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• The estimated U-factor of the studied algae window profile was 4.9 W m−2 K−1, VT and SHGC differ significantly according to the studied microalgae specie and concentration in the window system. • The maximum available energy saving of the studied algae window significantly differs according to the façade orientation and reached up to 20 kWh m−2 year−1 in the South façade. • Algae concentration, window size and the combination factor of algae concentration with window size were found to be statistically significant in affecting the energetic impact of the algae window, for each façade orientation. The present study focused on analyzing the potential impact of incorporating living microalgae to the built facades, Algae Window, on the energy consumption reduction of a building. Two microalgae species of Chlamydomonas reinhardtii and of Chlorella vulgaris were cultivated and the impacts cells density were studied on the light penetration and heat transfer. The experimentally measured impacts of the two studied microalgae species were used to calculate the U-factor (Thermal conductance), VT (Visible Transmittance) and SHGC (Solar Heat Gain Coefficient) of the Algae Window. Based on the empirical results, the impact of the algae window on the energy consumption was estimated by extensive simulation study within an office space in the LEED accredited Porter building in Tel-Aviv University, Israel. The results show that incorporation of the microalgae into the windows has the potential to improve the energy efficiency in the studied building under the conditions of the Mediterranean climate. The impact of the algae window on the energy consumption was estimated in comparison to single glazing and to double glazing, and was found to differ significantly according to the facade orientation in both microalgae species; at maximum concentrations in the algae window as compared to single glazing window, the energy saving reached up to 20 KWh m−2 year−1 in South, 8 KWh m−2 year−1 in East, 14 KWh m−2 year−1 in West, and energy increase up to 18 KWh m−2 year−1 in North. Three factors were found to explain the variance in the energy saving performance of the Algae Window, namely, the algae concentration, the window size and the combination factor of the algae concentration with the window size that yielded the largest effect on decreasing the energy consumption. This study suggests that incorporating microalgae cultivation in building windows can provide energy saving to the building and addresses the main design factors that can effect on the savings as well as on other energetic aspects involved in the system such as energy production from algal biomass that has multiple applications in the urban environment. [ABSTRACT FROM AUTHOR]