In 1997 several countries ratified the Kyoto protocol and so engage themselves to take into accountthe global warming, promote the sustainable development and act in order to reduce emission ofgreenhouse gases. Within this context, energy in buildings is known to be one of the first greenhousegases emitting sector. Consequently, determination of steady-state thermal transmission properties foreach kind of faade components becomes necessary in order to evaluate accurately energy loss, todevelop and enhance new products. Such determination should also lead to a better understanding ofphysics phenomenon. One way to determine these characteristics is the guarded hot box method.This experimental process, recognizes by the international scientist community, gives an accreditmeasure of conductance. The experimentation consists in placing a specimen between a hot and acold chamber in which the environmental temperatures are measured and controlled.In 2005, the Building Science Laboratory started the designing of a guarded hot box. Several elementsstep in this project. Indeed, validity of measure is certified under numerous conditions presented inISO 8990 Thermal insulation determination of steady-state thermal transmission properties Calibrated and guarded hot boxThe work described here is the designing phase of the box, which is oriented and thought in order toassume the best accuracy, obtain the necessary conditions of homogeneity and respect prescriptivecriteria list. The designing involved the creation of CFD pre-designing models as decision supportsystem. Thus, some predictive models were created in order to approximate the main dimension ofthe device and then designing the box integrating general constraints as for example weight or localdimension.