Energy glass in numbers: U-values Ug-values and dew point
How to read the technical data for a pane of glass
A modern window is a technical product and as a glazier you are constantly met with numbers that describe how well it insulates. Being able to read and explain those numbers is part of being a trusted advisor.
§U-value and Ug-value — what's the difference?
The Ug value describes only the glass itself—it is measured in the middle of the pane (center-of-glass) and tells how well the glass layers and the space between them insulate. The U value (also called Uw for the entire window) takes the whole element: glass frame casing and muntins. Two windows with the same glass can therefore have different total U value depending on how well the frame itself insulates.
| Window type | Typical Ug value (approx.) |
|---|---|
| Single glass | About 5.8 W/m²K |
| Thermal window, 2 layers of energy glass | About 1.1–1.3 W/m²K |
| 3-layer energy window | About 0.5–0.8 W/m²K |
§How to achieve a low U-value
- 01A low-emissivity coating (low-e) on one of the glass surfaces reflects heat radiation back into the room.
- 02The space between is filled with an insulating gas such as argon or krypton instead of ordinary air.
- 03The distance between the glass layers and the number of layers has a direct effect on insulation.
- 04A warm edge (warm edge spacer profile) in the perimeter zone reduces thermal bridging and risk of condensation along the edge.
§Dew point and condensation.
The dew point is the temperature at which air moisture begins to condense into water droplets. If you see condensation on the outside of an energy window in the morning, it's actually often a good sign — it means the outer glass is cold because the window insulates well. If you see condensation between the glass layers instead, it means the window is punctured and has lost its seal — then it needs to be replaced.
§Why it matters for you
The Building Regulations' energy requirements for the total U-value of windows are continuously being tightened as the green transition progresses and customers become more aware of their heating bills. If you can explain the difference between Ug and U what a low U-value requires and what condensation actually indicates you give advice the customer can act on — not just a sale.