Estimation of the thermal resistance of a building wall with inverse techniques based on rapid active in situ measurements and white-box or ARX black-box models

Classical in situ measurement methods of building wall thermal resistance are based on steady-state assumptions. This limits their applicability given their high sensitivity to outdoor conditions and the duration required to obtain accurate results. To overcome these limitations, the present paper introduces an in situ method for the assessment of building wall thermal resistance. It is based on rapid active measurements: indoor air is heated for several hours. In addition, unlike common active methods based only on temperatures, the proposed method is based on measurements of a heat flux and one or several temperatures. Two approaches are implemented and compared to estimate wall thermal characteristics from measurements. In the first approach, inverse technique are used based on "white-box" models derived from the heat equation. The best direct model to choose depends on the type of wall tested and weather conditions. In the second approach, ARX "black-box" models are used. The proposed methods were thoroughly tested on a full-size load-bearing wall equipped with a classical internal insulation system commonly found in french buildings. The wall was built inside a climate chamber in order to assess the robustness of the methods to various outdoor conditions. The results demonstrated that, for type of wall, both approaches lead to accurate and repeatable results if the external temperature remains roughly constant. With varying external conditions, white-box models remained accurate while ARX "black-box" models did not. (C) 2020 Elsevier B.V. All rights reserved.