Thermal performance of reflective insulation in enclosed airspaces: Effects of aspect ratio, installation defects, and airflow conditions

Reflective insulation systems are widely used to reduce radiative heat transfer across enclosed airspaces in building envelopes; however, their thermal performance is strongly influenced by airspace geometry, installation quality, and airflow conditions, which are not fully captured by standardized methods such as the ASHRAE Handbook—Fundamentals, based on HRP 32 data, and ISO 6946. This study presents a numerical investigation of the combined effects of airspace aspect ratio, installation defects, and air intrusion on the thermal performance of reflective insulation systems using a validated model that simulates coupled conduction, natural convection, and surface-to-surface radiation under steady-state conditions consistent with standard labeling procedures. The analysis considers representative airspace configurations for wall and roof applications with varying geometries, effective emittance values, and airflow conditions. The results show that thermal resistance increases significantly with airspace aspect ratio. For vertical single airspaces subjected to horizontal heat flow, increasing the aspect ratio from 1.1 to 27.4 increases the R -value from 0.33 to 0.65 m 2 ·K/W at an effective emittance of 0.03, corresponding to an increase of 98.9%, while for horizontal single airspaces subjected to downward heat flow, the R -value increases from 0.85 to 1.91 m 2 ·K/W, corresponding to an increase of 124.2%. Installation defects have a limited effect in long cavities (<3%); however, for multi-airspace configurations, non-ideal installation reduces thermal resistance from 1.25 to 0.96 m 2 ·K/W (31%). Air intrusion has a more pronounced effect, with reductions exceeding 100% for vertical airspaces. These results demonstrate that reflective insulation performance cannot be represented using constant R-values and must account for geometry, installation quality, and airflow conditions. The findings provide quantitative insight and practical guidance for improving performance evaluation and design of reflective insulation systems.