How the Style of Window Frame Effects Efficiency
Along with a U-factor for overall energy efficiency, window companies should also list the air leakage (AL) value on each model. The AL number rates the amount of air that passes through the window frame. It is typically measured in cubic feet per minute per square foot of frame area. Look for the lowest AL. Highly efficient windows have values as low as 0.01.
The AL varies widely according to the window style. Fixed non-opening windows allow very little air leakage. They also don't allow much ventilation, and should not be the only windows in a room. Hinged windows that swing open either vertically or horizontally are the next most efficient option. They provide a tight seal when closed. Sliding and double-hung windows are the least efficient style because they do not create a tight seal.