By now you have probably seen a fogged window or two. There are some that can be repaired, however it will not have the insulating gas between the panes of glass as it did originally. The only way to do this is to replace the window assembly.
A typical double pane window here in the south, will have an R value of somewhere between 2.0 & 3.0. After it looses it's seal this diminishes even more. To put this R value in perspective, insulation in your attic built locally here today would be an R38! Huge difference.
So, window manufactures also take into account U values which is how well or poorly it transfers heat
R-value, U-value or U-factor
– R-value indicates the resistance to heat flow through a window. U-value, the inverse of R-value, indicates a window’s tendency to transfer heat. Both R-value and U-value are measures of heat flow through a window.