Retrofitting cooling-dominant dwellings for future tropical climates: a multi-year study in Brunei Darussalam

<p dir="ltr">The increasing demand for cooling in tropical climates, driven by climate change and inefficient building design, highlights the urgent need for energy-efficient retrofitting of existing dwellings. In Brunei Darussalam, where cooling dominates residential energy demand, weak energy regulations and limited retrofitting efforts further exacerbate the issue. While Net Zero Energy (NZE) buildings have the potential to significantly reduce energy consumption and emissions, existing retrofit strategies often fail to account for increasing cooling loads and long-term climate variability in tropical regions. This study investigates the feasibility of retrofitting cooling-dominant dwellings in Brunei to achieve and sustain NZE status under both present and future climate scenarios.</p><p dir="ltr">To address the limitations of conventional retrofit studies, this research pioneers a multi-year (2015¿2100), scenario-based (SSP126, SSP245, SSP370, and SSP585) future weather file generation method, ensuring a robust assessment of NZE performance under climate variability and extremes. A novel combination of interpolation and quantile delta mapping techniques was developed to preserve the statistical integrity of projected climate trends, overcoming the limitations of traditional single-year weather files. These datasets were integrated into dynamic thermal modelling of five representative housing archetypes - detached, semi-detached, terrace, flat, and stilt dwellings - providing a comprehensive analysis of passive, active, and renewable retrofit strategies through parametric simulations. A detailed economic evaluation was conducted to assess the feasibility and cost-effectiveness of these strategies, identifying optimal retrofit combinations at medium (50%) and high (95%) performance levels before reaching the point of diminishing returns.</p><p dir="ltr">The results demonstrate that combined retrofits significantly reduce energy demand and enhance building resilience under both present and future climates. A medium-level retrofit (achieving 50% of the maximum possible energy reduction before reaching diminishing returns) reduces total energy consumption by 56.8%, while a high-level retrofit (achieving 95% of the maximum possible energy reduction, capturing near-maximal performance improvements) achieves reductions of 86.4%. Peak cooling loads followed a similar trend, decreasing by 64.4% and 84.0%, respectively. Additionally, retrofitting reduces solar PV system size requirements by 23.8% and 43.2%, improving feasibility of reaching NZE targets.</p><p dir="ltr">However, future climate conditions pose growing challenges to maintaining NZE. Under SSP585, a high-emission scenario, cumulative heatwave days are projected to reach 4,152 between 2015 and 2100, with the longest single heatwave lasting 326 days. Inter-annual temperature variability also increases significantly, from 0.5°C in SSP126 to 1.8°C in SSP585, indicating a more volatile climate that intensifies cooling demand fluctuations. Extreme weather years can drive up to a 10% increases in cooling energy consumption year-to-year, highlighting the critical role of resilience in NZE feasibility. The findings further show that all investigated archetypes failed to maintain NZE across all future years without PV expansion. On average, NZE self-sufficiency declined by 50% under SSP585, underscoring the growing challenge of maintaining energy independence as climate conditions worsen. Even with high retrofits, PV capacity would need to expand by 54.2% under SSP585 compared to SSP126 to sustain NZE year-round. While high retrofits reduce PV expansion needs by up to 50%, rooftop PV alone remains insufficient under higher-emission scenarios due to physical space constraints, necessitating alternative strategies beyond rooftop installations.</p><p dir="ltr">The economic analysis found that medium retrofits achieved a static payback period of 8.1 years, while high retrofits required 12.6 years due to higher capital costs. The average capital cost for medium retrofits was $19,091 (165.3 $/m²), increasing to $29,641 (256.3 $/m²) for high retrofits. However, under non-static conditions that incorporate maintenance and replacement costs, payback periods were unachievable across all scenarios. Additionally, Brunei's subsidised electricity pricing further extended these payback periods, reducing the financial attractiveness of NZE retrofits under current economic conditions.</p><p dir="ltr">This research makes two key contributions to knowledge. First, it identifies scalable NZE retrofit strategies tailored to tropical climates, providing practical insights for policymakers, energy planners, and building designers. These strategies balance energy savings, economic feasibility, and resilience, ensuring a realistic approach to NZE adoption in tropical regions. Second, it introduces an advanced multi-year, scenario-based future weather file generation method, enabling more accurate simulations of building energy performance under climate variability and extremes. This methodology is globally adaptable, providing a scalable framework for enhancing building resilience and supporting data-driven retrofitting policies in climate-vulnerable regions worldwide.</p>