New types of energy smart window curtains, with the aim to reduce the heat losses through windows have been studied. By covering windows with these curtains, for instance when buildings are not occupied, the energy performance can be improved and becomes more economical and sustainable. The paper aims to evaluate the energy performance of the curtains by in-situ measurements and simulations. In this paper, two different designs of these curtains have been studied. Curtain design (A) has the aim to increase the overall thermal resistances of window constructions and by that decrease the heat losses through windows. Curtain design (B) has an additional solar collector function which is aimed to capture some part of solar radiation striking windows and warm up the indoor environment. Warmer air is forced into the indoor environment by small fans inside the curtain. The paper investigates also the possibilities to optimize the design of the curtains regarding the emissivity of the material used in the design, using analytical models. During a 27 days measurement campaign, interior and exterior temperatures, intensity of solar radiation and energy consumptions in three identical test huts are continuously measured. The elevated interior temperatures are achieved using radiators. Results from the pilot study show that the total U-value of windows can be improved by using the curtains. For the case studied in this project, using the curtain design (A) improves the U-value of the window by approximately 50 %. For Curtain design (A) a surface to surface resistance of 0.65 m2K/W (3.69 ft2. h. °F 2/Btu) and for design (B) a resistance of 0.38 m2K/W (2.16 ft2.h. °F 2/Btu) is estimated.