The quantification of the overall "R-value" of building components is commonly achieved by using numerical models which are generally validated using the standardized Hot Box test. This test set-up follows a complex methodology specifically designed to deliver only the R-value. Modern building assemblies are of a level of complexity that many times a single parameter is insufficient to improve the design of the assembly. This paper proposes a simple thermal test set-up to analyse both transient and steady state heat flow processes, allowing for effective numerical fitting of parameters that describe all internal heat flow processes. As a result, the contribution of each element of an assembly can be evaluated on its overall insulating capabilities, thereby allowing for a truly optimised design solution. Two wall systems including significant thermal bridges have been chosen to illustrate this methodology. The proposed method, not only delivers a steady state thermal assessment as reliable as the standardized Hot Box procedure, but also allows a precise quantification of internal heat flows and the capability to conduct realistic transient state thermal assessments. (C) 2017 Elsevier B.V. All rights reserved.