The U-value, also called heat transfer coefficient, indicates how much heat per degree (Kelvin) of temperature difference, per second, per m² is transmitted through a structure. The lower the U-value, the better insulated the structure is. The U-value is used to calculate the insulation value of windows and frames.
The U-value varies by type of glass. Below is a listing of the different types of glass and their typical U-value:
The article The difference between HR, HR+, HR++ and HR+++ glass explains exactly what the difference is between the different types of insulating glass.
The U-value of the window frame is different for each type of window frame. Below you can see the u-value of different types of window frames:
