Yes. You need to replace them both.You can replace the glass in a double-hung window for about a third of the cost a professional would charge. In fact, the only time we'd hire a pro for this task is when the sash is so horribly encrusted with paint that you'd all but destroy it by removing it. Also, if you need to replace an insulated window, a trained specialist may be the best bet for a proper installation. Good Luck..