Quantitative application of Infrared Thermography



Some earlier posts referred to the importance of improving the thermal efficiency of existing buildings in order to reduce carbon emissions associated with heating.


Retrofit 2050 concentrated on methods of measuring the thermal performance of buildings when the thermal properties of the materials used are unknown.

Retrofit of buildings: citations and extracts referred to recent work on improving methods of thermal measurement.

Energy Efficiency in Buildings provided links to energy performance standards.

Many papers on thermal measurement focus on assessing the U-values of existing structures. Standard methods involve the use of both temperature and heat-flow measurements and are typically time-consuming and demanding. Effort has been directed to reducing the difficulties and producing quicker results, in some cases by eliminating the need for heat flow measurement. Reducing measurement time without loss of accuracy remains a goal.

The infrared thermography technique (ITT) has been widely used in thermal surveying as a qualitative method, intended to quickly identify areas of heat loss for further investigation. Brief notes follow on a 2018 Ph.D. thesis which investigates the use of ITT as a quantitative method.


by M. O’Grady 


It is focussed on a particular aspect of thermal assessment, that of thermal bridging, where certain parts of a building lose heat at a greater rate than their surroundings due to the method of construction in the local area. Window frames are a familiar example.
The thermodynamic principle underlying the work is surface energy balance; energy leaving a surface is evaluated from surface temperature measurements, and equated to energy reaching the surface. Thus temperature measurements on the exterior surface of a building can allow computation of energy loss from the building. One of the objectives of the work is to show that ITT used on the outside of a building can produce a thermal assessment of useful accuracy in a variety of weather conditions.
Thermodynamic literature is reviewed, with particular regard to heat transfer coefficients. It is claimed that the exterior convection coefficient hce, on which analysis depends, can be calculated with sufficient accuracy from the Nusselt number, Nu, the thermal conductivity of air k, and the appropriate characteristic length lch in the relation
hce = (Nu k)/lch
Post -processing of the infra red image allows the appropriate values of hce to be calculated and assigned pixel by pixel if required, and in some situations greater accuracy is achieved as a result.
Comparisons are made between ITT and laboratory measurements for a series of test cases, and good agreement is reported.
The results may be of interest to community energy workers involved in the retrofitting of buildings for fuel saving. The work described by O'Grady prompts consideration of whether practical thermal surveys can be carried out using ITT only, thus saving time and reducing the amount of equipment needed. However, the thesis described is specific in its focus on thermal bridging, and it may not follow that the ITT method alone is suitable for more general thermal assessment.


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