Replacement Window GuideA double-hung window has two sashes stacked in a single frame, and both of them move. That layout is the reason people pick this style for bedrooms, kitchens, and older homes where the classic look matters. The trade-off shows up on the energy label, because sliding sashes seal differently than a window that presses shut against a frame.
How a double-hung window operates
Andersen describes the double-hung as a window with two operating sashes, in contrast to a single-hung window where only the bottom sash moves. Because both sashes slide vertically inside the frame, you can lower the top, raise the bottom, or move both at once.
That two-sash design is what separates a double-hung from a single-hung. Andersen states that the top sash of a single-hung window is fixed, while a double-hung lets both the top and bottom operate.
Ventilation from the top, the bottom, or both
The main functional benefit of two moving sashes is airflow control. Andersen notes that opening both the top and bottom sashes can vent hot air out while letting cool air in, which is useful for rooms that heat up during the day.
Because you can crack the top sash instead of the bottom, a double-hung is also a common choice where a wide-open lower sash would be a safety or privacy concern.
Cleaning: sashes that tilt inward
Cleaning is where the double-hung earns a lot of its popularity. Andersen explains that the sashes tilt inside the home so you can clean both the interior and exterior glass without going outside.
That matters most on upper floors, where reaching the outside of a window otherwise means a ladder or a service call. The tilt-wash feature keeps the whole job at arm's length from inside the room.
What the energy label numbers mean
Every certified window carries an NFRC label. ENERGY STAR explains that the label reports performance in five categories, and the two headline numbers are U-Factor and Solar Heat Gain Coefficient. U-Factor measures the rate of heat transfer and tells you how well the window insulates, and the lower the U-Factor, the better the window insulates. Solar Heat Gain Coefficient, or SHGC, measures the fraction of solar energy transmitted, and the lower the SHGC, the less solar heat the window transmits.
Double-hung windows also carry an Air Leakage rating, which ENERGY STAR defines as the rate at which air passes through joints in the window, where a lower value means less air leakage. This is worth watching on a double-hung, because Marvin points out that double-hung windows are less airtight than casements since they do not have the same locking mechanism that pulls the entire sash in tight.