Inexpensive thermal panes and older ones lose their seal and allow moist air in them.Basically they are 2 panes of glass with a sealed spacer and inert gas between them. 3 cures1 Disassemble clean reassemble and replace inert gas. Not easy to do.2 Buy new thermal panes. Easier to do but not that cheap.3 Vent the thermal panes so the moist air can cycle out. This is the cheapest in the short run, it is a bit of work, and you loose some of the insulation value of the thermal pane. But if you plan to sell the house or just need time to afford new windows it works. Take the thermal panes out of the window frame.Measure the spacers between the panes of glass, should be 1/2 inch.With a 3/8 drill bit, drill four holes in the spacers between the panes 2 on top of the thermal pane and two on the bottom. Be careful not to hit the glass with the drill bit, best to make a pilot hole first. DO NOT DRILL ALL THE WAY THROUGH THE SPACER.If the spacer has solid foam in them continue drilling. If the spacers have little balls of foam in them. Ether take all the balls of foam out of the spacer or put a 3/8 liner in the hole to keep the balls of foam in place and use a smaller drill bit to continue drilling. Drill corresponding holes in the window frame and put the thermal pains back in the window frame. This can be a bit of a challenge if the frame is plastic or aluminum.