Thermal Transmittance (U-value) Evaluation of Innovative Window Technologies

Energy consumption in the construction sector has increased in recent years, as buildings account for more than 40 % of the energy consumption for most countries (IEA, 2013). In order to reduce this impact, it is relevant to promote stricter regulations focused on the developing of novel technologies on the building’s envelope. Fenestration elements play a particular role in this interaction, since they are the weakest point to uphold insulation in buildings. Windows allow bringing solar heat gain, natural light, ventilation and sound vibrations to interior spaces, and similarly they are a barrier to inclement weather conditions. These elements are extremely important to estimate the building energy performance since they act as an envelope for thermal comfort in spaces. In general, the assessment of the window’s thermal and optical properties utilises three parameters: visible transmittance, solar heat gain coefficient (SHGC), and thermal transmittance (or U-value) (Aguilar-Santana et al., 2020). Thermal transmittance is relevant for the analysis of insulating properties of glazing materials as per the main objective of this paper, and Figure 1 presents examples of their conventional values. An important part of the energy consumption is associated to the heating and cooling demands from buildings, especially those related to transparent surfaces (windows, skylights and roof lights). These elements contribute significantly to the energy losses in colder climates (Jelle, Kalnæs and Gao, 2015; Velasco-Carrasco et al., 2020). An inverse relation occurs in warmer climates, where these glazing elements contribute significantly to the cooling demand of HVAC systems due to the heat gains through poor insulating materials displayed by traditional glazing systems. Windows with materials of higher heat transfer rate report the highest U-values regardless of the window assembly; this as a result of the environment’s interaction through the building envelope in the building’s energy losses (Aguilar-Santana et al., 2020).