material emits infrared radiation in comparison with a surface at the same temperature that is an ideal blackbody (perfect emitter) [1]. The values of emissivity range between zero (perfect reflector) and one (perfect emitter) and are affected by variations of temperature, wavelength and surface condition [6]. In general, materials that have high emissivity values are non-metallic such as brick, paint and stone etc [6].

Alternatively, materials that have low emissivity values of less than 0.5 are usually clean, polished shiny metals like aluminium and steel. Due to their electron structure these materials are unable to emit infrared radiation adequately and so the information within the thermal image will be inaccurate and difficult to see [1]. Furthermore, it would not be possible to determine the correct temperature of the material. A way of solving this problem associated with low emissivity materials is to attach a small sized high emissivity material (usually black water based paint, emissivity value of approximately 0.9) to the low emissivity material being investigated [1].

3.Applications of infrared thermography within the construction industry

There are two different techniques for completing infrared thermography successfully, these are known as passive thermography and active thermography. Passive thermography is simply a method that identifies thermal anomalies of materials and structures that naturally have different temperatures than the ambient air temperature [4]. On the other hand, active thermography represents the process of using an external stimulus to create suitable temperature differences between the materials being tested and the surroundings [4]. Both of these methods are applied within the construction industry, however, passive thermography is generally used, as good temperature differences usually exist.

Within the last 25 years, [6] the development of infrared thermography within the construction industry has enabled it to become a useful diagnostic tool, as the process is extremely quick, with virtually no disruption or noise to the occupants of the building [4]. Also, it is a non destructive process as no contact with the target object is required and the technique is completely safe, as no harmful substances or radiation is involved [5].

Consequently, infrared thermography within the construction industry can be used effectively to identify numerous types of subsurface defects around the building envelope. These defects are found by using the thermograms produced by infrared cameras to identify thermal anomalies on the surface of the target object. These thermal images are then interpreted accurately to distinguish whether the thermal anomaly is in fact a building defect that needs repairing or replacing [1]. Possible thermal anomalies will appear on the thermogram as temperature differences of 1 or 2 C, whilst temperatures differences of 4 C and higher represent very strong evidence of a defect existing [3].

Thermograms (thermal images) are usually taken during dry nights and preferably during the winter months, as there is a greater temperature difference between the structure and ambient air temperature, this increases the chances of identifying defects. Additionally, it must be considered that some defects may not be identified as they are too small. The general rule states that in order for a defect to be identified by an infrared camera, the radius of the defect must be at least one or two times larger than the depth it is from the surface [3].

Typically, infrared thermography is applied within the construction industry by specialist companies. When a specialist thermographic company completes an infrared survey of the building, additional information may be required such as construction drawings, moisture meters etc so the results of the thermal images can be interpreted correctly. Usually a specialist company will produce an energy assessment of the building which means the energy efficiency of the structure is found by calculating (using software) the amount of unnecessary heat loss, cost (in respect to the utility bills) and carbon emissions lost through each defect [13]. Once defects have been repaired another infrared thermography survey is used to assess the quality of workmanship and check that the defect has been completely fixed [1].

Relevant building defects that are identified by infrared thermography include poorly fitted or damaged cladding panels, missing and damaged insulation, trapped moisture within the building envelope (i.e. flat roofing), thermal bridging (materials that are poor insulators that allow a direct path for heat to flow from the inside to out), delaminating render, hidden structural details (blocked up windows and doors), internal leaks (burst pipes) and air leakage pathways [1].

Air Leakage Detection

Uncontrolled ventilation through breaks and cracks in the building fabric are said to contribute to a major part of the overall heat transfer of the building [8]. Consequently, infrared thermography is combined with air leakage testing to improve the air tightness of structures to meet Part L of the building regulations. The 2006 version of this legislation states that for all new dwellings as well as commercial and industrial buildings the air tightness standard has to be equal to or better than 10 m³/(h.m²) at 50 Pa [8].

The air tightness test usually consists of a blower door system which is a variable flow portable fan that is installed into a doorway within the building's structure [8]. All windows and doors are closed and other accepted air pathways such as fireplaces and vents are sealed so no air can enter the building. Also, the wind speed should be below 6 m/s and the pressure difference between the inside and outside of the structure less than ±5 Pa [8]. Once these environmental conditions have been checked, the fan is switched on, therefore, the amount of air required to pressurise the building to 50 Pascals is found (known as air permeability). If the air permeability value is found to be equal to or lower than 10m³/(h.m²), the building will pass the test and a certificate that states that the building has met the relevant building regulations will be given to the owner of the building [8].

However, if the structure has an air permeability value above 10m³/(h.m²) the building will fail the air tightness test. As a result, either a smoke test or infrared thermography survey is used to identify air leakage paths so they can then be repaired [8]. An air tightness retest is then used to check that building is at the required air tightness.

Infrared thermography surveys are preferred over smoke tests for identifying air leakage paths as it is a much more reliable and accurate technique. Usually an infrared thermography survey is conducted before the air tightness test so any air leakage paths can be identified and repaired before the air tightness test commences. This usually enables the building to pass the air tightness test first time meaning no retest is required which saves money and time for the owner of the building [8].

An infrared thermography survey is able to identify air leakage paths very well by using the blower door equipment to depressurise the building [8]. This will cause cold air to flow through any unsealed areas and air leakage paths into the building. Both internal and external thermograms should be taken of the building before and after the pressure changes as this helps to distinguish between air leakage paths and thermal bridges [7]. From this the locations of any air leakage paths can be identified and repaired accordingly. Examples of internal thermograms identifying air leakage paths can be seen in Figure 3.1 and Figure 3.2.

Electrical Defects

A crucial use of passive infrared thermography is for predictive and preventative maintenance of electrical systems. As infrared thermography is a non contact technique, an electrical system does not have to be switched off whilst the diagnostic method is being taken place, therefore, there is no costly downtime. The benefit of identifying electrical defects before they cause a problem is it provides much better health and safety for the personal working around the equipment, as there is less chance of fires and electrical shocks [1, 13].

Infrared thermography can be used to detect any electrical defects as the heat generated from them will appear as red and white areas on the thermograms. Once the defect has been identified, concentrated maintenance can be completed quickly, keeping the downtime of the electrical system and repairs to a minimum [1]. An example of an electrical defect can be clearly seen in Figure 3.3.

4.Conclusion

Overall, infrared thermography is a particularly useful diagnostic tool that is applied within many different industries throughout the world. Recent development of this technique has enabled it to become a much more viable, cost effective and practical within the construction industry. Consequently, infrared thermography is increasingly used efficiently and effectively to identify many building defects within structures.

5.References

  1. Balaras, C.A. and Argiriou, A.A., 2002. Infrared thermography for building diagnostics. Energy and Buildings, 34(2), pp. 171-183.
  2. Snell, J., 2005. Breakthroughs in infrared camera prices and performance. Snell Infrared. Available at: http://www.affordablecomfort.org/images/Events/20/Courses/490/Snell_DIAG5.pdf [Accessed 9 November 2009]
  3. Maldague, X., 2000. Applications of infrared thermography in non-destructive evaluation. Available at: http://w3.gel.ulaval.ca/~maldagx/r_1123.pdf [Accessed 9 November 2009]
  4. Maldague, X., 2002. Introduction to NDT by Active Infrared Thermography. Materials Evaluation, 6(0), pp. 1060-1073 Also available online at: http://w3.gel.ulaval.ca/~maldagx/r_1221t.pdf [Accessed 10 November 2009]
  5. Lo, T.Y. and Choi, K.T.W., 2004. Building defects diagnosis by infrared thermography. Structural Survey, 22(5), pp. 259-263.
  6. Avdelidis, N.P. and Moropoulou, A., 2003. Emissivity considerations in building thermography. Energy and Buildings, 35(7), pp. 663-667.
  7. Grinzato, E., Vavilov, V. and Kauppinen, T., 1998. Quantitative infrared thermography in buildings. Energy and Buildings, 29(1), pp. 1-9.
  8. BSRIA Airtightness, 2007. Airtightness Testing for New Dwellings. The essential guide to Part L1 of the 2006 Building Regulations. Available at: http://www.bsria.co.uk/documents/airtightness-dwellings.pdf [Accessed 10 November 2009].
  9. Shi, X. and Choudhuri, D., 200-. Seeing into the building envelope. Infrared Thermography Evaluates without Destruction. Moore Knowledge. Available at: http://www.walterpmoore.com/downloads/knowledge/mooreknowledge/InfraredThermography.pdf [Accessed 10 November 2009]
  10. Chew, M.Y.L., 1998. Assessing building facades using infra-red thermography. Structural Survey, 16(2), MCB University Press, pp. 81-86.
  11. Anon, 199?. Thermography. Reduces maintenance costs and enhances reliability. NASA. Available at: http://engineer.jpl.nasa.gov/practices/at9.pdf [Accessed 10 November 2009]
  12. Kominsky, J.R., Luckino, J.S. and Martin, T.F., 200- . Passive Infrared Thermography - A Qualitative Method for Detecting Moisture Anomalies in Building Envelopes. Available at: http://www.tedfordhenry.com/admin/uploaded_files/article_1181567898.pdf [Accessed 10 November 2009]
  13. Lucier, R., 2000?... . Infrared Applications in Building Diagnostics. Based on article that appeared in November 2000 edition of Cleaning & Restoration magazine. Available at: http://www.irinfo.org/Articles/article_9_1_2006_lucier.html [Accessed 11 November 2009]
  14. Stockton, G. R. and Tache, A., 2006. Advances in Applications for Aerial Infrared Thermography. Available at: http://www.stocktoninfrared.com/PUBLISHED/PDF/aerialIRadvances.pdf [Accessed 11 November 2009]
  • Reference for Figure 1: Shi, X. & Choudhuri, D., n.d. Seeing into the building envelope. Moore Knowledge
  • Reference: http://www.bis.fm/assets/images/productphotos/P60_l.JPG http://www.stocktoninfrared.com/PUBLISHED/HTML/Beyond%20the%20Usual%20Applications%20for%20Infrared%20Thermography_files/Figure%205.jpg
  • Reference (Figure 3.1) = http://www.irtsurveys.co.uk/case-studies/cladding/
  • Reference (Figure 3.2) = http://www.irtsurveys.co.uk/images/houses2.jpg
  • Reference (Figure 3.3 and 3.4) = http://www.erac.ie/air-tightness-thermography/
  • Reference (Figure 3.5) = http://www.irtsurveys.co.uk/case-studies/flat-roofing/
  • Reference (Figure 3.6) = http://www.thermosurveys.ie/services.htm#thermal Gantt Chart!!

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