Replacement windows are designed to slip right into the spaces formerly occupied by old, poorly-performing windows. The windows come set in aluminum, wood, or vinyl frames. There are two basic options when replacing windows, a sash kit and total replacement. A sash kit allows the current glass pane to be reused, while total replacement involves tearing out the old window and installating a completely new unit. Replacement windows come in hundreds of sizes in order to accommodate your needs. Your window unit should be about one inch narrower and one half inch shorter than the opening into which it will be installed. The glass in most replacement windows has been improved by energy-saving technical advances, making the replacement windows very energy efficient. For more information about replacement windows, please contact a home improvement professional.