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Modern houses and constructions use energy efficient building materials, like metal shielding and energy saving windows, to improve the thermal insulation and efficiency. However, such energy efficient building materials not only block the thermal radiation but also substantially increase the building penetration losses of the radio signals, forming thus a challenging problem in mobile cellular networks where majority of network elements are still located outdoors while more and more of the mobile data use takes place indoors. In this article, the use of frequency selective surface (FSS) is studied as one potential means to reduce the penetration loss while still maintaining high thermal insulation of the buildings. FSS structures build on a combination of either conducting patches or apertures in a thin conducting sheet arranged periodically in one or two dimensional array, and can be etched on the metal coating of energy saving windows that behave as electromagnetic filter. Thus, such energy saving windows incorporating FSS structures can potentially allow transmission of radio signals while still essentially retaining its major thermal insulation properties. The aim of this paper is to investigate the applicability of such FSS structures in realworld scenarios with actual field measurements in real mobile networks. First, double square-loop FSS structure which is transparent to GSM 900MHz and UMTS/HSPA 2100MHz frequency bands is developed and analyzed. The developed FSS structure is then fabricated using aluminum foil and tested and measured in both laboratory conditions as well as in real mobile networks and real buildings. In the mobile network measurements, the developed FSS prototype shows substantial improvements of the indoor signal strengths, in the order of 8-13 dB for GSM 900MHz band and 2-5 dB for UMTS/HSPA 2100MHz frequency band, compared to the uniform metal coating. This indicates that FSS structures can provide useful and feasible means to enhance the indoor signal levels in future energy efficient built environments, and thus substantially enhance the indoor coverage and capacity of wireless networks.