The energy retrofit of historic buildings is a crucial strategy for achieving the European decarbonization targets. The revised Energy Efficiency Directive mandates an annual renovation rate of 3% of buildings owned by public bodies, aiming to upgrade them into at least nearly zero-energy or zero-emission buildings. Enhancing the energy performance of heritage buildings reduces environmental impact, diminishes operational costs, and increases indoor comfort, while ensuring buildings' functionality and adaptation to contemporary needs. However, this process presents unique challenges, particularly in preserving architectural integrity and addressing technical and regulatory constraints. In this context, retrofit strategies must harmonize energy efficiency goals with the safeguarding of cultural and architectural heritage, thus requiring tailored approaches and innovative proposals. Within this framework, this study aims to gain a comprehensive overview of the current retrofit options for historic buildings, with a focus on passive solutions. It explores innovative and non-invasive strategies and materials designed to improve the performance of historic buildings' envelopes and outdoor paving. A systematic literature review is conducted using the Scopus database, yielding 275 scientific articles. Preliminary analysis of these publications reveals a growing interest in the topic, highlights emerging research trends, and identifies the most promising non-invasive solutions and possible future developments. Among the most recent studies, this review identified three macro-groups of solutions, focusing on: i) the transparent envelope, leveraging solar gain or adaptive technologies - such as thermochromic or photoluminescent coatings - to reduce energy consumption; ii) the opaque envelope, using materials capable of increasing both thermal and moisture regulation; iii) horizontal surfaces (both building roofs and urban pavement) with cool materials effective in boosting building thermal performance while mitigating the Urban Heat Island (UHI) effects. The findings underscore the complexity of implementing energy-efficient passive solutions in heritage contexts, where technical, regulatory, and cultural challenges are often deeply connected. Achieving this delicate balance requires careful planning, close interdisciplinary collaboration, and context-sensitive design approaches that respect and enhance architectural heritage.
Peduzzi et al. (Tue,) studied this question.