Understanding the individual components is helpful in answering that very question.

THE THERMOPANE

The glass unit in a window is referred to as the thermopane, which is in itself a combination of vital components. An efficient thermopane will have not only two or three panes of glass, but also the following:

• LowE glass: Low Emissivity (LowE) glass has a metal-like coating, which reflects long-wave heat energy back into your home in the winter, while heat rays are reflected towards the outside in summer. LowE also prevents fading by filtering UV rays.
• Inert gas, such as Argon: Argon is a clear, non-toxic gas used to fill the thermopane. Heavier than air, argon offers greater resistance to heat loss and reduces exterior noise.
• Low-conductivity or “warm edge” spacer: A spacer keeps the glass panes in a window the correct distance apart, but also plays a larger role in creating a thermal break, insulating the edges of the pane and reducing heat transfer.

THE FRAME

The frame material also contributes to the overall efficiency of a window. Vinyl (PVC) windows are constructed with many air chambers that make it a good insulator. Wood is also a good insulator; however, aluminum window frames conduct heat very rapidly and therefore require a thermal break to improve insulation values.

NO AIR LEAKS HERE

Look for a window that has durable, multiple weather-stripping. Good-quality hardware is also vital to a tightly sealing window, as are solid, well-formed joints.

Casement and awnings are, by design, more efficient in terms of airtightness. The wind will push the casement sash against the frame, creating a tighter seal. That being said, innovations in hardware and design mean that good-quality sliding windows can now perform at similar levels.