What is a U-value, and why does it matter?

What U measures

The U-value shows how much heat passes through one square metre of a construction when the difference between inside and outside temperature is one degree. The unit is watts per square metre per kelvin, W/m²K.

The key rule is simple: the lower the number, the better. A window with U = 0.9 loses less heat than one with U = 1.3. That runs against the usual bigger-is-better logic, and it is where people often slip up.

Three different Us

• Ug is the glass unit only. Usually the most flattering number in a quote.
• Uf is the frame profile only.
• Uw is the whole window, with frame, glass and spacer bar together.

Only compare Uw with Uw. If one quote says 0.5 and another 0.9, the first may well be the Ug of the glass unit and the second the Uw of the whole window, in which case the second window is actually the warmer one. It is a common trick, and not always a deliberate one.

For comparison

To put the number in context: the uninsulated wall of a panel apartment block typically has a U of around 0.5–0.7. An old wooden window with two separate panes of glass is roughly 2.5–3. That shows why new windows make such a noticeable difference, and why the window is nearly always still the coldest part of the wall.

What the U-value does not tell you

It describes the window as a factory product tested in a laboratory. In the wall it works together with the installation joint, and if that joint is badly done the lab figure means little: heat escapes around the edge of the frame rather than through the glass.

Nor does U tell you anything about noise, which has its own separate rating, about draughts, or about how the window will behave in ten years. So "what is your U-value?" is a useful question, but not the only one.

We make our own double and triple glazed windows. For the figures that apply to the configuration for your opening, ask us when we measure: +372 5555 1682.