Replacement Window GuideA single-hung window looks like a double-hung but only the bottom sash moves. That one difference cascades into three practical effects: a lower price, less upkeep, and a fixed top half that gives leaking air fewer openings.
Only the bottom sash operates
The defining trait is the fixed upper sash. Andersen states that a single-hung window only opens from the bottom and that the top sash of a single-hung window is fixed.
That simpler build is the reason the style is common in budget-conscious projects. Andersen notes the design is simpler with fewer moving parts and pieces due to the fixed top sash.
Cost, maintenance, and air leakage
Fewer operating parts translate to lower cost and upkeep. Andersen states that single-hung windows cost a bit less to produce, and this is reflected in the price, and that fewer operable parts mean less care will be required over time.
The fixed top also helps the energy picture. Andersen notes the top half of a single-hung window is fixed, meaning it doesn't open, so there is less opportunity for leaking air.
What the energy label still decides
A fixed top sash lowers leak potential, but the label still reports the full performance. ENERGY STAR states that U-Factor measures the rate of heat transfer and tells you how well the window insulates, with lower being better.
Those ratings are independent. ENERGY STAR states that the energy performance of all certified windows must be independently tested, certified, and verified, so compare labels on the exact single-hung model and glass package you are quoted.