Remote measurements of thermal transmittance without approximating a steady state

The thermal transmittance of the building envelope is a key parameter that determines the energy performance of a building. In order to estimate the potential of refurbishment options, accurate information about the thermal transmittance is desirable. This information should be obtained with as little impact on the use of the building and in as little time as possible. The use of thermography for this purpose has the advantage that it allows for remote measurements without installation of sensors on the envelope. A model calibration approach is proposed to increase the accuracy under varying environmental conditions and to reduce the measurement time for thermography from the outside. A simplified physical model with a small number of thermal resistor and capacitor elements is used to represent the building wall and simulate the heat flow under transient conditions. Its parameters are varied and chosen such that the best reproduction of the measured surface temperature is obtained. The uncertainty of the determined transmittance depends on the thermographic measurements and on the uncertainties in the boundary conditions which describe convective and radiative heat exchange and also have to be determined by measurements. The uncertainty analysis is performed using a Monte-Carlo sampling of all measured quantities.