Ug, Uf and Uw: read window thermal performance
Published on 4 August 2026 · Verified on 18 September 2026 · The Mestravaux.lu team
A thermal value helps only when its scope is known. Glazing alone and a complete window are different objects. Request the symbol, unit, product and conditions behind each figure.
The essentials in 30 seconds
- Ug concerns glazing, Uf the frame and Uw the whole window.
- Compare values based on compatible calculations.
- Separate product performance, installation and grant criteria.
Identify the coefficient’s scope
Buildwise distinguishes Ug for glazing, Uf for the frame and Uw for the whole unit. Uw also accounts for geometry and glazing edges; it is not simply copied from Ug.
| Symbole | Périmètre | Usage |
|---|---|---|
| Ug | vitrage seul | comparer les compositions |
| Uf | cadre seul | comparer les profilés |
| Uw | fenêtre complète | comparer les produits |
| Pose | liaison au mur | évaluer le chantier |
Cette grille prépare la comparaison ; elle ne remplace pas le diagnostic du support et du produit précis.
Ask which basis is used
Clarify reference dimensions or project configuration. The documentation distinguishes these purposes. Do not dismiss reference data as useless; check its purpose and whether quotes use comparable bases.
Link the sheet to the order
Match system reference, opening type, glass composition and options to the quote. Request evidence for the value. Where a variant changes, confirm the corresponding figure rather than retaining the catalogue’s best value.
Your project deserves an accurate estimate — it's free.
Request a quoteSeparate decisions
Thermal selection alone does not resolve acoustics, light, solar protection or wall junctions. Request data matching each room’s priorities. Check the applicable scheme for grants; this guide establishes no eligibility from an isolated coefficient.
Related guides to continue
- Double or triple glazing: compare variants for your homePane count alone cannot select a window. Request identified variants and compare performance and cost within the same project. No average premium or payback is claimed without verifiable offers.Read the guide
- PVC, aluminium or timber-aluminium windows: compare proposed systemsVisible material does not describe the whole window. Compare profile sections, performance data and maintenance requirements for the actual models. This guide assigns no unsupported ranking for average price, lifespan or property resale value.Read the guide
- Window Klimabonus: prepare the application and check the rateWindow replacement may qualify under Klimabonus 2026 conditions. Pane count or an eligible label is insufficient. Prepare the application before work and confirm the regime matching your circumstances.Read the guide
- Understand a window quote: product, work and exclusionsFirst check whether offers describe the same openings and finished result. This guide provides a comparison method, not average prices or a legal contract template.Read the guide
Frequently asked questions
Are Ug and Uw interchangeable?
No. Their scopes differ.
Must values always use actual dimensions?
First establish purpose and basis. Product comparisons may use reference dimensions; project calculations have their own scope.
Does the figure guarantee installed performance?
Ask about substrate and junctions too. A product sheet does not replace site checks.
Take action
Your project deserves an accurate estimate — it's free.
Request a quote