Windows were traditionally used as a means of building access and egress for RF signals. However, the drive towards building energy efficiency now means that windows are frequently coated with thin layers of Transparent Conductors (TCs). TCs can let visible light energy pass virtually un-attenuated, but reflect longer wavelengths (typically from the infrared region), to keep buildings warmer in colder climates and cooler in warmer climates. However, the use of TCs has a negative impact on wireless propagation, which this paper reports on. Three commercially available windows are examined, with results showing that less than 1% of the signal passes in either direction. This result is taken from a series of measurement over a range of frequencies from 800 MHz to 6 GHz, both in a large hall and in the WIT anechoic chamber.