Short answer
Fog or a cloudy haze between the two panes means the window's seal has failed and moisture has gotten inside the sealed unit. Cleaning can't fix it, because the haze is between the panes where no one can reach. The fix is replacing the glass unit (or the window), not cleaning it.
How to tell if it's inside or out
Wipe a small spot on each side of the glass. If the haze is still there, it's between the panes. Seal failure often shows up as fog that comes and goes with temperature, then becomes a permanent white film or mineral streaks.
What we do when we see it
We clean the inside and outside surfaces like any other window, and let you know which units look like failed seals so you're not surprised the haze remains. We don't repair or replace glass, but that list is handy when you call a window company.
Is it urgent?
Not usually. A failed seal reduces insulating value and looks cloudy, but it isn't dangerous. Many homeowners replace several units at once when it's convenient.
Rather have a pro handle it?
Pro Solutions does window cleaning across Boise and the Treasure Valley. Quotes are free, itemized, and usually texted the same day.