A U-value is a measurement that indicates a window’s glass or other material’s thermal efficiency – the lower a material’s U-value is, the longer it takes for heat to pass through.
With an average U-Value of just 0.7W/(m²K), retrofitted sash windows with vacuum double glazing are at the top of the market for energy efficiency.
These efficient windows ensure that properties provide a cosy place to keep warm throughout the winter and prevent energy bills from getting too high. Read more here https://chameleon-decorators.co.uk/blog/u-values-for-windows/