The aims of this BRE Trust report are to support the implementation of building regulations on conservation of fuel and power and legislation on the energy performance of buildings, to help raise awareness and understanding of U-values (thermal transmittances) and kappa-values (thermal mass values) and to encourage a unified, consistent and up-to-date approach to calculating these. The calculation methods are explained using worked examples for a number of wall, roof and floor designs. The examples can also be used to support training programmes for practitioners carrying out energy assessments and using U-value calculation software. Information is given about calculating U-values using standard simplified methods.