Transforming Residential Lightwells into High-Performance Daylighting Systems Through Optical and Geometric Retrofit Optimization

Narrow internal lightwells are often featured in multi-story residential buildings in hot arid climates in order to daylight rooms where direct façade access is not possible, but privacy-driven window closure strongly limits the daylighting potential of these shafts. This paper examines whether three retrofit daylighting strategies—(1) increased shaft wall reflectance, (2) multi-level tubular daylighting devices (MTDDs), and (3) inclined lightwell remodeling with clerestory openings—can retrofit a common residential lightwell in Ha’il City, Saudi Arabia for high performance, high-efficiency daylighting performance under typical privacy-constrained urban conditions. A calibrated daylight model was developed using field measurements of the existing lightwell conditions, and the three retrofit strategies were then evaluated through Radiance-based simulations. The calibrated Radiance model showed good agreement with observations (MBE = −9.6%, RMSE = 17%). It was found that the aged lightwell received less than 0.5% of outdoor daylight in clear summer conditions and that the performance was highly seasonal and orientation-dependent, with 15–20% higher summer peaks in one monitored room, while both rooms remained largely below 100 lx in winter and overcast periods. Simulations determined that the base case results in the ground and first-floor areas remaining comparatively lowly illuminated (UDI<100 lx = 96–100%). The wall reflectance contributed only moderate improvements, while the MTDD strategy was notably effective in improving the daylight availability of lower-floor levels without increasing annual sunlight exposure. The biggest gains (first-floor mean illuminance increasing to 516–952 lx and 100% sDA300,50% on the second floor) were for the lightwell inclined with clerestory opening type, but an increased risk of over-exposure at upper levels was activated. These findings demonstrate how balancing the needs of daylight autonomy with exposure control shapes the performance of daylight retrofits.