Building Surveying: Understanding Thermography

The ability to view the heat that is being emitted from an object can have a number of benefits when surveying a property. Being able to quickly visualise differential surface temperatures can assist in checking the condition of buildings by seeking out hidden weak spots and defects across the building fabric in a non-destructive way.

In this blog, I will be looking at the basics of Thermography and the use of thermal cameras for diagnosing building defects. This blog should give the reader a good start in the topic of thermography and provide a bunch of tips to get started.

This blog is aimed at the maintenance professional who is interested in picking up a thermal camera and wants to know whether it will be a benefit to them in diagnosing defects.

Background

Sir John Herschel created a thermograph using carbon and alcohol in 1840 after his father discovered the infrared portion of the electromagnetic spectrum. Over the following decades, the knowledge and understanding of thermal radiation was enhanced by the likes of Josef Stefan, Ludwig Bolzmann and James Clerk Maxwell amongst other prominent physicists. These physicists paved the way for the commercially available products which we see today which are used in a variety of industries.

The photograph produced following the conversion of infrared radiation to visable light viewable on a display is called a thermogram. Infrared thermography produces thermograms by detecting radiation in the long-infrared range of the electromagnetic spectrum. The electromagnetic spectrum is the range of frequencies of electromagnetic radiation and their respective wavelengths and photon energies. The spectrum goes from Gamma ray – X-ray – Ultraviolet – Visable – Infrared – Microwave – Radio.

For IR imaging the spectral ranges typically used are as follows; 0.9 – 1.7 μm (micrometre) for shortwave (SW), 3 – 5 μm for midwave (MW) and 7 – 14 μm for longwave (LW).

electromagnetic spectrum main

As infrared radiation is generally invisible to the human eye, thermographic cameras are able to open up a whole new view. The higher the temperature, the increased amount of radiation emitted and thus a thermal image can be produced by looking at the variations. All objects above absolute zero emit IR. Thermography is therefore defined as a technique for visualising an object or material using the thermal energy which discharges from its surface.

This surface temperature viewed by the camera is referred to as the apparent temperature which is the temperature that is apparent to the camera dependant on the conditions at the time.

Basics of Emissivity and Thermal Reflections

Emissivity is the efficiency that an object can radiate thermal energy and is shown as a value between 0 -1. Shiny metals like silver, copper, and aluminium, for example, have a very low emissivity and do not radiate their heat very well whereas concrete, paint, and brickwork have a much higher emissivity.

So the shiny metal will show on the thermal image as being cool when in fact it might be a lot warmer. So if you are carrying out a quantitative survey that requires accurate temperatures you need to know the emissivity of the surface and any thermal reflection that might be affecting the surface.

Thermal reflection can also present an issue. If you are looking at a reflective surface, the temperature you will see is that of the reflection, not the actual surface temperature. So if you standing exactly perpendicular from a reflective surface you may pick your own body temperature. You also don’t want to aim the camera at an angle greater than 45-50 ° as this can affect the results. So viewing at the right angle is key as well as getting as close as possible to the object under observation whilst maintaining a safe distance, especially during inspections of electrical installations.

for opaque objects, the emissivity and reflectivity go hand in hand. objects high in reflectivity are low in emissivity and vise versa. The best way to explain the above concepts is with the below picture.

emmisivity

Type of checks

There are a number of ways to conduct and analyse the results from a thermographic survey, these include:

  • Measurement method

    • Quantitive measurement is the process of taking a detailed look at the stats and assessing what they mean taking into account any variables and site conditions to accurately assess the data. You may also see the term semi-quantitive bounded about which deals with an approximation of the results which falls short of a quantitive result.

    • Qualative measurement is the evaluation of the thermal colour paterns to check for anomalies. The process of quickly looking at the variations in temperature which works well in a variety of situations.

  • Inspection technique

    • Compararative thermography. Comparing the results of similar components under similar conditions.

    • Baseline thermography. Comparing thermal images against a reference point or baseline. This can be used on equipment, the baseline image will be when the equipment is known to be in good working order in normal operating conditions.

    • Thermal trending. Repeat thermography over time which can detect the declining performance of building materials or equipment.

  • Analysis Scheme

    • Passive analysis is the process of observing the building materials without manipulating the thermal environment.

    • Active analysis is the process of controlling the thermal environment whilst observing the effect on the building fabric.

  • Application method

    • Aerial
    • Pass-by
    • Walk-past
    • Walk-through
    • Time-lapse
  • Location

    • Internal
    • External
    • Both

The thermographic survey type which I think is most pertinent to the maintenance surveyor is the qualitative passive internal walk-through survey. It is clearly demonstrated in a 2016 research paper from the University of Plymouth that internal walk-through thermography is much more successful at detecting a broader range of thermally significant building defects than external surveys.

thermal images

Practical application

Thermography has a number of potential applications such as:

  • Diagnosing damp

  • Detecting and tracking subsurface pipework

  • Assessing heat loss

  • Detecting defects within the insulation and identifying properties with unfilled cavities

  • Electrical safety checks

Infrared cameras can be used as a tool for diagnosing building defects but should not be used in isolation but as part of a suite of tools using a holistic approach. When evaluating building defects in building elements, thermography should be combined with other devices such as moisture meters to achieve reliable results and to avoid misinterpretations. 

Things that can have a direct influence on the results you are seeing include:

  • Environmental thermal conditions

  • Thermal influences like shading, reflections, surface finishes etc

  • Emissivity (how well a material emits thermal radiation)

  • Other factors like air temperature, distance and camera settings.

This is why it is best to use the results obtained as an indicative measure only to be used in conjunction with other tools and methods to diagnose defects. Also when carrying out a damp inspection, just because an area is showing up as cold does not mean it is necessarily wet. This is why using the thermal camera might be the first step in a holistic survey, not the only step.

Thermography can be used throughout the life cycle of a building from assisting design, assessing workmanship to evaluating failures. From new builds right through to building refurbishments and restoration projects there is a potential use case for thermal cameras.

Below is an interesting example of cold spots on a ceiling where there was missing insulation leading to condensation forming on the ceiling. The thermal image, in this case, proved very useful in visualising this defect and further investigation of the loft space confirmed the missing area of insulation.

thermal internal survey

Tips for Conducting Thermal Surveys

As mentioned before, qualitative surveys are just the process of looking at thermal patterns on a surface, so not caring too much about the exact temperature but more so the patterns. This is the type of survey that I use my thermal camera for when looking at issues like dampness.

Detection on heating pipes should preferably be carried out just as the system is being switched on or heating up. Some floor construction types will not allow for this type of survey to be completed which is worth keeping in mind.

For me, a qualitative damp survey does not require so much rigmarole and is pretty much a case of turning up and getting the camera to use. Internal damp areas are usually quite easy to spot. Wet insulation on a flat roof should also be visible as it cools down at a slower rate to dry insulation so an inspection at the end of the day might show up some interesting results.

Thermal surveys can be very useful for assessing electrical installations and are sometimes carried out as part of an EICR contract.  This is where a knowledge of emissivity and thermal reflection will become invaluable as you are more likely to encounter shiny metals.

For internal surveys to assess wall heat loss, a more active analysis scheme could deliver better results.  Some of the things that you might need to consider will include:

  • Heating the property consistently for 12 hours in advance to at least 20 °C

  • Cut out air currents from windows, vents, air ducts, and fans

  • Ensuring furniture, coverings and curtains are moved away from walls or taken down 12 hours before measurement

  • Turning off the heating 1 hour before measurement

  • Noting down areas of surface moisture as condensation will have an effect on surface temperature

  • Avoid surveying buildings where most of the internal surfaces are obscured, for example by raised floors and false ceilings.

overheated switch

Which Device to Use?

There is a bunch of good infrared camera’s out there which vary massively in price depending on the spec from hundreds of pounds to over £40k. The higher the thermal resolution, the more expensive the camera is going to be. The thermal resolution of your camera of choice will impact how accurate the image is going to be. There are four main areas of thermal resolution which are:

Spectral resolution– the particular wavelength within the electromagnetic spectrum being tested. The FLIR One Pro has a spectral range of 8 – 14μm.

Radiometric resolution (thermal sensitivity) refers to the smallest temperature differential, which can be perceived by the camera’s pixels. The FLIR One Pro has a thermal sensitivity of 150 degrees millikelvin (mK).

Temporal resolution– this relates to the image refresh frequency of the camera. The FLIR One Pro has a frame rate of 8.7Hz, some recommend a higher frame rate of around 25-30Hz.

Spatial resolution refers to the smallest discernable target that the detector can measure. The pixel resolution will have a big impact on this, The FLIR One Pro has a pixel size of 12 μm and a pixel resolution of 160×120, however, some recommend a minimum of 640×480 for external surveys. The field of view is another factor and refers to the total area which can be seen by the camera.

The below image gives an idea of the difference between resolutions. Common resolutions for building diagnostics are (464 × 348), (384 x 288), (320 x 240), (240 × 180), (160 × 120) and (80 x 60). 

Thermal resolution

I have used a FLIR E40bx and the FLIR One Pro for internal surveys, you can check out the full review of here. For me, the FLIR One Pro wins out of these two devices, the reasons for which you can catch in the review. The FLIR One Pro is fast to set up and really easy to transport in its compact carry case.

The photographs you take can be edited and analysed on-site or when you get back to the office. With the FLIR companion app, you can switch between the standard and thermal images, alter the palettes, add temperature reference points and change the IR scale. see the below video for the app overview:

For professional external surveys you might want to consider a device with a higher thermal resolution, these higher resolution cameras can be much more expensive so this is something that you will want to research to make sure you are getting the right product to meet your needs.

Do you use thermography to survey building defects? Feel free to reach out to me if there is anything I have missed or if you need any clarification on points mentioned. Due to the length of this blog, I have taken out the section on external surveys which I will publish as a separate blog.

Further Reading

Research papers:

  • Building defect detection: External versus internal thermography
  • Thermography Methodologies for Detecting Energy Related Building Defects
  • Time-lapse Thermography for Building Defect Detection
  • Infrared thermography for building diagnostics

Books:

  • Infrared Thermal Imaging; Fundamentals, Research and Applications
  • Infrared Thermography for Building Moisture Inspection

Other

  • FLIR, Thermal Imaging Guidebook for Building and Renewable Energy Applications
  • BSRIA, Thermal Imaging Powerpoint
  • Energy Saving Trust, The use of thermal imaging to support area-based energy efficiency schemes

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