A new framework to select energy-efficient retrofit schemes of external walls: A case study

Retrofitting energy-inefficient external walls would improve the energy performance of building envelope by utilizing new insulation materials to decrease heat transfer. However, previous researches mainly based on environmental and economic considerations are still partial and could hardly provide a holistic perspective for the optimal selection of external walls retrofit techniques. To address this problem, this paper proposes a novel heterogeneous multi-criteria group decision-making (MCGDM) framework in which the ratings of schemes are described with real numbers and linguistic variables and the weights of criteria are unknown. Firstly, an evaluation index system is established covering criteria of thermal insulation property, technique, durability, and economic efficiency. Secondly, building information modeling (BIM) is utilized to determine multiple retrofit schemes and derive the heat transfer coefficient. Thirdly, after heterogeneous evaluation values are unified, a combined weighting approach is developed based on best worst method (BWM) and the similarity method before fuzzy VlseKriterijumska Optimizacija I Kompromisno Resenje (VIKOR) method is applied to rank the alternatives. Fourthly, the applicability of the proposed framework is demonstrated by a case study of the external wall retrofit scheme selection in Beijing, which reveals that extruded polystyrene (XPS) and expanded polystyrene (EPS) insulation schemes are the compromise choices with aggregating index Q_k of 0.0032 and 0.0967 respectively, while rock wool insulation scheme is the last option with Q_k of 0.7182. The stability of ranking results is proved with the changing weight of maximum group utility, and the effectiveness of the proposed framework is verified by a comparative analysis. This study can facilitate the implementation of building envelope retrofit by providing useful guidance for selection of external wall retrofit schemes.