This document specifies a test method for the in situ testing of the thermal performance of building structures especially built for the purpose of the test. This document also specifies the apparatus to be used and the measurement procedures to collect the data and the reporting format for the apparatus including the building test structure and the test conditions. NOTE The analysis of the data and the reporting format for the analysis are referred to in prEN 17888-2. This document does not apply to: existing buildings; building structures allowing direct solar gains through glazing surfaces; the determination of the thermal performance of a specific building product, material, component or element.