Since 1960's concrete based wall panel systems have become more popular especially in multi-storey residential buildings. The main components of concrete are water, cement and aggregates and especially water-to-cement ratio has a significant influence on the pore structure of the concrete and therefore, on its electrical properties. This paper presents a study where the effect of moisture on the RF attenuation of the outermost concrete sandwich element is examined based on material samples. The frequency range used in the measurements was 7 GHz to 13 GHz, but the results are expandable for other frequencies based on the stable permittivity value. In particular, at higher frequencies, the attenuation of concrete layer increases rapidly and already at lower frequencies of 5th Generation (5G) networks, the wetting of the outermost layer of the concrete element can cause an 6-12 dB additional signal attenuation.