BUILT ENVIRONMENT JOURNAL

Conducting a successful thermographic survey

There are a number of common issues you need to be aware of when carrying out a thermographic building inspection if you don't want to miss critical anomalies

Author: Richard Bedford

28 October 2021

External thermographic inspection of a commercial building

External thermographic inspection of a commercial building © Richard Bedford Surveying Limited

Whether qualitative or quantitative, internal or external, a thermographic survey should be treated in the same way as any other instruction you undertake as a surveyor. The service you are offering, its extent and any exclusions must all be expressly stated in your formal terms and conditions.

A thermographic survey is undertaken using thermal imaging cameras, which detect infrared light that is not visible to the human eye. Everything above absolute zero emits infrared light, and as a result it is possible to detect variations in the temperatures of different surfaces.

This can in turn help identify anomalies such as damp ingress, cold bridging, air infiltration and missing thermal insulation in buildings. It is a particularly useful inspection technique because it is completely non-invasive, and enables large areas of walls, ceilings and floors to be inspected safely in a relatively short period of time.

Clarity for client and cover

Once initial discussions have taken place with your client, perhaps the most useful first step is to detail in no uncertain terms the type of survey you propose, its benefits, the extent, the scope of any subsequent written report, likely timings and the costs involved.

I have found it most helpful to provide clients with a further checklist of specific requirements ahead of the survey, along with a brief but succinct description of building thermography and how it can be used to identify thermal and other anomalies.

Professional indemnity insurance (PII) providers should also be informed before proceeding with any level of thermographic service. My experience is that PII providers require no additional premiums once advised that you are performing this service for clients; however, I am a certified thermographer, and conditions may be different for surveyors who are not.

Making the right arrangements

Confirming a suitable inspection date may not be as straightforward as it first appears, because obtaining useful information depends to a large extent on the basic principles of heat transfer. The second law of thermodynamics states that thermal energy will always transfer from warm to cold bodies, and never vice versa.

When conducting building thermography, therefore, there needs to be a suitable temperature differential between the inside and outside of a building, since thermal imaging cameras are not able to detect slight heat transfers. Where temperatures are similar, there will be no heat transfer but thermal equilibrium instead.

In the UK, heat transfer conditions suitable for thermographic surveys generally occur in the winter months, when buildings tend to be heated to at least 20C and external air temperatures drop below 10C. This will typically be the case from late October to March or April, making these months the building thermographer's season.

Minimum temperature differential

As a thermographer, you must always look for a minimum temperature differential of 10C to be sure of sufficient heat transfer. Confirming adequate conditions can be done before the date of the inspection by monitoring weather forecasts.

The necessary temperature differential also means that external thermographic surveys should only be conducted at night, allowing sufficient time for any solar saturation from the day to dissipate from surfaces.

Table 1 shows satisfactory temperature differentials for heat transfer, giving an idea of when this occurs even outside the traditional thermographic season in the UK.

'Thermographers always need to look for a minimum temperature differential of 10C to be sure of sufficient heat transfer'
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Table 1: Sample internal and external temperatures suitable for conducting thermographic building surveys

Internal temperature (C) External temperature (C) Temperature differential (C)
20 10 10
22 10 12
25 12 13
18 1 17
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Table 1: Sample internal and external temperatures suitable for conducting thermographic building surveys

Internal temperature (C) External temperature (C) Temperature differential (C)
20 10 10
22 10 12
25 12 13
18 1 17

It is possible to force the situation where the outside air temperature is greater than 10C by increasing the internal air temperature to ensure a sufficient differential. However, this will create discomfort for occupants and increase their heating costs – especially given that this level of heat will need to be maintained for at least three days to be effective.

You should obtain on-site confirmation of the ambient air temperature with the use of a thermal imaging camera, a suitably calibrated hygrometer or both. This information should be formally recorded as part of the overall inspection.

External thermographic inspection of a commercial building

External thermographic inspection of a commercial building, with heat transfer indicated by red areas

The following internal conditions are required, and can be included in a checklist you give clients before the inspection:

  • constant internal temperature of 20–22C for at least 72 hours before inspection
  • a minimum temperature differential of 10C
  • clear internal faces to external walls; note that if there is a void behind a surface you will not record any useful infrared emissions from that source
  • internal doors left open to encourage even heating
  • no windows or external doors left open, as this would encourage cold air ingress.

All of the above are necessary conditions for heat transfer. They will also allow you sufficient access to exposed surfaces to record any potential thermal or other building anomalies.

All of the above are necessary conditions for heat transfer

Internal thermographic inspection of an office with clear internal surfaces. The hottest area in the image – shown in white – represents an active computer monitor

External inspections conditions

Meanwhile, there are particular conditions under which an external inspection should not be undertaken.

Rain and medium or strong wind can both cool surfaces to the extent that any anomaly will no longer be visible using thermography, so inspections should not be carried out in these conditions. It is possible to inspect during light wind, though; you should use an anemometer to measure wind speed, but bear in mind that anything above 5m/s will render the inspection pointless.

External thermographic inspections should not be undertaken in the daytime either, since solar loading or saturation caused by the sun – as indicated in the image below – will completely obscure the presence of thermal anomalies on any external surface.

Thermal image showing solar saturation of surfaces
Photograph showing solar saturation of surfaces

Thermal image and photograph showing solar saturation of surfaces. Note building orientation and cooler building surfaces facing away from direct sunlight: the hottest areas are shown in white, followed by red then green, with blue and purple being the coldest. The inside of the building is evenly heated

Common issues

Despite asking the client to ensure the building is adequately heated, you may find that your hygrometer readings indicate less than the required internal air temperature. This can be deeply frustrating, especially when they offer to turn up the heating after you arrive.

Speed heating will however only warm the nearby air rather than the building fabric, which is what is actually required. Under such circumstances, your inspection may not be able to proceed. Provided you have agreed robust terms and conditions with your client that clearly set out what you require of them, you will be well within your rights to refuse to conduct a survey in such a situation.

A more common issue, but one that is less of an obstacle, is presented by closed internal doors and furniture that obscures surfaces to be inspected. While doors can easily be opened, you have to be realistic about moving heavy furniture as this may not always be possible.

You will also come across situations where furniture, curtains and the like have only been moved just before you arrive, but you are not informed of the fact. In these instances you will likely record colder surfaces because warming convective currents will not have been able to circulate. Misidentification of anomalies can occur as a result. Bear in mind that, even though the checklist you provide the client should mention the need to clear surfaces in good time ahead of your inspection, this does not always happen.

As a building thermographer, you are also likely to find yourself in buildings of all descriptions, ages and sizes. Consequently, some internal spaces will prove extremely awkward to inspect properly. It is therefore strongly recommended that you use a thermal imaging camera with a sensor of 320 by 240 pixels resolution and a 42-degree lens. This will allow you to inspect as much of the surface area as possible without being too far away from it. In certain cases, this may be the only way you can obtain a reading at all.

Internal thermographic inspection of an office with unclear surfaces

Internal thermographic inspection of an office with unclear surfaces

When it comes to external inspections, there are other common issues. Photographs of the building from your client ahead of inspection might not show the full extent of vegetation, trees and planting that actually obscure large areas of external walling. Such features will affect your ability to conduct a full external inspection, but your terms and conditions should again have allowed for this fact.

External thermographic inspection of a commercial building close to a main road

External thermographic inspection of a commercial building close to a main road. Note that working on the roadside at night is a clear health and safety issue, for which you need to plan

Despite all reasonable attempts you make to determine the prevailing weather conditions, localised rainfall could still occur or wind pick up. It is therefore recommended that you carry an anemometer to measure wind speeds while undertaking external inspections. Should you experience either wind or rain, you may find such an inspection is not possible. Equally important is protecting your sensitive thermal imaging camera, although if you cover it with a thin black plastic bag you can still obtain accurate readings.

Other external features that can affect thermal imaging include the proximity of large buildings and trees. A clear night sky meanwhile can be extremely cold, and windows may reflect low-temperature infrared emissions, resulting in misleading readings.

Safety is paramount

Finally, as surveyors we all need to keep health and safety firmly in mind. You should therefore consult the current edition of Surveying safely: health and safety principles for property professionals RICS guidance note.

As inspecting thermographers, we should also consider and plan for potential issues such as:

  • working alone
  • working outside, and at night
  • working in an unknown environment
  • working at night near main roads
  • working on an active building site.

This article offers only a brief overview of thermographic building inspections, following my introduction to the subject in a previous article. I therefore strongly advise that you gain formal training and certification in thermography before offering such services to clients.

There is after all a huge potential market for building thermography, ranging from residential to commercial and institutional building owners. So although the initial investment in equipment and training may seem daunting, the rewards from offering this in-demand specialist service to your clients can be considerable.

Richard Bedford MRICS is managing director at Richard Bedford Surveying Limited
Contact Richard: Email

Related competencies include: Building pathology, Inspection

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