Summer thermal consequences of EU retrofit Strategies: An integrated microclimate and indoor comfort study in Düsseldorf

• Realistic building archetypes alter simulated outdoor heat stress by up to 1.5 °C compared with model defaults. • T mrt explains over 91% of UTCI variance at peak hour, confirming radiation and shading as the dominant drivers. • Urban greenery cools peak UTCI by up to 6.0 °C, clearly outperforming insulation-only retrofits. • A crossover effect shows that winter-optimized retrofits increase daytime indoor overheating by up to 3.8 °C. • A one-way ENVI-met–CitySim linkage reveals a mitigation–adaptation trade-off in current European residential retrofit policies. Building retrofits in temperate European climates primarily target winter heat losses, yet their dynamic implications for outdoor microclimate and indoor thermal safety under heatwaves remain poorly understood. This study investigates how winter-oriented retrofits modify both outdoor microclimate and indoor thermal conditions during July 2022 Central European heatwaves, using a one-way, offline integration of ENVI-met and CitySim. A dense residential neighborhood in Düsseldorf, Germany, was simulated under multiple retrofit and greening scenarios, incorporating realistic building data derived from the EU TABULA archetype database to ensure reliable façade–microclimate interactions. Methodologically, results show that using realistic archetypes alters simulated outdoor heat stress by up to 1.5 °C compared to generic defaults. In terms of mitigation, façade retrofits alone slightly reduce outdoor Universal Thermal Climate Index (UTCI) by up to 0.8 °C, while green infrastructure provides the strongest cooling, lowering local UTCI by up to 6 °C through shading and evapotranspiration. Typically, indoor simulations revealed a crossover effect, where retrofitted buildings remained cooler at night but became up to 3.8 °C warmer during daytime peaks, increasing short-term overheating risks in the absence of active cooling. These findings highlight a mitigation–adaptation trade-off in EU retrofit strategies: measures optimized for winter energy savings can exacerbate summer heat stress if not combined with greenery and summer-oriented design measures. The proposed framework offers a scalable workflow to support climate-resilient, human-centric retrofit planning in other TABULA-covered cities.