There are many kinds of insulated window these days. The frame could be wood, vinyl metal or mixed of them. The glass could be glazed or unglazed. There could be filled with Low-conductivity gas instead or air. The R valued ranges for R2 to R6. It is difficult to choose one since there are variety of price and quality.
The space between two glasses should be sealed. If the seal is broken, the window may fog up. Only way to fix it is to replace it. For regular non Low-conductivity gas window, you don’t lose much thermal value even it fog up, it just looks bad.
Because the way they make the window, the seal will fail eventually even for best quality window. Not if but when it will fail. You should able to tell the quality of the window according to their warranty. A 20 years warranty should be better than 10 years one.
You should look into warranty more carefully; some 20 years warranty only applies to original purchaser. This is same as fraud. In North American, average people move every seven years. If warranty is not transferable, they basically get away with most of their responsibility.
Never replaced window, nothing to recommends. Hope the information could help you.
