Starting from the measurement principle for the determination of thermal resistance of a building element in the laboratory (guarded hot box method) and in-situ, possible testing errors are considered, in particular those due to temperature nonuniformity and imbalance between the metering and guard section. Almost all in-situ measurements are carried out in non-steady-state conditions, nevertheless imbalance problems are similar to those of such steady-state test methods as the guarded hot box.
Temperature non-uniformity on the surfaces of the specimen generates an uncertainty in the definition of the surface temperature itself, but, as a consequence, there is an associated potential temperature imbalance.
The scope of this work is to verify the semi-empirical expressions introduced in two documents of the European Committee for Standardization (CEN, Comité Européen de Normalisation) with those for guarded hot plate apparatus, analytically obtained and experimentally verified by Woodside and those analytically obtained for guarded hot box apparatus. Next, the application of the CEN expression to assess the effect of temperature non-uniformities was checked: for this purpose some homogeneous and layered structures have been analysed by the finite difference method.