The gap found in double glazed windows runs from 6 mm to 16 mm (1/4 to 5/8 inch). Double glazed windows use two panes of glass that are separated by a gap. The gap is filled with air or other gases, known as insulating gas. The width of the gap will affect the insulation properties of the window, as well as noise reduction.
The gap between the two panes of glass is very important, as this serves as insulation, stopping heat from escaping out the window. For double glazing that contains air in the centre, the gap should be the largest gap size available, at 16 mm (5/8 inch). The larger the gap's size, the less chance that the air will conduct heat across the gap and release it to the outside. The larger the gap, the less heat loss across the window.
The gap size will also affect the noise reduction abilities of the double glazed windows. A gap size of four to eight inches is recommended for noise reduction. This is much larger than what is typically available for double glazing, but it is possible for triple glazing. Think about whether you want noise reduction to factor into your window choices, or if you are mainly after additional insulation and draft removal.
The current depth of your wall will play a role into the size of the gap that can be used for double glazed windows in your home. The gap used in the double glazing will be constrained by your current wall width. If you cannot install double glazing that contains a large enough gap, then consider secondary glazing. The second pane of glass that makes a gap can be fastened to the current window frame or sash. This may be a good option to achieve the amount of insulation you desire if wall depth is an issue.
Compare the U factor for each of the double glazed widows you are considering. The U factor is the rate of heat lost through the window, and includes the window and window frame material as well as the gap size. Glass, plastic and special coatings can all be used for double glazing. The different materials have different heat and light transfer rates. Therefore, a window with a low heat transfer rate can use a smaller gap than a window that uses glass, and both will have the same U factor.