The energy performance of buildings has become increasingly important, driven by efficiency, awareness, and sustainability goals. While attention often focuses on passive or zero-emission buildings, upgrading the thermal insulation of existing stock remains essential. In Hungary, many buildings still fail to meet current standards, from 19th-century apartments in Budapest to post-war panel blocks designed without thermal considerations. This study examined mineral-based insulation boards made from autoclaved aerated concrete under controlled exposure scenarios: cold winter (−20 °C), humid spring/autumn (10 °C, 90% RH), and hot summer (70 °C, 20% RH). Standardized tests assessed mass variation, compressive strength, thermal conductivity, and short-term water absorption. Results show that high humidity caused up to 14% loss of compressive strength and a 17–18% increase in thermal conductivity after 14 days, severely reducing insulation capacity. In contrast, extreme cold and heat induced only minor changes. Moisture was identified as the most critical factor compromising both mechanical stability and energy efficiency. The findings provide reproducible evidence of environmental sensitivity and underline the importance of durability assessments in designing and retrofitting energy-efficient buildings.