A single hung window consists of two sashes, one on the top that does not move and another one on the bottom that does move vertically. Contrast single-hung windows with fixed windows (consisting of one sash that does not move) or double-hung windows (consisting of two sashes, both of which move up and down).
Single-hung windows are available in both new construction or replacement window styles. The chief advantage of the fixed window is that it limits air intrusion, since there are fewer seams that must be blocked with window seals. As time goes by, window seals begin to dry and hard, creating cracks which allow for air seepage. Also, some fixed windows may be less expensive than double-hung or casement windows, due to fewer mechanical parts.