Complete technical guide to the Uw thermal coefficient of windows: what values Romanian law requires in 2026, Salamander PVC and Alumil aluminium profile comparisons, glass types and real heating savings calculation.
What is the Uw Thermal Coefficient of a Window?
Uw (thermal transmittance) measures how much heat a window loses — the lower the value, the better the insulation. It is expressed in W/(m²K) and includes the profile (Uf), glass (Ug) and spacer (Ψg).
Calculation formula:
Uw = (Ag × Ug + Af × Uf + lg × Ψg) / (Ag + Af)
In practice: a window with Uw = 0.72 W/(m²K) loses 6.5 times less heat than an old one with Uw = 4.8 W/(m²K).
What Thermal Coefficient Does Romanian Law Require in 2026?
According to HG 765/2016 and Standard NP 048, maximum permitted Uw values differ by climate zone:
| Climate Zone | Maximum Uw | Areas |
|---|---|---|
| Zone I | 1.10 W/(m²K) | Dobrogea, coast |
| Zone II | 1.00 W/(m²K) | Bucharest, Ilfov, Muntenia |
| Zone III | 0.85 W/(m²K) | Transylvania, Moldova |
| Zone IV | 0.70 W/(m²K) | High mountain areas |
For Bucharest and Ilfov: Uw ≤ 1.00 W/(m²K) is the minimum legal limit for any new construction or permitted renovation.
Thermal Coefficient by Window Type
PVC Salamander Windows
| Profile | Uf (W/m²K) | Uw with triple 44mm | Uw with triple 52mm |
|---|---|---|---|
| bluEvolution 92 | 0.95 | 0.72 | 0.66 |
| bluEvolution 82 | 1.00 | 0.80 | 0.74 |
| GreenEvolution 76 | 1.10 | 0.88 | 0.82 |
Alumil Aluminium Windows
| System | Uf (W/m²K) | Uw with triple 44mm |
|---|---|---|
| S77 Supreme | 1.30 | 1.00–1.15 |
| SF85 (door) | 1.40 | 1.10–1.25 |
Thermal Insulating Glass Types
| Glass type | Ug (W/m²K) | Thickness | Uw impact |
|---|---|---|---|
| Double argon standard | 1.00 | 32 mm | Uw ≈ 1.2–1.4 |
| Triple argon 44mm | 0.50 | 44 mm | Uw ≈ 0.72–0.88 |
| Triple argon 52mm | 0.40 | 52 mm | Uw ≈ 0.66–0.80 |
What Uw Means in Money: Real Savings Calculation
Practical example — 50 m² apartment, 6 windows (7.2 m²), Bucharest:
Heat loss formula: Q = U × A × ΔT × t
- Average ΔT cold season: 20°C | Cold season hours: 3,500h
| Configuration | Uw | Loss kWh/year | Cost €/year |
|---|---|---|---|
| Old timber windows | 4.8 | 2,419 | ~€290 |
| Double argon standard | 1.4 | 706 | ~€85 |
| Triple Salamander 92 | 0.72 | 363 | ~€44 |
Saving vs double standard: 343 kWh/year = ~€41/year per apartment.
The Warm Edge Spacer — Why It Matters for Uw
The spacer (metal frame between glass panes) has a major impact on final window Uw:
- Standard aluminium spacer: Ψg = 0.10 W/(mK) — creates thermal bridge, promotes condensation
- Warm Edge spacer (Swisspacer, TGI): Ψg = 0.03 W/(mK) — reduces Uw by 0.05–0.10 W/(m²K)
At Neofort BIZ, all triple glazed windows are delivered with Warm Edge spacer as standard.
FAQ — Window Thermal Coefficient
What Uw should windows have for nZEB?
Per HG 765/2016, Uw ≤ 1.00 W/(m²K) for Zone II (Bucharest). Recommendation for nZEB homes: Uw ≤ 0.85 W/(m²K). Best available system: Salamander bluEvolution 92 with triple 52mm glazing — Uw = 0.66 W/(m²K).
What does Uf mean for windows?
Uf is the thermal coefficient of the profile (window frame), excluding glass. Salamander bluEvolution 92 has Uf = 0.95 W/(m²K) — the best in the PVC range commercially available in Romania.
What does Ug mean for windows?
Ug is the thermal coefficient of the glass unit. Double argon glass has Ug = 1.0 W/(m²K), triple argon has Ug = 0.5 W/(m²K) — 50% better.
Is Uw 1.1 acceptable for Bucharest?
No — the legal limit for Bucharest (Zone II) is Uw ≤ 1.00 W/(m²K). A window with Uw = 1.1 does not comply with regulations for new buildings.
What is the best Uw available in Romania?
Salamander bluEvolution 92 + triple 52mm + Warm Edge spacer + Blaugelb Triotherm+ preframes: Uw = 0.66 W/(m²K) — PassivHaus certified.
Conclusion — Which Uw Value to Choose
| Scenario | Recommended Uw | Optimal profile |
|---|---|---|
| Old apartment renovation | ≤ 1.00 | Salamander GreenEvolution 76 |
| Standard new construction | ≤ 0.85 | Salamander bluEvolution 82 |
| Certified nZEB home | ≤ 0.72 | Salamander bluEvolution 92 + triple |
| PHI passive house | ≤ 0.66 | bluEvolution 92 + triple 52mm + Blaugelb |
Related: nZEB Windows Guide Romania 2026 · Triple vs Double Glazing Savings · PVC vs Aluminium nZEB 2026
Neofort BIZ supplies Salamander PVC and Alumil aluminium windows with Uw from 0.66 W/(m²K). Request free technical consultation.
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