Window performance depends on frame, glazing, glass edge and installation. Chamber count or glazing Ug alone does not describe an installed window.
Parameters
Uw describes the window, Ug the glazing and Uf the frame. Check the specimen dimensions, configuration and assessment method behind each value.
Solar gains and daylight
Solar and visible transmittance serve different purposes from U-value. Assess them against orientation, external shading and daylight needs.
Envelope and installation
Assess installation psi separately from the product rating. Supports, reveals and sills must preserve structural reliability and envelope continuity.
Air, water and maintenance
Seals, drainage and wall-barrier connections need inspectable details. After installation, check operation, closing and drainage against product documentation.
Common mistakes
Ordering by the best Ug ignores the frame and junction. Component certification does not automatically prove correct installation or building certification.
| Item | Parameter | Check / result |
|---|---|---|
| Ug | Glazing | Not the whole window |
| Uw | Window | Size and configuration |
| Installation psi | Junction | Separate assessment |
What this means for your project
Request the same parameter set for the actual window sizes in each quotation. Include installation details and summer performance in the comparison.
Sources and scope of review
- Passive House Institute — Window frames and component assessment
- NFRC — Certified product label verification
- PHI Passipedia — Window installation thermal bridges
Related reading
- Installing Windows in the Insulation Layer
- Thermal Bridges: Where They Occur and Why They Matter
- Summer Overheating: Why Highly Insulated Homes Can Still Overheat