They typically use modelling. It may involve infrared, but often it can be captured just by a thorough engineer going through the site and looking at the wall structure and the sizes, at the vintage of the technology, the vintage of the building. Once you've modelled it, then you can do all sorts of iterations and what-ifs. What if I put a separate skin on? What if I change the boiler? What if I improve it? They're really doing a fairly detailed modelling exercise.
On March 9th, 2017. See this statement in context.