First commercially available thermally insulated glass unit (IGU) was made in USA in 1930 and was named ‘’Thermopane’’. This is why nowadays most of the insulated glass units are called ‘’thermopane glass’’. During the following decades the development of insulated glass units progressed, and the air between the panes was replaced by inert gases such as argon and krypton. There was a problem with gases leaking from the glass unit and it was complicated to achieve such tightness that would prevent gas leakage and consequently the loss of thermal insulation and occurrence of condensation between glass panes. IGUs today provide better tightness and minimize the gas leakage. European standard EN 1279 demands that only 1 % of gas is allowed to leak in 1 year, which in theory means that it takes 100 years before the gas will be fully gone. In practice, the expected lifetime is shorter since already from the beginning the space is filled with only 95 % of inert gas and 5 % of air. The main indicator for inappropriate tightness of IGU is the presence of condensation in cavity between the panes. IGU manufacturers provide a 10-year guarantee on their tightness.