  The \emph{in situ} estimation of the thermal properties of existing building
wall materials is a computationally expensive procedure. Its cost is highly
proportional to the duration of measurements. To decrease the computational
cost a methodology using a D-optimum criterion to select an optimal experiment
duration is proposed. This criterion allows to accurately estimate the thermal
properties of the wall using a reduced measurement plan. The methodology is
applied to estimate the thermal conductivity of the three-layer wall of a
historical building in France. Three different experiment sequences (one, three
and seven days) and three spatial distributions of the thermal conductivity are
investigated. Then using the optimal duration of observations the thermal
conductivity is estimated using the hybrid optimization method. Results show a
significant reduction of computational time; and reliable simulation of
physical phenomena using the estimated values.
