The Window U-Value Explained: How Well Do New Windows Insulate?

When comparing window quotes, you come across abbreviations such as Uw, Ug and Uf everywhere. The most important figure is the Uw value – it tells you how much heat is lost through the entire window. We explain what is behind the numbers, which requirements typically apply and how much you can save with better windows.

What does the Uw value mean?

The U-value (thermal transmittance) indicates how much heat energy in watts is lost per square metre of a building component for every degree of temperature difference between inside and outside. The unit is W/(m²K). The rule is: the lower the value, the better the window insulates. The “w” stands for “window” – so the Uw value describes the complete window made up of frame, glass and glass edge.

Uw, Ug, Uf and Psi – the building blocks

• Ug (glazing): insulation value of the glass. Double low-E glazing is around 1.1, triple glazing 0.5 to 0.7 W/(m²K).
• Uf (frame): insulation value of the frame. Modern PVC profiles reach around 0.9 to 1.2 W/(m²K).
• Psi (Ψ): thermal bridge at the edge of the glass. A plastic “warm edge” spacer instead of aluminium reduces this loss.

The Uw value is calculated from these values and the proportions of the areas. Manufacturers usually state it for a reference window of 1.23 × 1.48 m. Small windows with a high frame proportion have a slightly worse value, large windows often a better one.

Which requirements apply?

Requirements depend on the country. In Germany, for example, the Building Modernisation Act (GModG), which replaced the Building Energy Act in July 2026, requires a Uw value of no more than 1.3 W/(m²K) when windows are replaced. Anyone applying for BAFA funding must achieve a much better Uw ≤ 0.95 W/(m²K). In Poland, the technical building regulations (WT 2021) call for 0.9 W/(m²K). Passive houses use windows of around 0.8 W/(m²K) and below.

Example: how much does a better Uw value save?

A detached house has 15 m² of windows. The old insulated glass windows have a Uw value of around 2.8, the new triple-glazed windows 0.8 W/(m²K). The difference of 2.0 W/(m²K) across 15 m² means 30 watts less heat loss per degree of temperature difference. Over a typical Central European heating season, this amounts to roughly 2,000 to 2,500 kWh less heating energy per year – several hundred euros, depending on energy prices. On top of that comes comfort: the inner glass surface stays warmer, there are fewer draughts and less condensation.

Note: the example is a simplified estimate. The actual saving depends on the building, heating habits and solar gains.

Tips for your quote

• Ask for the Uw value for your actual window sizes, not just for the reference window.
• Pay attention to the Ug value and the warm edge – both directly affect the Uw value.
• A good Uw value only pays off if the installation is airtight. Read more in our article on RAL installation.

Our PVC windows with triple glazing reach Uw values down to 0.74 W/(m²K) (VEKA Softline 82 MD). You will find details on our page PVC windows. We explain the differences between glazing types in Double or triple glazing.