This is one service that is so unique and so useful that I had to include it in my recommended products section of my site. No they don't pay me. I should work on that.
When condensation shows up inside a thermal pane double window (between the two pieces of glass), you know that the seals around the edge have been compromised, allowing moisture to get inside. These cannot really be re-sealed so the standard response to this problem is to replace the glass unit. That is expensive.
If the window frame and the mechanisms are in good shape that you are considering changing just the thermal glass unit rather than the entire window, you might want to consider a new and growing service that takes something that we used to do with great fear of breaking glass (see my web entry on moisture in windows), drill some holes to let the sun dry out the window.
In fact the professional services process does much more than just drilling holes in the windows. They wash out the space inside the thermal pane window and put in a permanent valve that keeps the window clear even though the seals are broken. They can even fix scratches on the glass just like you have done to the windshield of your car. The process won't bring every old foggy window back to life, for instance when the moisture has etched into a Low-E coating on the inside, that cannot be repaired. That is one reason to have windows repaired as soon as you see the first signs of moisture inside between the two pieces of glass.
You will probably find this service more expensive than you expected so before you ask for an estimate for this clearing and venting process, get an estimate for the cost of replacing the glass unit or replacing the whole window so you will know which choice of the three is most economical in your case. Although it is generally less expensive than replacing the entire window, it is not worthwhile doing if the rest of the window is reaching retirement.
Yes windows will be slightly less efficient with a vent valve installed than if that space were effectively filled with special insulating gasses -- but it will be more efficient than a window filled with humidity as moisture inside the window actually conducts heat towards the outdoors.
One of the great aspects of this process is that it works continually, so if more moisture wants to sneak into the window, it is kept clear, and it keeps an even efficiency over time equal to a newly installed window that did not have insulating gas.
Remember to do your financial math. If the window or door frame is really shot, don't bother to spend the money on getting the glass clear. If you have several years service left in the frame and hardware, then an investment in clearing the window should be worth it. In one place at one time (prices change all the time) having a new thermal pane installed in a patio door cost about $350, while having the old one professionally cleared with a 10 year guarantee was about $200. A whole new patio door installed was close to $1,000.