Driven by rising energy costs, building regulations and Carbon Emission Reduction Targets (CERT), thermography of building envelopes is the fastest growing application of thermography. The intention of this article is to inform potential customers of the various approaches to building thermography and provide an understanding of the benefits and limitations of each of these approaches. This will enable the reader to specify the type of survey they require to meet their desired outcomes.
Qualitative or Quantitative?
Approaches to building thermography can classified under two categories; qualitative and quantitative. Qualitative approaches rely on the thermographer’s knowledge and experience of thermography and building science to identify potential defects with a building envelope. This approach does not rely on temperature measurements but rather the differences in the levels of radiosity (differences in colours shown on a thermal image) to ascertain areas of excess heat loss. Whereas quantitative approaches use the same approach to identify the defects but go one step further and use the temperatures measured to ascertain how much heat is being lost.
The type of approach used will therefore depend on what it is you are trying to achieve. If you are only looking identify the defects within the building envelope, then a qualitative survey will deliver the desired results, however if you are looking to identify potential cost savings/CO2 reductions, then a quantitative approach is needed.
External or Internal Survey?
The choice of survey approach does not stop there. The next question is should the survey be carried out externally, internally or both?
An external survey, invariably is the simplest approach to implement. It is relatively straight forward to capture thermal images of all of the building envelope, whereas images captured internally require the walls to be clear of furniture, pictures and other equipment, if confidence is to gained from the survey. On the face of it then, an external survey would be the best approach to implement.
However, it may be the simplest, but it may not give the desired results. An external survey is very much affected by the environmental conditions at the time of the survey, clear night skies and windy conditions make any kind of quantitative survey far too inaccurate. Also, depending on the building construction, not all building defects will be evident from images taken externally, e.g. poorly fitted loft insulation in a house with a pitched roof, can only be detected with an internal survey.
Conversely, an internal survey is less affected by environmental conditions, as air temperature and convective currents are relatively constant making quantitative assessments more accurate. But internal surveys are far more time consuming, with many more images needed to be captured due to the number of floors/rooms and limitation of the lens angle.