What does a window’s Uw-value mean

Client asks:

When choosing windows, Uw-values are mentioned quite often. What does this actually mean, and why should it matter?

Aru Grupp answers:

The Uw-value is one of the most important indicators when assessing a window’s thermal insulation. Simply put, it shows how much heat a window lets through. The lower the Uw-value, the better the window retains heat, making the whole building more energy-efficient.

Uw-values are measured in W/m²K, which accounts for both the window glass and frame, as well as how they work together. This means the Uw-value always reflects the thermal performance of the entire window, not just the glass.

In technical window specifications, you may also see other values:

• Ug-value – thermal insulation of the glass unit
• Uf-value – thermal insulation of the window frame
• Uw-value – thermal insulation of the complete window

In practice, Uw is what really matters. Good glass cannot compensate for a poor frame, and vice versa – the key is balance, where all components work together.

Why is Uw-value important?

• 🔥 Lower Uw = lower heating costs – a good window helps maintain stable indoor temperatures and reduces heat loss.
• 🌡️ Better indoor climate – reduces cold bridges and drafts, creating a comfortable temperature near the window.
• 💧 Less condensation – warm glass minimizes moisture buildup.
• 🏡 Impact on overall building energy efficiency – the right windows improve the building’s energy performance and energy class.

What is a good Uw-value?

• Standard modern window: ≈ 1.0–1.2 W/m²K
• Energy-saving solution: ≤ 0.9 W/m²K
• Highly energy-efficient / low-energy building: ≤ 0.8 W/m²K

It’s important to understand that the lowest number isn’t always the best choice. You should also consider window size, location, building use, and architecture.

What affects a window’s Uw-value?

Several factors influence thermal performance: glass unit type (double- or triple-glazed), selective coating, argon gas between panes, frame material and construction, as well as window size and layout. Therefore, Uw cannot be considered in isolation – a window always functions as a complete solution where every detail matters.

Aru Grupp’s experience shows that the number alone is not enough. A good Uw-value is important, but the right window must be balanced: it should match the building’s architecture, suit its intended use, be technically correct and durable, and provide real energy savings, not just a good number on paper.

A high-quality wood or wood-aluminium window is an investment that pays off over the years – both by reducing energy costs and increasing the building’s value.

Uw-value shows how well a window retains heat, but the best results come from a thoughtful, balanced solution. The right window improves indoor comfort, reduces energy consumption, and enhances the overall value of the building.

Explore Aru Grupp’s window selection here:
👉 https://arugrupp.ee/en/windows/

📧 aken@arugrupp.ee
📞 +372 329 5650
🌐 www.arugrupp.ee