Using the HIL-building as a case study, the results highlight the critical role of embodied emissions in retrofit scenarios. Bio-based materials such as cellulose and straw consistently perform better than conventional options like EPS. Lowering the U-value improves the global warming potential (GWP) for all materials—except EPS—up to around 0.25 W/m²·K (in line with SIA 380/1). For EPS, emissions increase as soon as additional insulation is added.
Reducing the window-to-wall ratio proves more effective for lowering emissions than increasing insulation thickness. Improving glazing reduces operational emissions by about 14%, while switching insulation material at the same U-value leads to only minor gains around 2%. Façade orientation also has a significant impact, meaning that uniform design strategies across all façades are not effective.
As buildings transition to low-carbon energy systems, optimising HVAC systems becomes increasingly important—material changes alone are not sufficient. Overall, the study shows that thermal retrofitting should not be seen as a purely technical upgrade. Instead, it offers a strategic opportunity to align carbon reduction, comfort, and spatial quality in pursuit of net-zero goals.
To maximise impact, it is essential to prioritise material choice, façade and spatial design, and system integration.
