Buildings significantly impact climate change, accounting for approximately 26% of energy- related greenhouse gases globally in 2021. As a result, improving their energy efficiency has become one of the primary policy goals for many countries worldwide; the EU, for example, plans to retrofit 35 million buildings by 2030 to improve their energy efficiency. This has resulted in the ongoing widespread adoption of insulation systems in building stocks. Many existing buildings employ solid walls which absorb and release liquid and water vapour both internally and externally. Introducing insulation systems can alter this hygric behaviour across the wall section, increasing the risk of moisture-related structural and health problems. Despite these risks, there is a deficit of concise scientific, evidence-based guidance and regulation on the topic, even though numerous studies exist on the hygrothermal behaviour of solid-walled structures. This review reports and collates the existing international literature on the influence of insulation types and systems on the hygrothermal performance in solid wall structures in different climates, focusing on moisture movement and thermal performance. The scope includes laboratory, test cell and field trial experiments as well as simulation studies. Targeted hygrothermal performance, key wall locations and threshold parameters are reported. The strengths and weaknesses of the different approaches are highlighted and research gaps identified. The work will lay the basis for the development of an experimental and simulation study of the hygrothermal behaviour of different insulation types and systems for solid walled structures in a temperate maritime climate, using Ireland as a case study.