sliding windows: gliding and double-hang

Sliding windows are less energy efficient than casement windows (and awing and hopper). They lack the compression seals, and their design turns them potentially much more leaky (see: Sliding Windows Weatherstripping).

Gliding windows Advantages

Gliding (horizontal sliding) windows are very common, in part because they are easy to open, close and clean. And also because when they are open they are usually less obstructive than casement, awning and hopper windows.

Pros of gliding windows

Besides being easy to open, close and clean, horizontal sliding windows can be used in big sizes, without any restrictions.

Cons of horizontal sliding windows

Horizontal sliding windows tend to be less energy efficient than casement windows (and awing and hopper). They lack the compression seals, and their design turns them potentially much more leaky.

Casement (and awning and hopper) windows are a much better option in terms of energy efficiency.

Double-hung windows

Double-hung windows have top and bottom sliding sections (sashes), and can be opened by pulling down the upper sash or by pulling up the lower sash. Double-hung windows are popular, largely due to their aesthetics.

Pros of double-hung windows

The advantages of these windows reside in their aesthetics and in small details: they are easy to open, and to clean, and have a classical and attractive design.

Cons of double-hung windows

These windows have the same disadvantage of gliding windows: they are potentially too leaky. In other words: there are better window options in terms of energy-efficiency.