Steel truss structures, covered with a PVC thin sheet of matt light colour, were used in recent time for the construction of three new classrooms in the University of Catania (southern Italy). Although the construction was fast and cheap, the classrooms proved to be uncomfortable in relation to thermal and luminous environment. A first analysis revealed that the drawbacks were caused by the ultra-lightweight of the structure as a whole, resulting in thermal discomfort. Furthermore, the matt light colour gave raise to an excessively uniform luminous environment, with occurrences in discomfort glare. To correct the architectural errors, a simple technical solution was adopted in one of these classrooms. Before and after the intervention, the thermal and luminous parameters were recorded. In addition the thermal sensations experienced by the students were collected by a questionnaire. The comparison of the two set of data shows that the new design resulted in beneficial effects. Final remarks are made about the relationship between ultra-lightweight structures and local climate.