What Thermal Coefficient Should Windows Have? Uw Explained (2026)

What is the Uw thermal coefficient of a window?

Uw (Wärmedurchgangskoeffizient) is the value that measures how much heat a window loses — the lower it is, the better the window insulates. It is expressed in W/(m²K) and covers the profile (Uf), the glass (Ug) and the spacer (Ψg).

Calculation formula:

Uw = (Ag × Ug + Af × Uf + lg × Ψg) / (Ag + Af)

In practice: a window with Uw = 0.72 W/(m²K) loses almost 7 times less heat than an old one with Uw = 4.8 W/(m²K).

What thermal coefficient does Romanian legislation require in 2026?

The methodology for calculating the energy performance of buildings, Mc 001-2022 (approved by MDLPA Order 16/2023), gives reference values for Uw depending on the type of building and work:

For a new house in Bucharest, Uw ≤ 1.11 W/(m²K) is the reference value. nZEB compliance is then checked on the energy performance of the whole building, not just the windows.

Neofort BIZ recommendation: Uw ≤ 0.85 W/(m²K) — a clear safety margin below the reference value, for guaranteed long-term performance.

The thermal coefficient by window type

Salamander PVC windows

Alumil aluminium joinery

Types of insulating glass

What Uw means in money: a real savings calculation

Practical example — 50 m² apartment, 6 windows (7.2 m²), Bucharest:

Heat loss formula: Q = U × A × ΔT × t

• Average ΔT in the cold season: 20°C | Cold-season hours: 3,500 h | Energy price: ~0.12 EUR/kWh

Saving compared with standard double glazing: 343 kWh/year, or ~41 EUR/year per apartment. Compared with old wooden windows: ~2,056 kWh/year, or ~245 EUR/year.

The Warm Edge spacer — why it matters for Uw

The spacer (the frame between the panes) has a major impact on the window's final Uw:

• Standard aluminium spacer: Ψg = 0.10 W/(mK) — creates a thermal bridge and encourages condensation
• Warm Edge spacer (for example Swisspacer): Ψg = 0.03 W/(mK) — lowers Uw by 0.05–0.10 W/(m²K)

At Neofort BIZ, the Warm Edge spacer is fitted on request at a separate cost; about 90% of customers choose it.

Frequently Asked Questions — the thermal coefficient of windows

What Uw should windows have for nZEB?

Mc 001-2022 gives a reference value of Uw ≤ 1.11 W/(m²K) for new houses. For nZEB houses we recommend Uw ≤ 0.85 W/(m²K). The best system we offer: Salamander bluEvolution 92 with triple glazing, with Uw from 0.70 W/(m²K).

What do Uf and Ug mean for windows?

Uf is the thermal coefficient of the profile (the window frame), without glass: bluEvolution 92 has Uf = 0.95 W/(m²K), the best in the Salamander range we offer. Ug is the thermal coefficient of the glass: double argon 1.0–1.1 W/(m²K), triple argon 0.5–0.6 W/(m²K), roughly half.

Is Uw 1.1 OK for Bucharest?

It complies: the Mc 001-2022 reference value for a new house is Uw ≤ 1.11 W/(m²K), and 1.20 for renovations. For an nZEB house we still recommend Uw ≤ 0.85 W/(m²K), with a safety margin.

What is the best Uw we can achieve?

Salamander bluEvolution 92 with triple glazing and a Warm Edge spacer: Uw from 0.70 W/(m²K); the bluEvolution system holds Passivhaus certification. Installation with Blaugelb Triotherm+ pre-frames does not change the window's Uw, but it preserves its performance once built into the wall.

How do I check a window's Uw?

Ask for the Declaration of Performance (DoP) and the IFT Rosenheim or TÜV certificate for the profile used. The Uw must be calculated according to EN ISO 10077 and stated in the technical documents.

Conclusion — which Uw value to choose

The simple rule: always choose a Uw with a clear margin below the reference value. For a new house in Bucharest, that means Uw ≤ 0.85 W/(m²K), against a reference value of 1.11.

If you are installing a heat pump, this value becomes decisive: see why Uw matters for the pump's efficiency.

If you want to improve insulation without replacing the windows, there are a few solutions: How can I improve window thermal insulation in Bucharest? Complete guide 2026.

Related articles:

• Which windows should you choose for an nZEB house in Romania?
• Triple vs double glazing: how much do you really save on heating?
• PVC or aluminium for nZEB homes in 2026?

Neofort BIZ supplies Salamander PVC and Alumil aluminium windows with Uw from 0.70 W/(m²K). Ask for technical advice. See the Salamander PVC joinery we offer. Useful pages: Saint-Gobain SGG insulating glass and the Warm Edge thermal spacer.