The U-value is the most important measure of how much heat escapes through a window. We explain the difference between Ug, Uf and Uw, how the values are calculated and how replacing windows affects your energy bill, with an example.
The "U-value", which appears often in window quotes and technical documents, is the most important indicator of a window’s energy performance. A significant share of the energy you spend on heating in winter and cooling in summer is lost through windows. Understanding the U-value helps you choose the right window and predict what your investment will bring you. In this article we explain the U-value in plain language and give a simple calculation example.
What Is a U-Value?
The U-value (thermal transmittance) expresses the amount of heat that passes through one square metre of a building element per second when there is a temperature difference of one degree between the inside and outside. Its unit is W/m²K. The rule is simple: the lower the U-value, the better the insulation.
For example, a window with a U-value of 2.8 W/m²K loses roughly twice as much heat under the same conditions as a window with a U-value of 1.4 W/m²K.
Ug, Uf and Uw: Three Different Values
A window as a whole consists of the glass, the frame and the edge zone where the two meet. That is why the U-value is expressed in three separate ways:
- Ug (glazing): The U-value of the glass alone. For standard air-filled 4+16+4 double glazing it is about 2.7–2.9 W/m²K, while with a Low-E coating and argon fill it drops to around 1.0–1.1 W/m²K.
- Uf (frame): The U-value of the frame, i.e. the frame and sash profiles. For good-quality PVC profiles it is generally in the range of 1.0–1.4 W/m²K.
- Uw (window): The U-value of the whole window. It takes into account the glass, the frame and the thermal bridge at the glass edge (the ψ value). This is the value you should use when comparing windows.
How Is Uw Calculated?
The Uw value is obtained from the area-weighted average of the glass and frame plus the thermal bridge at the glass edge:
Uw = (Ag × Ug + Af × Uf + lg × ψg) / (Ag + Af)
Here Ag is the glass area, Af the frame area, lg the total length of the glass edge and ψg the linear thermal transmittance at the glass edge. Using a warm-edge spacer instead of an aluminium spacer lowers ψg and improves Uw.
Today, with a good-quality PVC profile, Low-E argon-filled double glazing and a warm-edge spacer, the Uw value usually falls in the range of 1.0–1.4 W/m²K. For old single-glazed windows this value is above 5 W/m²K.
A Simple Heat Loss Calculation
The heat lost through a window can be found approximately with this formula: Q = U × A × ΔT. Here A is the window area (m²) and ΔT is the difference between the indoor and outdoor temperatures.
For example, take a 2 m² window with an indoor temperature of 21 °C and an outdoor temperature of 5 °C (ΔT = 16 K):
| Window type | Approximate Uw (W/m²K) | Instantaneous heat loss |
|---|---|---|
| Old single-glazed timber/aluminium | 5.0 | 160 W |
| Standard air-filled 4+16+4 | 2.6 | 83 W |
| Low-E + argon + warm edge | 1.3 | 42 W |
This difference is for a single window over a single hour. When all the windows in the house and the entire heating season are considered, the savings from replacing old windows become significant. The same logic applies in summer: windows with a low U-value also slow down the transfer of outdoor heat indoors, so the air conditioning runs less.
Is Looking at the U-Value Alone Enough?
The U-value describes heat conduction, but not the heat that enters through solar radiation. In regions with very sunny summers such as Ortaca and Dalaman, the glass’s solar energy transmittance (g-value) is at least as important as the U-value. Moreover, even the best window loses its performance with a leaky installation. That is why the U-value, the g-value and installation quality should be considered together.
Effect on Condensation and Comfort
The benefit of a low U-value is not reflected only in the bill. The inner surface of poorly insulated glass stays cold in winter; humid indoor air touching this surface condenses, forming water droplets on the glass and frame and, over time, mould stains. In a window with a low U-value, the inner surface temperature of the glass is closer to room temperature, so the risk of condensation drops noticeably. In addition, a person sitting in front of the window feels less of the "draught" caused by a cold surface, which makes it possible to run the heating at a lower setting.
What to Ask When Getting a Quote
- What is the Uw value of the window offered; is it calculated for the whole window, not just Ug?
- Does the glass have a Low-E coating and argon fill?
- Is the spacer aluminium or warm edge?
- Are insulating foam and sealing tapes used during installation?
Conclusion
The U-value is the basic measure of how much heat escapes through a window, and the lower it is, the greater the energy savings. When comparing, look at the Uw value, which describes the whole window, rather than the Ug value of the glass. During a free site survey our team examines your existing windows and assesses with you the gains you can achieve by replacing them.
