The thesis investigates detailed calculation methods for evaluating heat loss through building envelope constructions, or more specific, thermal bridges. First a detailed description of the calculation methods, i.e. both calculation programs and guidelines, for calculating typical thermal bridges in building envelope constructions is given. After this a validation of both programs and guidelines is presented. The validation is performed by comparing calculated U-values with Guarded Hot Box measurements. The last part of the thesis discusses the possibilities of utilising the results of detailed calculations in more operational and applicable projecting tools, e.g. thermal bridge catalogues or U-value tables.