Renovating homes: is a light or deep retrofit better for saving the planet?

The residential sector accounts for a large share of global energy use and CO₂ emissions. Therefore, retrofit strategies—targeted renovation measures that boost efficiency—are essential to meet climate goals. A new Irish study combined dynamic energy simulations with life‑cycle assessment (LCA) to evaluate the total impact of thirty retrofit scenarios across different housing types. Interventions were grouped into three levels: keeping the original fabric, shallow retrofits (e.g., high‑efficiency heat pumps), and deep retrofits that overhaul the envelope and install advanced HVAC systems. Results reveal that deep retrofits can cut operational emissions by up to 80 % but bring the highest embodied carbon from materials. In contrast, shallow retrofits deliver almost comparable energy savings with far lower embodied impacts. Moreover, lowering heating set‑points reduces energy demand more than improving system efficiency, especially in low‑performance buildings. Over a 25‑year horizon, shallow retrofits outperform deep ones in overall carbon efficiency, achieving up to 76 % total emissions reduction versus 74 % for deep scenarios. As buildings approach near‑zero‑energy standards, the share of embodied carbon rises, making LCA indispensable for more informed design decisions.

The integrated analysis proposed in the study relies on a calculation chain that links dynamic energy simulation (software such as EnergyPlus or IESVE) with a life‑cycle assessment model compliant with ISO 14044. The simulation provides seasonal energy consumption profiles, accounting for Irish climate variables, household occupancy, and heating set‑points. These data feed the LCA, which quantifies carbon emissions embodied in materials (cement, steel, insulation) and in production, transport, and disposal processes. For each scenario, operational emissions (CO₂‑oper) and embodied emissions (CO₂‑inc) are summed to obtain total carbon (CO₂‑tot) over a 25‑year service life, applying a de‑contamination factor that considers component efficiency decay. Results show that deep retrofits can have CO₂‑inc representing up to 30 % of CO₂‑tot, whereas shallow retrofits keep it below 10 %. This ratio varies with the baseline building performance: poorly insulated homes benefit most from envelope upgrades, drastically cutting CO₂‑oper. Sensitivity analysis reveals that lowering heating set‑points by 2 °C reduces CO₂‑oper by 12 %, while adopting a high‑efficiency heat pump (COP ≈ 4) yields a 9 % reduction. The interaction of these factors is crucial for optimizing the intervention mix, especially when national energy decarbonisation scenarios lower CO₂‑oper but increase the relevance of CO₂‑inc. Finally, the study introduces a “carbon efficiency index” (ECI) that normalises total emissions per habitable square metre, facilitating comparisons across building types and retrofit strategies at the policy level. This methodological framework can be replicated in other climatic contexts by adapting local energy consumption parameters and material emission factors, guiding more sustainable, evidence‑based retrofit decisions.