Windows with double glazing are constructed using 2 sheets of glass with a gap between them. This is an isolated window that is installed on the outside of your property. Meanwhile, secondary glazing is a glazing unit (that can be single or double glazed) that is installed within your existing frame and usually on the internal side of the sill. Since there can be such a large gap between the external window and the secondary glazing, you’ll probably notice a dramatic difference in how much outside noise travels into your home. But unless the noise pollution in your area is severe and unbearable, most people avoid installing secondary windows. This is because they don’t tend to look very nice, and it greatly reduces the amount of sill space you have.