Window symbols describe different calculation boundaries. Compare numbers only after establishing what they represent.
Ug and Uf
Ug refers to glazing and Uf to the frame. A better component value does not guarantee a better whole-window result with different dimensions and frame proportions.
Uw and glass edge
Uw accounts for the window configuration under the relevant method, including frame proportion and edge effects. Check whether the rating applies to your configuration.
Installation psi
Installation psi concerns the wall-to-window junction. Do not confuse it with the glazing-edge coefficient or add it to Uw without the proper lengths and areas.
Air and moisture
A thermal rating does not prove installation airtightness or rain drainage. Assess surface temperature and local moisture risk using geometry and conditions.
Common mistakes and review
Comparing one supplier’s Ug with another’s Uw is invalid. Retain dimensions, configuration, method and date rather than a number in a message.
| Item | Parameter | Check / result |
|---|---|---|
| Ug | Glazing | W/(m²K) |
| Uf | Frame | W/(m²K) |
| Uw | Window | W/(m²K) |
| ψ | Linear junction | W/(mK) |
What this means for your project
Use separate procurement columns for Ug, Uf, Uw and installation details. An empty column identifies information to request.
Sources and scope of review
- Passive House Institute — Window frames and component assessment
- PHI Passipedia — Window installation thermal bridges
Related reading
- Installing Windows in the Insulation Layer
- Thermal Bridges: Where They Occur and Why They Matter
- Energy-Efficient Windows: Which Parameters Really Matter