Energy retrofit of non-monumental, historic buildings: limitations and opportunities of technological advances

The new European ‘Green Building Directive’ requires all Member States to develop their own national building renovation plan addressing the bloc’s biggest emission sources associated with old buildings. Member States are expected to implement an extended renovation strategy to achieve a 55% reduction in energy consumption. This ambitious target challenges countries like Italy, where most residential buildings were built before energy efficiency policies, and 60% of them are ranked in the two lowest energy classes. The most common energy retrofit strategies for buildings built after the Second World War range from increasing thermal insulation of the envelope to window replacement with high-performance frames and glazing. However, this approach is not viable for historic buildings of which Italy is rich. Their cultural significance largely outweighs their poor energy performance, as the form and aesthetic of these buildings considerably contribute to defining the character of Italian cities—thus, they must be preserved. The high quality of the original exterior finishing of these historic buildings, often fronting onto public urban spaces, strongly limits the addition of external wall insulation, while installing internal insulation, which dramatically reduces the dimensions of interior spaces, would cancel out the beneficial effect of thermal mass in summer without addressing thermal bridges. Consequently, actual interventions in non-monumental, historic buildings are often limited to roof insulation, the adoption of insulated shutters in front of windows and the use of higher-performance glass of limited thickness and weight to replace the original single pane glass while maintaining the existing frames. This paper discusses the limitations and opportunities of energy retrofit practice of non-monumental, historic buildings in temperate, continental climates of northern Italy, focusing on high-performance glass technologies, to estimate potential energy savings while evaluating the aesthetic impact of their application. The paper quantifies the energy savings of high-performance glass solutions such as vacuum-insulated glass mounted in existing frames and triple-glazed windows; it uses a case study research design to qualitatively investigate new technological solutions and their limitations, as well as quantitively evaluate the energy performance of two, non-monumental, historic buildings in Milan and Novara, when subject to different retrofit interventions, to demonstrate their effectiveness in both winter and summer. Results are then compared to provide guidance for effective and economically viable energy retrofit of non-monumental, historic buildings in the specific Italian context and construction market.