If you're experiencing high energy bills, a lot of draftiness, or a lot of condensation, you may want to consider at least a partial window replacement — whether that's replacing an insulating glass unit or sash (in the event of window seal failure, indicated by haze or fog), the sashes along with covers and hardware (insert window replacement), or absolutely everything (full frame replacement). Full frame replacement can be a pricey proposition however, so it's best to know when it's absolutely necessary!