Orientation-Dependent Daylight–Sunlight Trade-Offs in a Tropical Single-Aspect Studio Apartment: Effects of Window Configuration and Visible-Light Transmittance

In single-aspect studio apartments, daylight sufficiency and sunlight exposure constrain window design simultaneously. A studio reference room in Bangkok, Thailand was evaluated using Radiance through Ladybug Tools and a TMYx weather file. Six main-window configurations, three visible-light transmittance (VLT) levels (0.45, 0.65, 0.88), and four orientations formed a complete factorial set of 72 cases, assessed with spatial daylight autonomy (sDA300/50%), useful daylight illuminance (UDI100–3000), and annual sunlight exposure (ASE1000,250). The room was also simulated with the balcony door alone, giving a simulated baseline. Within the investigated design space, orientation accounted for 93.16% of the variation in ASE and 63.36% in UDI, but only 3.39% in sDA, which was governed instead by VLT (69.10%) and window configuration (23.23%). sDA ranged from 52.17% to 100.00% and 19 cases did not reach 75%; ASE ranged from 1.09% to 53.26%. The door alone gave a mean sDA of 47.10%, and the smallest window raised it by 25.00 percentage points. A screening rule informed by LEED v5 retained 29 cases and no west-facing case; substituting finer-grid values for four near-threshold cases reduced this to 28, testing that subset rather than grid independence across the dataset. Orientation and solar exposure should be assessed first, transmittance second, and window area last.