HomeHow much heat escapes through the entire window (frame + glass + spacer)
The lower the value, the better the window insulates
The window thermal transmittance (Uw) is a key parameter indicating how much heat escapes through 1 m² of a window at a temperature difference of 1 Kelvin. The lower the value, the better the window insulates; for new buildings, the standard is Uw ≤ 0.85 W/(m²K) (new windows should be even better), which reduces heating costs and increases comfort. The Uw value is influenced by the glazing, the frame (profiles), and the installation, with smaller windows typically having a poorer value than larger ones.
How to achieve a low Uw value
- Multiple glazing: Triple glazing is more thermally efficient than double glazing.
- Low-emissivity coatings: Additional glass coating reduces heat transfer.
- Insulating glass cavity filling: Noble gases (argon, krypton) between the panes improve insulation.
- Quality frame: Choice of profile system (plastic, aluminum)
- Spacer bar
- Dimensions
Unit
Watt per square meter Kelvin W/(m²K)
Related values
Uf: Thermal transmittance of the frame (glazier’s dictionary)
Uw: Thermal transmittance of the window (glazier’s dictionary)
Ug: Thermal transmittance of the insulating glass (glazier’s dictionary)
Ψg (psi): Linear thermal transmittance at the frame and glass junction (spacer bar) – link
