Hygrothermal performance of a self-insulated exterior wall with various exterior/interior insulation thicknesses in two climate zones in China: A novel moisture-energy-environment-economic (M3E) method in insulation thickness optimization

Abstract

The accumulation of moisture inside the envelope under high-humid climates can strongly affect the hygrothermal properties and can lead to moisture-related damage / moulds to the materials, especially in severe weather conditions. However, the optimal insulation thickness of the exterior envelope is typically determined by factors such as overall heat transfer performance, carbon dioxide production and economic cost. The effect of insulation thickness on the annual average moisture content (AAMC) of a self-insulated exterior envelope is often neglected and seldom considered in the optimization process. In this study, a coupled heat, air and moisture transfer (HAM) model was derived and validated. Theoretical indices for water vapour transfer were implemented, such as moisture storage coefficient and moisture inertia factor. A simplified model for liquid water penetration and evaporation was also implemented. A novel four-dimensional moisture-energy-environment-economic (M3E) method that incorporates moisture content analysis was proposed for insulation thickness optimization. A case study was conducted on a self-insulated aerated concrete wall in two climate zones: the hot summer and cold winter (HSCW) zone and the cold zone. The insulation thickness interval investigated were 0–50 mm for interior thermal insulation (ITI) and 0–60 mm for exterior thermal insulation (ETI). The results of the case study showed that the AAMC of the entire envelope varied up to 15.2% (HSCW zone) and 10.0% (cold zone) for different insulation thicknesses. In the HSCW zone, the lowest AAMC of the outer part of the envelope adjacent to the atmosphere was found at 30 mm (ITI) or 20 mm (ETI), indicating the presence of turning thicknesses; while in the cold zones, the lowest AAMC was found at 50 mm (ITI) or 10 mm (ETI) with no turning thickness. The proposed theoretical implementations accounted for changes of AAMC in different interior/exterior insulation thicknesses and were verified with the results of numerical calculation. The proposed theoretical indices revealed an opposite effect of water vapour transfer and liquid water transfer on the AAMC for different insulation thicknesses, which can explain the presence of turning thickness. Furthermore, the selected optimal insulation thicknesses from Pareto fronts were found around 25–29 mm for ITI and 36–38 mm for ETI in two climate zones. The results from the M3E method showed a reduction in moisture risks (up to 62.91%) and mould growth risks (up to 4.84%) in the case study, successfully meeting the acceptable range of moisture risk and mould growth risk. The findings of this paper can help generate a new understanding of moisture transfer, and the proposed M3E method can be applied in all cases to reduce the moisture/mould growth risks in the design stage.