What is thermography?
Thermography is a visual method of illustrating invisible heat energy. This is typically captured using an infrared camera, which displays a map of the temperature variations emitted by all objects with a surface temperature above absolute zero (-273°C).
Objects are then illustrated as thermal patterns, using different colours or shades of grey depending on the palette selected. The thermal patterns illustrate the variations of heat energy being emitted from the surface of the object.
The subsequent interpretation of the thermal patterns requires an understanding of the environmental conditions at the time of capturing the image, as well as surface characteristics of the materials being viewed.
Emissivity (an object’s ability to emit infrared radiation) and reflectivity (the property of reflecting radiation) are the two characteristics of materials which determine how reliable the thermal pattern is in the image. Materials with high emissivity have a low reflectivity and vice versa.
Only materials with a high emissivity provide a reliable reading. This is because materials with a low emissivity have a tendency to reflect the temperature of surrounding objects.
Typically, materials such as bricks and plaster have a high emissivity, whilst metals and glass have a low emissivity. Given that the fabric of many older buildings will be made up from predominantly high emissivity materials, thermography can be a really useful tool for assessing the fabric of new and existing buildings, in particular retrofitted energy efficiency interventions.
Thermography as a retrofit tool
The main advantage of using thermography as a tool is that you can see what is going on under the skin of a building without having to take it apart. Having the ability to see invisible heat energy provides valuable data for assessing thermal performance.
Quantitative thermographic assessments of heat loss require precise climatic conditions, which rarely occur, particularly in external environments. As a result, typically only qualitative thermographic building surveys provide robust and reliable data.
Qualitative thermographic building surveys can provide many useful insights, such as: the effectiveness of insulation as a barrier to heat loss; occurrences of thermal bridging; sources of air-leakage; moisture and damp; and hidden components, such as pipes and wall ties.
There is a clear benefit to using non-destructive infrared thermography in traditional buildings. For example, intrusive investigations – drilling holes or dismantling construction elements – would otherwise be required to identify missing insulation and the location of thermal bridges. Furthermore, air leakage tests only identify that there is air infiltration, not its location. And moisture and damp often goes unnoticed until it appears on the surface.
However, being able to achieve robust and reliable results from qualitative thermography requires professional training and certain environmental conditions to be achieved at the time of the survey.
Getting the most out of thermography
Being able to use an infrared camera and accompanying software, as well as understanding and interpreting thermal images, requires a suitable level of knowledge and experience about the survey process and the building being assessed.
The correct survey process includes ensuring that the current environmental conditions are suitable, as this will maximise the accuracy of the data being collected. Although camera technology has improved over the years, along with the process itself, results still depends on the type of building fabric being surveyed. Therefore having knowledge and experience about building materials and construction methods are an essential prerequisite for a successful survey.
The criteria for the environmental conditions also vary depending on whether an internal or external thermographic survey is required. The latter is more challenging. For example, external surveys require: no solar radiation on the surface before and during the survey; no precipitation before and during the survey; a temperature difference between the inside and outside the building of at least 5°C – more commonly 10°C – is recommended; and wind speeds of no more than 10 meters per second (m/s), or preferably 6 m/s are recommended. And the length of time with no solar radiation and precipitation prior to undertaking a survey depend on the absorptance (the effectiveness of absorbing heat energy) of the building material.
To overcome the need to avoid solar radiation, many experts currently recommend undertaking thermographic surveys in the dark, although others suggest that a cold overcast day is sufficient. It is also often advocated that thermographic surveys should only be undertaken in the winter, which could be a significant limitation to its use.
However, in my previous work I have achieved reliable results to identify thermal bridging using thermography during the early hours of the morning during the summer. The most important consideration is taking due account of the environmental conditions during interpretation.
Internal surveys have fewer restrictions. With the exception of temperature, most of the criteria used for external climatic conditions do not apply. However, the temperature difference between the inside and outside of the building is critical for both internal and external thermographic surveys. Subject to the restrictions imposed by the resolution of the infrared camera, the greater the temperature difference, the better the detail that is visible in the thermal image.
Camera resolution is critical for undertaking thermographic surveys of building fabric, particularly when needing to capture a whole façade. Sometimes it is also necessary to use a wide angle lens. And occasionally images have to be stitched together, depending on the size of the building. The cost of cameras rises significantly in line with increases of the resolution, as well as with the addition of any accessories, which can be a further significant limitation.
But despite this, one great advantage of using thermography during retrofit is for communicating what has been done. Capturing the whole façade before and after insulating is a great way of visually highlighting the improvements that have been made. More detailed and close up images are also useful for identifying any gaps and subsequent thermal bridging.
When interpreting thermal images taken inside the building, colours representing cold areas are an indication of heat loss. Whereas for external thermal images, it is the colours representing warm and hot areas that are an indication of heat loss. These areas of heat loss can be either expected as part of the design, or unexpected and indicating a potential problem. Issues could include inappropriate installation, poor workmanship, or other unforeseen consequences.
Following a series of complaints by residents approximately 12 months after they had received retrofitted external wall insulation (EWI) to their homes, the Carbon Trust was commissioned to undertake an independent investigation using thermography. The residents’ concerns centred on feeling colder and experiencing draughts since the EWI had been installed, as well as the quality of the installations.
The homes are of non-traditional British Iron and Steel Federation (BISF) construction and required extensive structural repairs as part of the retrofit works. To ensure structural issues were addressed appropriately, the installer commissioned a structural engineering consultancy to develop detailed designs for the replacement of structural elements, at the same time as incorporating the EWI. These designs also aimed to address the requirement for a continuous covering of EWI to minimise the occurrence of thermal bridging, wherever possible.
A visual inspection indicated that the installations had been undertaken to a high standard. However, it was observed that the plinth (base) of the external walls had not been insulated and the installer advised that it had not been possible to insulate lofts due to the risk of asbestos. The funding did not allow for these locations to be insulated, as it would have significantly increased the costs of installing EWI to the main structure of the dwellings.
Not insulating the plinths appears to have resulted in the solid concrete ground floor slab providing a passage for heat loss from the building. This meant that there was a significantly lower air temperature recorded at this level when compared to waist level. This was compounded by the decision not to insulate the loft, which meant that the roof was also a source of significant heat loss. When combined these two factors appeared to have provided the ideal conditions for creating air movement through the building, which were the draughts the residents were experiencing.
Taking a whole-house approach, rather than just focusing on installing EWI, may have prevented this from occurring.
A version of this article first appeared in the Institute of Historic Building Conservation Journal’s 2017 special issue on green retrofit.
About the author
Dr Jo Atkinson leads the Carbon Trust’s housing-related work. Jo is an architectural technologist and qualified thermographer. Her doctorate focused on the evaluation of retrofitted external wall insulation, where she developed a methodology for assessing its construction quality, which included the use of thermography.