Thermal imaging – handle with care

This Issue This is a part of the Retrofit and renovation feature

By - , Build 116

Thermal (or infrared) cameras are becoming more and more affordable, and their use in building surveys is becoming widespread to identify problems and areas for improvement. However, understanding the results involves more than just looking at pretty pictures.

Infrared image of a wall showing air convection around insulation that is thinner than the depth of the wall cavity. There is also a gap between the bottom of the insulation and the dwang.
Conventional (left) and infrared (right) images of a hallway. The cold areas (purple) show the missing ceiling insulation.

Thermal cameras are still a significant cost, although prices are coming down. Entry level models currently cost about $5,000, with top of the range versions rising to approximately $100,000. BRANZ has recently purchased a thermal camera and is exploring its capabilities.

Detecting what the eyes can’t

The human eye is sensitive to radiation that has a wavelength of between 0.4 and 0.8 micrometres (μm). Given that the electromagnetic spectrum covers wavelengths from 1 x 10-¹² m (gamma rays) up to hundreds of metres (radio waves), one can get a feeling for the narrow view of the universe our eyes actually provide.

Thermal cameras are sensitive to radiation that has a wavelength within the range of 2–15 μm, which are longer wavelengths than our eyes can see. These waves represent a large portion of the heat emitted by surfaces with near-ambient temperatures. The primary benefit of infrared cameras is that they allow thermal differences to be located, and they can also allow the actual temperature to be determined.

Many applications but pitfalls

There are literally dozens of applications for thermal cameras in building surveys, including:

  • detection of insulation defects or areas of heat loss
  • detection of air leakage
  • detection of moisture
  • examination of heating systems
  • examination of electrical systems.
Conventional (left) and infrared (right) images of a hallway. The cold areas (purple) show the missing ceiling insulation.

This sounds great, but thermal cameras are not a panacea for building inspections – in reality, they will complement other tools that building surveyors possess, not least their own knowledge of the built environment. It is crucial to remember that there is skill in both conducting a thermal survey and in interpreting the resultant thermal images.

The main pitfall for a user of a thermal camera is assuming that ‘seeing is believing’. There are many things that can result in temperature differences across a surface, and they may not be what the user is actually looking for.

Underlying principles

It is essential that users of infrared cameras are aware of the underlying principles of their use. All objects radiate energy, with the amount dependent on both the temperature of the object and its emissivity.

Emissivity is defined as the ratio of how much radiation an object can emit compared to some baseline measurement (known as a black body). In more simplistic terms, emissivity is the ability of a surface to emit thermal radiation.

It is very hard to see thermal patterns from low emissivity surfaces, for example, shiny metals. Fortunately, most building materials have a high emissivity, but it is not a constant, and different materials or finishes may change it. Where an infrared camera is used on a low emissivity surface, most of the image will actually be reflected radiation from other surfaces.

Infrared image of a wall showing air convection around insulation that is thinner than the depth of the wall cavity. There is also a gap between the bottom of the insulation and the dwang.

Detecting defects in insulation

One obvious use of thermal cameras in building surveys is for detecting insulation defects. When performing such a survey, it is necessary to have a reasonable temperature difference (≈10°C) between the inside and outside. If conducting the survey from the inside and assuming that the interior is warmer than the exterior, defective insulation will be shown as cold spots on the wall. If it is the exterior that is warmer, defects will show up as hot spots.

Even in the relatively simple case of identifying insulation defects in light-weight walls, there are still errors that could go unnoticed by an inexperienced user. For example, local temperature differences on a wall could also be caused by things such as airflows into the room, emissivity variations, partial shading of the wall and surface moisture.

Further issues must be considered when the inspection is conducted from outside, such as performing the survey at the right times to ensure optimal conditions for thermal camera use.

Experienced operator needed

The capability of the person using a thermal camera is just as important as the technical capability of the camera itself. It is recommended that practitioners should be working to a minimum of Level 2 (ASNT equivalent) certification and have experience of building surveys.

Note

This introduction to thermography is likely to be followed by other Build articles on the use of thermal cameras.

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Articles are correct at the time of publication but may have since become outdated.

Infrared image of a wall showing air convection around insulation that is thinner than the depth of the wall cavity. There is also a gap between the bottom of the insulation and the dwang.
Conventional (left) and infrared (right) images of a hallway. The cold areas (purple) show the missing ceiling insulation.

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