Glass is construction of two or more glass, between which there is an air gap that occurred with different size spacers and hermetically closed. 
Good glass must be double barrier. One barrier in the glass is butylene affixed on both sides of the spacer and the second is silicone, polysulfides or tiokolat filling the space between the panes and the spacer. In operable windows can not penetrate water outside and inside the closed moisture with air in making it is absorbed by molecular sieve, which is filled spacer. Thus, between the panes can not be formed condensate.
Glazing may be several types and for different requirements: low-emission, energy saving double glazing, triple glazing, insulation, sound insulation, argon.