Insulating glass units (IGUs) are composite structures typically consisting of glass plates, spacers, and internal gas. The edge spacers enclose the gas, which plays a key role in providing effective thermal and acoustic insulation, as well as contributing to energy efficiency. However, most research has treated these structures without adequately accounting for the real influence of the internal gas, which can undergo density variations due to climatic factors, temperature changes, and, more broadly, climate change. For this, in many cases IGUs are also used by combining with photovoltaic panels. In this paper, an accurate analytical and numerical analyses have been carried out to quantify the dynamic behaviour of IGUs, as a function of the glass and cavity thickness, glass size, temperature variations, gas density variation, gas type, etc. Some scenarios and benchmark examples have been analysed. Results shown that the gas presence can produce a system frequency greater of ~5.0 than the fundamental frequency of the glass. More significant effects were noted for higher vibration modes. Additionally, results shown that, due to daily variations of solar radiation and environmental temperature, the gas is subjected to temperature variations in the range about ±20.0 °C, leading to density variations that significantly impact the frequency response of IGUs.