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').