Thermal insulation solutions for opaque envelope of low-energy buildings: A systematic review of methods and applications

It is crucial to develop thermal insulation solutions for opaque envelopes of low-energy buildings that match the global sustainable development vision. With the popularity of low-grade energy and renewable energy in the building sector, the opaque envelopes have been regarded as multifunctional elements (e.g., energy and structural aspects), providing a transition opportunity for the thermal insulation solutions from high to zero-carbon attributes. However, the current thermal insulation solutions for opaque envelopes focus more on the static type with constant thermal properties and the dynamic type with switchable thermal properties, which is the foundation of most existing reviews. For the burgeoning dynamic solutions relying on adjusting the core layer temperature inside the envelope and thus controlling the temperature difference forming the enclosure heat load, few people or research communities explicitly incorporate them into the technical framework of thermal insulation and raise their importance to the same level as static type solutions and the dynamic type solutions with switchable thermal properties, leading to the relevant research on this topic is still unstructured and fragmented. Therefore, this study aims to clarify the differences in concept and technical principles of various thermal insulation solutions for opaque envelopes through a unique classification method based on the formation principle of the enclosure heat load. Based on the proposed classification methodology, this paper reports a systematic review of the static type and dynamic type thermal insulation solutions in terms of characteristics, performance, application, and future pathways. • Studies on thermal insulation solutions for opaque envelopes are reviewed. • A novel classification method based on formation principle of heat load is proposed. • Properties and performances are analyzed qualitatively and quantitatively. • The η U is 1.5–47.8 for R -based type and can be an arbitrary value for Δ T -based type. • Research trends, challenges, and future research opportunities are identified.