As the thermal and sound resistance of windows and doors is lower than of other construction elements and materials, more and more often insulating glass units are preferred. They increase the thermal and sound resistance of windows, make windows simpler and more compact, they are easier to maintain and they improve the aesthetic look of a window.
An insulating glass unit may consist of two or more glass panes. Glass panes are hermetically bonded on an aluminium spacer with butyl sealant and adhesive mastics. The function of butyl sealant is to make the insulating glass unit hermetic and to resist moisture entry in the insulating glass unit. At the same time butyl sealant interrupts heat transfer between glass panes and an aluminium spacer. Adhesive mastics forms an elastic bond between the glass panes and the spacer and bonds these strongly to one another. Air or gas remaining between the glass panes is dried with moisture absorbing substance (absorbent) inside the spacer. An insulating glass unit may be composed of plane float glass panes and decorative glass panes, also of different glass panes with structural surface cover, toughened and laminated glass panes. As the choice of glass panes is very wide, it is important that the choice of an insulating glass unit be based on its function.
The choice of an insulating glass unit should be based on the following:
• thermal transmittance U-value, i.e. K-factor (W/m²K);
• sound insulation index Rw (dB);
• if required, blocking UV-radiation;
• also the properties of the external (in some cases also internal) glass pane regarding reflection and absorbing of solar energy should be considered;
• depending on the location also security requirements should be considered – whether glass pieces should not cause injuries when the glass pane is broken, whether the glass panes should be resistant to vandalism, burglary and bullet, whether the glass panes should be fire safe – simply fire-retardant (E) or in addition also blocking radiation heat transfer (EI);
• thickness of glass panes – the glass panes should resist their width and height (and ratio thereof) considering with the imposed load (wind, snow, etc.).
There are various types of glass panes to choose from:
• Antisolar glass panes
• Fire retardant glass panes
• Security glass panes