With respect to thermal performance, windows are the weakest component of the building envelope, essentially because the U-value is usually higher than the opaque envelope. This would allow the highest heat conductance of the building envelope. However, it also helps buildings to gain useful solar heat during winter. Therefore, it has been generally accepted that passive buildings would have small windows towards the poles and large windows facing the equator (Persson, Roos, and Wall 2006). In spite of this guideline, large or fully glazed facades have been used in modern architecture. The intensive use of air conditioning is the result of overheating and high thermal loss problems, which otherwise would lead to thermal discomfort. This extensive use of large windows associated with high energy consumption has motivated researchers to study this building component.

Window areas were investigated by Persson et al. (Persson, Roos, and Wall 2006) on 20 terraced houses with larger windows facing the equator and built in Gothenburg. The building envelope was well insulated and fitted with energy efficient windows. It was found that energy efficient windows do not have a major influence on the heating demand in the winter, but it is relevant for the cooling need in summer. Therefore, reduced indoor illuminance due to small windows can be solved by enlarging them in order to obtain relevant daylighting conditions. When efficient windows are designed for a warm climate, as in Mexico, reducing heat flux and solar transmittance indoors was the best option for energy savings (Aguilar et al. 2017). However, reducing solar transmittance influences the indoor illuminance, which was not analyzed.

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Author notes

1. Centre of Materials and Building Technologies (CMADE-UBI), Universidade da Beira Interior, Rua Marquês D'Ávila e Bolama, 6200-001, Covilhã, Portugal, jcarlos@ubi.pt