Heritage buildings in hot-arid climates face an acute dilemma: conservation doctrine prohibits the envelope modifications and system upgrades typically required for sustainability, while their poor thermal performance often consumes 30-45% more energy than contemporary buildings, creating a seemingly irreconcilable tension between cultural preservation and environmental responsibility. Failure to resolve this tension forces practitioners into binary choices that sacrifice either heritage authenticity or sustainability objectives, resulting in suboptimal outcomes that neither adequately preserve cultural significance nor meet contemporary performance standards. This research addresses this challenge by demonstrating that interior design strategies-prioritized for their inherent reversibility and minimal impact on historically significant fabric-can successfully bridge this divide. Through an integrated mixed-methods approach combining quantitative building performance simulation with qualitative heritage assessment across twenty adaptive reuse projects in Saudi Arabia, this study develops and validates a comprehensive framework specifically calibrated for hot-arid climate conditions. A key methodological innovation is a non-destructive testing (NDT) protocol using micro-sensors that evaluates building envelope performance without compromising historical fabric. The results demonstrate that carefully selected interior interventions can achieve substantial improvements: non-invasive technological solutions reduced energy consumption by 35-42%, while passive interior design strategies improved occupant comfort by 28%. Critically, adaptive reuse projects showed 55.5% lower total embodied carbon compared to new construction, with this advantage more than offsetting a 13.8% increase in operational energy to yield 6.1% lower overall life cycle carbon. By proving that sustainable interventions can significantly enhance environmental performance without compromising cultural and historical significance, this research provides practitioners with an evidence-based methodology that resolves the heritage-sustainability tension rather than perpetuating it.