Over the past years, increasing efforts have been devoted to understand how people perceive the indoor environment with the goal to improve building design and operation. Studies have separately addressed the effects of the four main aspects that influence the perception of the indoor environment, i.e., visual appearance, thermal condition, acoustic ambiance, and air quality. However, building occupants are simultaneously exposed to multiple indoor stimuli and their perception of the indoor environment depends on the mutual combination and interaction of such stimuli. Despite the increasing interest in the topic, few studies on the interactions between indoor factors are available. Most of them have focused on interactions between thermal and visual factors due to their large implications for energy consumption and building design. In this context, electric lighting has been primarily used as visual stimulus, leaving a knowledge gap on interaction effects in daylit environments.

To address this challenge, this doctoral thesis focuses on the effects of visual and thermal interactions on human responses, with the use of daylight as the sole source of light. Two types of visual-thermal combinations are investigated: temperature and "daylight color" (i.e., daylight transmitted through colored glazing) and temperature and daylight quantity (i.e., illuminance levels). The aim of this work is threefold: (i) understand the effect of daylight color and quantity on thermal perceptual and physiological responses; (ii) investigate the influence of indoor temperature on visual perception of daylight; and (iii) explore the effect of daylight and temperature combination on the overall comfort perception. To address the previous aspects, four experiments were conducted. Two of them were carried out in an office-like test room to investigate both visual-thermal combinations. One field study was performed to further explore temperature and daylight quantity interactions. One experiment was conducted in a test room with the use of virtual reality to further investigate temperature and "daylight color" interactions.

The results of this thesis show that daylight and temperature do interact, influencing human responses in a perceptual rather than in a physiological way. The interaction effects are bi-directional, as temperature influences visual perception, and daylight color and quantity affects thermal perception. Daylight conditions and indoor temperatures simultaneously and equally contribute to the overall comfort perception, but only after a relatively long exposure to the visual stimulus.

The findings of this research suggest that indoor factors should not be considered separately but in combination. The integration in both research and practice of interaction effects between indoor factors will ultimately contribute to the improvement of comfort conditions inside buildings. Potentially, this integration will also allow to reduce the energy consumed in buildings to guarantee occupants' comfort due to the extension of the comfort ranges that currently neglect interaction effects between indoor environmental factors. Disentangling for the first time the complex relationship between daylight and indoor temperature, this research constitutes an additional step towards understanding interactions of indoor factors and contributes to the establishment of a comfortable and energy-efficient built environment.