HomeWhat Does This Value Indicate? Heat Loss Through the Frame.
Lower Value, Warmer Window.
The frame heat transfer coefficient, Uf (from English ‘frame’), indicates how well the window frame itself insulates. A lower number signifies better thermal insulation, which is crucial for the overall thermal efficiency of the window (Uw), also influenced by glazing (Ug) and other factors.
What Does Uf Mean?
Uf (f = frame)
The rate of heat loss through the window frame.
Unit
Expressed in watts per square meter Kelvin (W/m²K).
Impact on Overall Window (Uw)
The Uf value, along with Ug (glass) and Ψg (spacer frame), determines the resulting Uw value for the entire window. Smaller windows have a poorer Uw value because, compared to the frame, they have a larger glass area, which insulates better.
Why Is It Important?
A good (low) Uf prevents heat loss through the frame, which is critical, especially for aluminum and older window types.
For passive and low-energy buildings, windows with a very low Uw are ideal, and this also requires a high-quality Uf (e.g., below 0.8 W/m²K).
Related Values
Ug: Heat transfer coefficient through glazing (window glossary)
Uw: Heat transfer coefficient for the entire window (window glossary)
Ψg (Psi): Linear heat transfer coefficient at the frame-glass junction (spacer) (window glossary)
