The secondary window traps an insulating mass of air concerning the internal and outer glass which minimizes warmth reduction, increases U-values, and when a minimal-E glass is used it can reflect back again infrared radiation and so retain more heat from the space. The isolated frames are considerably less conductive. https://manuelkevkx.blogsumer.com/26707476/the-2-minute-rule-for-affordable-secondary-glazing-bayswater