It should be applied between the sash and the frame, but should not interfere with the operation of the window. The type and quality of the window frame usually affect a window's air infiltration and heat loss characteristics. When properly installed, fixed-pane windows are airtight and inexpensive and can be custom designed for a wide variety of applications. But, because they cannot be opened, fixed-pane windows are unsuitable in places where ventilation is required. Casement, awning, and hopper windows with compression seals are moderately airtight and provide good ventilation when opened.
Casement windows open sideways with hand cranks.
Awning windows are similar to casement windows except that their hinges are located at the tops of the windows instead of at the sides. Hopper windows are inverted versions of awning windows with their hinges located at the bottom. Windows with compression seals allow about half as much air leakage as double-hung and horizontal sliding windows with sliding seals. Although they are among the most popular type of window, double-hung windows can be inefficient because they are often leaky. Single-hung windows are somewhat better because only one sash moves.