The thermal insulation coefficient value U is calculated according to the following European standards:
- EN 10077-1: Thermal performance of windows, doors and shutters – Calculation of thermal insulation coefficient – Part 1: General
- EN 10077-2: Thermal performance of windows, doors and shutters – Calculation of thermal insulation coefficient – Part 2: Numerical method for frames
These standard provide for 7 categories, classified in ascending order of thermal insulation (U):
Class T6 thus obtains the best thermal insulation.
N.B. => Legally defined value in Belgium: T4 ≤ 2.0 W/m²K!