Uw, Ug, Uf: Understanding Coefficients Before Signing a Window Quote

Why Are Uw, Ug, and Uf Coefficients Essential in a Window Quote?

When you receive a quote for replacing your window frames, you are faced with a multitude of figures and technical terms. Among them, the Uw, Ug, and Uf coefficients are the most important. They determine the energy performance of your windows, your indoor comfort, and your heating consumption. In Luxembourg, where winters are harsh and energy is costly, understanding these values is a prerequisite before signing a quote.

This article explains in simple terms what these coefficients mean, how to interpret them, and what values to aim for to benefit from the Klimabonus Wunnen 2026 (the program that replaced PRIMe House in January 2026). You will be able to ask the right questions to your contractor and compare offers with full knowledge.

What Is the U-Value?

The U-value (or thermal transmittance) measures the heat transfer through a building element. It expresses the amount of heat that escapes through one square meter of surface for a temperature difference of one Kelvin between inside and outside. Its unit is W/m²K (watts per square meter kelvin).

The lower the U-value, the better the insulation. A window with a Uw of 1.0 W/m²K loses less heat than one with a Uw of 2.0 W/m²K. Thus, to reduce heat loss, you should aim for the lowest possible U-values.

In the window domain, three coefficients are distinguished:

• Uw: coefficient of the complete window (glazing + frame + spacers).
• Ug: coefficient of the glazing (the glass part).
• Uf: coefficient of the frame (the profile surrounding the glazing).

These three values are linked by a formula that accounts for dimensions and edge effects (see below).

Ug: The Glazing Coefficient

The Ug is the U-value of the glazing alone. It depends on the type of glass, number of panes, filling gas (argon, krypton), and the nature of the spacers. For standard double glazing, the Ug is around 1.1 W/m²K. High-performance triple glazing can achieve 0.5 W/m²K or less.

In Luxembourg, to qualify for the Klimabonus, the replaced glazing must have a Ug ≤ 1.0 W/m²K. This is a minimum requirement. For a passive house, aim for a Ug ≤ 0.6 W/m²K.

Note: A very low Ug is not sufficient if the frame is poorly insulated. The Uw is the overall value that truly matters.

Uf: The Frame Coefficient

The Uf measures the insulation of the window frame (wood, PVC, aluminum, or composite). Solid wood frames typically have a Uf between 1.2 and 1.5 W/m²K. PVC profiles with multiple chambers can achieve 0.8 to 1.2 W/m²K. Aluminum, an excellent conductor, requires a thermal break to obtain acceptable values.

The Uf influences the Uw more when the frame area is large relative to the glazing area. For example, a small window will have a larger frame proportion than a large picture window. Therefore, it is important to check the stated Uf and prefer frames with low conductivity.

Uw: The Complete Window Coefficient

The Uw is the value that characterizes the overall performance of the window, i.e., the combination of glazing, frame, and spacers. It is calculated according to the standard EN ISO 10077-1. The simplified formula is:

Uw = (Ag × Ug + Af × Uf + Lg × Ψ) / (Ag + Af)

• Ag: glazing area
• Af: frame area
• Lg: glazing perimeter
• Ψ: linear thermal transmittance due to the spacer (edge effect)

In practice, the Uw is always lower than the weighted average of Ug and Uf, because spacers create a thermal bridge. That is why it is essential to check the stated Uw, not just the Ug.

For a standard renovation in Luxembourg, a Uw ≤ 1.3 W/m²K is recommended. For a passive house, aim for a Uw ≤ 0.80 W/m²K. Some manufacturers offer windows with Uw = 0.60 W/m²K, but they are more expensive.

Klimabonus Wunnen 2026 Requirements for Windows

The Klimabonus Wunnen 2026 program (managed by Klima-Agence) encourages energy-efficient renovation of homes. It replaces the former PRIMe House since January 2026. For window replacement, the aid is 70 EUR per m² of replaced glazing (glazing area, excluding frame). This aid is subject to technical conditions:

• The glazing must have a Ug ≤ 1.0 W/m²K.
• The complete window (frame) must have a Uw ≤ 1.3 W/m²K (indicative value, to be verified on technical datasheets).
• Work must be carried out by a qualified professional.
• The dwelling must be more than 2 years old (to benefit from the reduced VAT rate of 3%).

Additionally, window replacement can be combined with other measures (insulation, ventilation) to achieve an overall bonus. Remember that Klimabonus is cumulative with the super-reduced VAT of 3% (instead of 17%) for renovation works in a dwelling older than two years.

For exact conditions and to apply, consult the website of Guichet.lu or Klima-Agence.

How to Read a Window Quote?

A professional quote must clearly state the Uw, Ug, and Uf values for each proposed window model. Here are the points to check:

• Stated Uw: It should be ≤ 1.3 W/m²K to be eligible for Klimabonus (and ideally ≤ 1.0 for optimal comfort).
• Ug of the glazing: Verify it is indeed ≤ 1.0 W/m²K.
• Uf of the frame: Ask for the exact value, especially if you choose an aluminum frame.
• Solar factor (g): To benefit from solar gains in winter, a g ≥ 0.5 is recommended.
• Light transmission (TL): For natural brightness.
• Standards and certifications: Values must be certified by a laboratory (e.g., IFT Rosenheim).

Beware of vague quotes that only mention "double glazing" or "reinforced insulation window" without providing figures. Always request the technical datasheets of the proposed products.

Illustrative Example: Calculating Aid for Window Replacement

Consider a typical case: you replace 10 windows in a single-family house. The total glazing area (glass part) is 15 m². With Klimabonus, you will receive 70 EUR/m² × 15 m² = 1,050 EUR. If the total cost of the windows (supply and installation) is 12,000 EUR excl. VAT, you will also benefit from the 3% VAT instead of 17%, saving about 1,680 EUR. The total aid (subsidy + VAT) thus amounts to approximately 2,730 EUR.

Of course, these figures are indicative. The exact amount depends on the actual glazing area and the conditions of your project. Consult a Klima-Agence advisor for a personalized estimate.

Common Mistakes to Avoid When Choosing Windows

Here are the most common pitfalls:

• Focusing only on price: A cheap window with a high Uw will cost you more in heating bills in the long run.
• Neglecting installation: A high-performance window poorly installed can create thermal bridges. Insist on careful installation and possibly an air-tightness test.
• Ignoring orientation: Highly insulating windows with a low solar factor can reduce free heat gains in winter. Adapt the glazing according to orientation.
• Forgetting ventilation: Airtight windows reduce fresh air supply. Plan mechanical ventilation (MVHR) to prevent humidity and mold.

Conclusion: Clear Figures for a Sustainable Investment

Understanding the Uw, Ug, and Uf coefficients allows you to communicate effectively with contractors and compare offers on an objective basis. In Luxembourg, the Klimabonus Wunnen 2026 offers substantial aid for glazing replacement, provided you meet performance requirements. Before signing a quote, ensure that the Uw, Ug, and Uf values are clearly stated and meet the eligibility criteria. Do not hesitate to seek advice from an independent professional or a Klima-Agence advisor. Investing in high-performance windows is an investment in your comfort, health, and wallet.