Demands for improved energy conservation of windows and glazing and for solar shading systems installed in the cavity between glass panes mean that the cavity in insulating glass units have to be increased. However, there are tight physical constraints because larger cavities lead to greater climatic stresses of the glass panes and edge seal and hence to breakage and leaks. These constraints can be eliminated by changing the construction of the glass units by equalising the pressure from insulating glass units. The thickness of conventional insulating glass units with their hermetically sealed cavities is limited owing to climatic stresses resulting from the construction. In order to avoid these and the climatic stress, a connection had to be established between the cavity and the ambient air pressure. Glass units with this type of construction are also referred to as pressure-equalised insulating glass units (PE-IGUs). Advantages of equalising the pressure between cavities and the surrounding atmosphere: 1. it is easier to integrate various components in the cavity (e.g. solar shading systems); 2. it is possible to construct insulating glass units with more than three panes without significant restriction of the cavity width; 3. glass units can have greater depth, thus reducing the effect of the geometric thermal bridges at the connection with the building fabric; 4. thermal transmittance is reduced compared to conventional double and triple glazing; 5. improved air-borne sound insulation; 6. reduced risk of glass breakage and extended service life; 7. potential reduction in glass thickness. A physical calculation model was developed for dimensioning possible “pressure equalisation systems” which, in the form of a simulation tool, is easy to use in practice. The durability of the selected pressure equalisation systems was tested in laboratory and open air tests. In addition, the moisture entering the insulating glass units is much reduced. Depending on the format, construction, climate exposure and intended degree of pressure equalisation, a service life expectancy of over twenty years for capillaries and of over forty years for valves appears realistic.