Search for the optimal window-to-wall ratio in office buildings in different European climates and the implications on total energy saving potential

• Window-to-wall ratio (WWR) that minimizes total energy use in four European climates.
• Integrated thermal-lighting simulations on a low-energy office building.
• Most optimal WWR values in the range of 0.30–0.45 – but not for south-facing façades.
• With best façade technologies, moderate impact of WWR on energy performance.

This paper deals with the search for the optimal window-to-wall ratio (WWR) in different European climates in relation to an office building characterized by best-available technologies for building envelope components and installations. The optimal WWR value is the one that minimizes, on an annual basis, the sum of the energy use for heating, cooling and lighting.By means of integrated thermal and lighting simulations, the optimal WWR for each of the main orientations was found in four different locations, covering the mid-latitude region (35° to 60° N), from temperate to continental climates. Moreover, the robustness of the results was also tested by means of sensitivity analyses against the efficiency of the building equipment, the efficacy of the artificial lighting and the compactness of the building.The results indicate that although there is an optimal WWR in each climate and orientation, most of the ideal values can be found in a relatively narrow range (0.30 < WWR < 0.45). Only south-oriented façades in very cold or very warm climates require WWR values outside this range. The total energy use may increase in the range of 5–25% when the worst WWR configuration is adopted, compared to when the optimal WWR is used.

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