How are windows designed to hold moisture?
Desiccant is an absorption material which is used in all windows. 
This material is in the spacer bar and functions much like a sponge. 
As 
Solar Pumping naturally produces moisture within the window,
the desiccant absorbs it. 

It then dries out as a sponge would sitting on a window sill. 
Overtime, the desiccant, much like a sponge, it can not absorb
as much moisture as it originally was designed to do. 

The result is moisture forming on the glass (i.e. a fogged window). 

Every windows has a finite capacity of how much water it can hold ('hide').